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https://github.com/RPCS3/llvm-mirror.git
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7fbb587058
As a follow-up to https://reviews.llvm.org/D104129, I'm cleaning up the danling probe related code in both the compiler and llvm-profgen. I'm seeing a 5% size win for the pseudo_probe section for SPEC2017 and 10% for Ciner. Certain benchmark such as 602.gcc has a 20% size win. No obvious difference seen on build time for SPEC2017 and Cinder. Reviewed By: wenlei Differential Revision: https://reviews.llvm.org/D104477
3074 lines
119 KiB
C++
3074 lines
119 KiB
C++
//===- JumpThreading.cpp - Thread control through conditional blocks ------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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//
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// This file implements the Jump Threading pass.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar/JumpThreading.h"
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#include "llvm/ADT/DenseMap.h"
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#include "llvm/ADT/DenseSet.h"
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#include "llvm/ADT/MapVector.h"
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#include "llvm/ADT/Optional.h"
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#include "llvm/ADT/STLExtras.h"
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#include "llvm/ADT/SmallPtrSet.h"
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#include "llvm/ADT/SmallVector.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Analysis/AliasAnalysis.h"
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#include "llvm/Analysis/BlockFrequencyInfo.h"
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#include "llvm/Analysis/BranchProbabilityInfo.h"
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#include "llvm/Analysis/CFG.h"
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#include "llvm/Analysis/ConstantFolding.h"
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#include "llvm/Analysis/DomTreeUpdater.h"
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#include "llvm/Analysis/GlobalsModRef.h"
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#include "llvm/Analysis/GuardUtils.h"
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#include "llvm/Analysis/InstructionSimplify.h"
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#include "llvm/Analysis/LazyValueInfo.h"
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#include "llvm/Analysis/Loads.h"
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#include "llvm/Analysis/LoopInfo.h"
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#include "llvm/Analysis/MemoryLocation.h"
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#include "llvm/Analysis/TargetLibraryInfo.h"
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#include "llvm/Analysis/TargetTransformInfo.h"
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#include "llvm/Analysis/ValueTracking.h"
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#include "llvm/IR/BasicBlock.h"
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#include "llvm/IR/CFG.h"
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#include "llvm/IR/Constant.h"
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#include "llvm/IR/ConstantRange.h"
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#include "llvm/IR/Constants.h"
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#include "llvm/IR/DataLayout.h"
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#include "llvm/IR/Dominators.h"
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#include "llvm/IR/Function.h"
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#include "llvm/IR/InstrTypes.h"
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#include "llvm/IR/Instruction.h"
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#include "llvm/IR/Instructions.h"
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#include "llvm/IR/IntrinsicInst.h"
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#include "llvm/IR/Intrinsics.h"
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#include "llvm/IR/LLVMContext.h"
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#include "llvm/IR/MDBuilder.h"
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#include "llvm/IR/Metadata.h"
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#include "llvm/IR/Module.h"
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#include "llvm/IR/PassManager.h"
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#include "llvm/IR/PatternMatch.h"
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#include "llvm/IR/Type.h"
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#include "llvm/IR/Use.h"
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#include "llvm/IR/User.h"
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#include "llvm/IR/Value.h"
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#include "llvm/InitializePasses.h"
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#include "llvm/Pass.h"
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#include "llvm/Support/BlockFrequency.h"
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#include "llvm/Support/BranchProbability.h"
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#include "llvm/Support/Casting.h"
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#include "llvm/Support/CommandLine.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/raw_ostream.h"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Transforms/Utils/BasicBlockUtils.h"
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#include "llvm/Transforms/Utils/Cloning.h"
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#include "llvm/Transforms/Utils/Local.h"
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#include "llvm/Transforms/Utils/SSAUpdater.h"
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#include "llvm/Transforms/Utils/ValueMapper.h"
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#include <algorithm>
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#include <cassert>
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#include <cstddef>
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#include <cstdint>
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#include <iterator>
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#include <memory>
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#include <utility>
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using namespace llvm;
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using namespace jumpthreading;
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#define DEBUG_TYPE "jump-threading"
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STATISTIC(NumThreads, "Number of jumps threaded");
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STATISTIC(NumFolds, "Number of terminators folded");
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STATISTIC(NumDupes, "Number of branch blocks duplicated to eliminate phi");
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static cl::opt<unsigned>
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BBDuplicateThreshold("jump-threading-threshold",
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cl::desc("Max block size to duplicate for jump threading"),
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cl::init(6), cl::Hidden);
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static cl::opt<unsigned>
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ImplicationSearchThreshold(
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"jump-threading-implication-search-threshold",
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cl::desc("The number of predecessors to search for a stronger "
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"condition to use to thread over a weaker condition"),
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cl::init(3), cl::Hidden);
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static cl::opt<bool> PrintLVIAfterJumpThreading(
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"print-lvi-after-jump-threading",
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cl::desc("Print the LazyValueInfo cache after JumpThreading"), cl::init(false),
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cl::Hidden);
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static cl::opt<bool> JumpThreadingFreezeSelectCond(
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"jump-threading-freeze-select-cond",
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cl::desc("Freeze the condition when unfolding select"), cl::init(false),
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cl::Hidden);
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static cl::opt<bool> ThreadAcrossLoopHeaders(
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"jump-threading-across-loop-headers",
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cl::desc("Allow JumpThreading to thread across loop headers, for testing"),
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cl::init(false), cl::Hidden);
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namespace {
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/// This pass performs 'jump threading', which looks at blocks that have
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/// multiple predecessors and multiple successors. If one or more of the
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/// predecessors of the block can be proven to always jump to one of the
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/// successors, we forward the edge from the predecessor to the successor by
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/// duplicating the contents of this block.
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///
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/// An example of when this can occur is code like this:
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///
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/// if () { ...
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/// X = 4;
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/// }
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/// if (X < 3) {
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///
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/// In this case, the unconditional branch at the end of the first if can be
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/// revectored to the false side of the second if.
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class JumpThreading : public FunctionPass {
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JumpThreadingPass Impl;
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public:
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static char ID; // Pass identification
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JumpThreading(bool InsertFreezeWhenUnfoldingSelect = false, int T = -1)
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: FunctionPass(ID), Impl(InsertFreezeWhenUnfoldingSelect, T) {
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initializeJumpThreadingPass(*PassRegistry::getPassRegistry());
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}
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bool runOnFunction(Function &F) override;
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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AU.addRequired<DominatorTreeWrapperPass>();
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AU.addPreserved<DominatorTreeWrapperPass>();
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AU.addRequired<AAResultsWrapperPass>();
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AU.addRequired<LazyValueInfoWrapperPass>();
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AU.addPreserved<LazyValueInfoWrapperPass>();
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AU.addPreserved<GlobalsAAWrapperPass>();
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AU.addRequired<TargetLibraryInfoWrapperPass>();
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AU.addRequired<TargetTransformInfoWrapperPass>();
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}
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void releaseMemory() override { Impl.releaseMemory(); }
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};
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} // end anonymous namespace
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char JumpThreading::ID = 0;
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INITIALIZE_PASS_BEGIN(JumpThreading, "jump-threading",
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"Jump Threading", false, false)
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INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
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INITIALIZE_PASS_DEPENDENCY(LazyValueInfoWrapperPass)
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INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
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INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
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INITIALIZE_PASS_END(JumpThreading, "jump-threading",
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"Jump Threading", false, false)
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// Public interface to the Jump Threading pass
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FunctionPass *llvm::createJumpThreadingPass(bool InsertFr, int Threshold) {
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return new JumpThreading(InsertFr, Threshold);
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}
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JumpThreadingPass::JumpThreadingPass(bool InsertFr, int T) {
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InsertFreezeWhenUnfoldingSelect = JumpThreadingFreezeSelectCond | InsertFr;
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DefaultBBDupThreshold = (T == -1) ? BBDuplicateThreshold : unsigned(T);
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}
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// Update branch probability information according to conditional
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// branch probability. This is usually made possible for cloned branches
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// in inline instances by the context specific profile in the caller.
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// For instance,
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//
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// [Block PredBB]
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// [Branch PredBr]
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// if (t) {
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// Block A;
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// } else {
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// Block B;
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// }
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//
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// [Block BB]
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// cond = PN([true, %A], [..., %B]); // PHI node
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// [Branch CondBr]
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// if (cond) {
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// ... // P(cond == true) = 1%
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// }
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//
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// Here we know that when block A is taken, cond must be true, which means
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// P(cond == true | A) = 1
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//
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// Given that P(cond == true) = P(cond == true | A) * P(A) +
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// P(cond == true | B) * P(B)
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// we get:
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// P(cond == true ) = P(A) + P(cond == true | B) * P(B)
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//
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// which gives us:
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// P(A) is less than P(cond == true), i.e.
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// P(t == true) <= P(cond == true)
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//
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// In other words, if we know P(cond == true) is unlikely, we know
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// that P(t == true) is also unlikely.
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//
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static void updatePredecessorProfileMetadata(PHINode *PN, BasicBlock *BB) {
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BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
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if (!CondBr)
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return;
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uint64_t TrueWeight, FalseWeight;
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if (!CondBr->extractProfMetadata(TrueWeight, FalseWeight))
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return;
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if (TrueWeight + FalseWeight == 0)
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// Zero branch_weights do not give a hint for getting branch probabilities.
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// Technically it would result in division by zero denominator, which is
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// TrueWeight + FalseWeight.
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return;
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// Returns the outgoing edge of the dominating predecessor block
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// that leads to the PhiNode's incoming block:
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auto GetPredOutEdge =
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[](BasicBlock *IncomingBB,
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BasicBlock *PhiBB) -> std::pair<BasicBlock *, BasicBlock *> {
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auto *PredBB = IncomingBB;
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auto *SuccBB = PhiBB;
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SmallPtrSet<BasicBlock *, 16> Visited;
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while (true) {
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BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator());
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if (PredBr && PredBr->isConditional())
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return {PredBB, SuccBB};
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Visited.insert(PredBB);
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auto *SinglePredBB = PredBB->getSinglePredecessor();
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if (!SinglePredBB)
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return {nullptr, nullptr};
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// Stop searching when SinglePredBB has been visited. It means we see
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// an unreachable loop.
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if (Visited.count(SinglePredBB))
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return {nullptr, nullptr};
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SuccBB = PredBB;
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PredBB = SinglePredBB;
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}
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};
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for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
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Value *PhiOpnd = PN->getIncomingValue(i);
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ConstantInt *CI = dyn_cast<ConstantInt>(PhiOpnd);
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if (!CI || !CI->getType()->isIntegerTy(1))
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continue;
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BranchProbability BP =
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(CI->isOne() ? BranchProbability::getBranchProbability(
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TrueWeight, TrueWeight + FalseWeight)
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: BranchProbability::getBranchProbability(
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FalseWeight, TrueWeight + FalseWeight));
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auto PredOutEdge = GetPredOutEdge(PN->getIncomingBlock(i), BB);
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if (!PredOutEdge.first)
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return;
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BasicBlock *PredBB = PredOutEdge.first;
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BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator());
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if (!PredBr)
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return;
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uint64_t PredTrueWeight, PredFalseWeight;
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// FIXME: We currently only set the profile data when it is missing.
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// With PGO, this can be used to refine even existing profile data with
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// context information. This needs to be done after more performance
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// testing.
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if (PredBr->extractProfMetadata(PredTrueWeight, PredFalseWeight))
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continue;
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// We can not infer anything useful when BP >= 50%, because BP is the
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// upper bound probability value.
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if (BP >= BranchProbability(50, 100))
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continue;
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SmallVector<uint32_t, 2> Weights;
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if (PredBr->getSuccessor(0) == PredOutEdge.second) {
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Weights.push_back(BP.getNumerator());
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Weights.push_back(BP.getCompl().getNumerator());
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} else {
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Weights.push_back(BP.getCompl().getNumerator());
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Weights.push_back(BP.getNumerator());
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}
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PredBr->setMetadata(LLVMContext::MD_prof,
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MDBuilder(PredBr->getParent()->getContext())
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.createBranchWeights(Weights));
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}
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}
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/// runOnFunction - Toplevel algorithm.
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bool JumpThreading::runOnFunction(Function &F) {
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if (skipFunction(F))
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return false;
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auto TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
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// Jump Threading has no sense for the targets with divergent CF
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if (TTI->hasBranchDivergence())
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return false;
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auto TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(F);
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auto DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
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auto LVI = &getAnalysis<LazyValueInfoWrapperPass>().getLVI();
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auto AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
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DomTreeUpdater DTU(*DT, DomTreeUpdater::UpdateStrategy::Lazy);
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std::unique_ptr<BlockFrequencyInfo> BFI;
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std::unique_ptr<BranchProbabilityInfo> BPI;
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if (F.hasProfileData()) {
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LoopInfo LI{DominatorTree(F)};
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BPI.reset(new BranchProbabilityInfo(F, LI, TLI));
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BFI.reset(new BlockFrequencyInfo(F, *BPI, LI));
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}
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bool Changed = Impl.runImpl(F, TLI, LVI, AA, &DTU, F.hasProfileData(),
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std::move(BFI), std::move(BPI));
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if (PrintLVIAfterJumpThreading) {
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dbgs() << "LVI for function '" << F.getName() << "':\n";
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LVI->printLVI(F, DTU.getDomTree(), dbgs());
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}
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return Changed;
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}
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PreservedAnalyses JumpThreadingPass::run(Function &F,
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FunctionAnalysisManager &AM) {
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auto &TTI = AM.getResult<TargetIRAnalysis>(F);
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// Jump Threading has no sense for the targets with divergent CF
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if (TTI.hasBranchDivergence())
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return PreservedAnalyses::all();
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auto &TLI = AM.getResult<TargetLibraryAnalysis>(F);
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auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
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auto &LVI = AM.getResult<LazyValueAnalysis>(F);
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auto &AA = AM.getResult<AAManager>(F);
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DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
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std::unique_ptr<BlockFrequencyInfo> BFI;
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std::unique_ptr<BranchProbabilityInfo> BPI;
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if (F.hasProfileData()) {
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LoopInfo LI{DominatorTree(F)};
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BPI.reset(new BranchProbabilityInfo(F, LI, &TLI));
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BFI.reset(new BlockFrequencyInfo(F, *BPI, LI));
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}
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bool Changed = runImpl(F, &TLI, &LVI, &AA, &DTU, F.hasProfileData(),
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std::move(BFI), std::move(BPI));
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if (PrintLVIAfterJumpThreading) {
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dbgs() << "LVI for function '" << F.getName() << "':\n";
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LVI.printLVI(F, DTU.getDomTree(), dbgs());
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}
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if (!Changed)
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return PreservedAnalyses::all();
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PreservedAnalyses PA;
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PA.preserve<DominatorTreeAnalysis>();
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PA.preserve<LazyValueAnalysis>();
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return PA;
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}
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bool JumpThreadingPass::runImpl(Function &F, TargetLibraryInfo *TLI_,
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LazyValueInfo *LVI_, AliasAnalysis *AA_,
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DomTreeUpdater *DTU_, bool HasProfileData_,
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std::unique_ptr<BlockFrequencyInfo> BFI_,
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std::unique_ptr<BranchProbabilityInfo> BPI_) {
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LLVM_DEBUG(dbgs() << "Jump threading on function '" << F.getName() << "'\n");
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TLI = TLI_;
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LVI = LVI_;
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AA = AA_;
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DTU = DTU_;
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BFI.reset();
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BPI.reset();
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// When profile data is available, we need to update edge weights after
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// successful jump threading, which requires both BPI and BFI being available.
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HasProfileData = HasProfileData_;
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auto *GuardDecl = F.getParent()->getFunction(
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Intrinsic::getName(Intrinsic::experimental_guard));
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HasGuards = GuardDecl && !GuardDecl->use_empty();
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if (HasProfileData) {
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BPI = std::move(BPI_);
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BFI = std::move(BFI_);
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}
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// Reduce the number of instructions duplicated when optimizing strictly for
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// size.
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if (BBDuplicateThreshold.getNumOccurrences())
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BBDupThreshold = BBDuplicateThreshold;
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else if (F.hasFnAttribute(Attribute::MinSize))
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BBDupThreshold = 3;
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else
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BBDupThreshold = DefaultBBDupThreshold;
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// JumpThreading must not processes blocks unreachable from entry. It's a
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// waste of compute time and can potentially lead to hangs.
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SmallPtrSet<BasicBlock *, 16> Unreachable;
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assert(DTU && "DTU isn't passed into JumpThreading before using it.");
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assert(DTU->hasDomTree() && "JumpThreading relies on DomTree to proceed.");
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DominatorTree &DT = DTU->getDomTree();
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for (auto &BB : F)
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if (!DT.isReachableFromEntry(&BB))
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Unreachable.insert(&BB);
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if (!ThreadAcrossLoopHeaders)
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findLoopHeaders(F);
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bool EverChanged = false;
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bool Changed;
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do {
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Changed = false;
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for (auto &BB : F) {
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if (Unreachable.count(&BB))
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continue;
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while (processBlock(&BB)) // Thread all of the branches we can over BB.
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Changed = true;
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// Jump threading may have introduced redundant debug values into BB
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// which should be removed.
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if (Changed)
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RemoveRedundantDbgInstrs(&BB);
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// Stop processing BB if it's the entry or is now deleted. The following
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// routines attempt to eliminate BB and locating a suitable replacement
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// for the entry is non-trivial.
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if (&BB == &F.getEntryBlock() || DTU->isBBPendingDeletion(&BB))
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continue;
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if (pred_empty(&BB)) {
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// When processBlock makes BB unreachable it doesn't bother to fix up
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// the instructions in it. We must remove BB to prevent invalid IR.
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LLVM_DEBUG(dbgs() << " JT: Deleting dead block '" << BB.getName()
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<< "' with terminator: " << *BB.getTerminator()
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<< '\n');
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LoopHeaders.erase(&BB);
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LVI->eraseBlock(&BB);
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DeleteDeadBlock(&BB, DTU);
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Changed = true;
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continue;
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}
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|
|
// processBlock doesn't thread BBs with unconditional TIs. However, if BB
|
|
// is "almost empty", we attempt to merge BB with its sole successor.
|
|
auto *BI = dyn_cast<BranchInst>(BB.getTerminator());
|
|
if (BI && BI->isUnconditional()) {
|
|
BasicBlock *Succ = BI->getSuccessor(0);
|
|
if (
|
|
// The terminator must be the only non-phi instruction in BB.
|
|
BB.getFirstNonPHIOrDbg(true)->isTerminator() &&
|
|
// Don't alter Loop headers and latches to ensure another pass can
|
|
// detect and transform nested loops later.
|
|
!LoopHeaders.count(&BB) && !LoopHeaders.count(Succ) &&
|
|
TryToSimplifyUncondBranchFromEmptyBlock(&BB, DTU)) {
|
|
RemoveRedundantDbgInstrs(Succ);
|
|
// BB is valid for cleanup here because we passed in DTU. F remains
|
|
// BB's parent until a DTU->getDomTree() event.
|
|
LVI->eraseBlock(&BB);
|
|
Changed = true;
|
|
}
|
|
}
|
|
}
|
|
EverChanged |= Changed;
|
|
} while (Changed);
|
|
|
|
LoopHeaders.clear();
|
|
return EverChanged;
|
|
}
|
|
|
|
// Replace uses of Cond with ToVal when safe to do so. If all uses are
|
|
// replaced, we can remove Cond. We cannot blindly replace all uses of Cond
|
|
// because we may incorrectly replace uses when guards/assumes are uses of
|
|
// of `Cond` and we used the guards/assume to reason about the `Cond` value
|
|
// at the end of block. RAUW unconditionally replaces all uses
|
|
// including the guards/assumes themselves and the uses before the
|
|
// guard/assume.
|
|
static void replaceFoldableUses(Instruction *Cond, Value *ToVal) {
|
|
assert(Cond->getType() == ToVal->getType());
|
|
auto *BB = Cond->getParent();
|
|
// We can unconditionally replace all uses in non-local blocks (i.e. uses
|
|
// strictly dominated by BB), since LVI information is true from the
|
|
// terminator of BB.
|
|
replaceNonLocalUsesWith(Cond, ToVal);
|
|
for (Instruction &I : reverse(*BB)) {
|
|
// Reached the Cond whose uses we are trying to replace, so there are no
|
|
// more uses.
|
|
if (&I == Cond)
|
|
break;
|
|
// We only replace uses in instructions that are guaranteed to reach the end
|
|
// of BB, where we know Cond is ToVal.
|
|
if (!isGuaranteedToTransferExecutionToSuccessor(&I))
|
|
break;
|
|
I.replaceUsesOfWith(Cond, ToVal);
|
|
}
|
|
if (Cond->use_empty() && !Cond->mayHaveSideEffects())
|
|
Cond->eraseFromParent();
|
|
}
|
|
|
|
/// Return the cost of duplicating a piece of this block from first non-phi
|
|
/// and before StopAt instruction to thread across it. Stop scanning the block
|
|
/// when exceeding the threshold. If duplication is impossible, returns ~0U.
|
|
static unsigned getJumpThreadDuplicationCost(BasicBlock *BB,
|
|
Instruction *StopAt,
|
|
unsigned Threshold) {
|
|
assert(StopAt->getParent() == BB && "Not an instruction from proper BB?");
|
|
/// Ignore PHI nodes, these will be flattened when duplication happens.
|
|
BasicBlock::const_iterator I(BB->getFirstNonPHI());
|
|
|
|
// FIXME: THREADING will delete values that are just used to compute the
|
|
// branch, so they shouldn't count against the duplication cost.
|
|
|
|
unsigned Bonus = 0;
|
|
if (BB->getTerminator() == StopAt) {
|
|
// Threading through a switch statement is particularly profitable. If this
|
|
// block ends in a switch, decrease its cost to make it more likely to
|
|
// happen.
|
|
if (isa<SwitchInst>(StopAt))
|
|
Bonus = 6;
|
|
|
|
// The same holds for indirect branches, but slightly more so.
|
|
if (isa<IndirectBrInst>(StopAt))
|
|
Bonus = 8;
|
|
}
|
|
|
|
// Bump the threshold up so the early exit from the loop doesn't skip the
|
|
// terminator-based Size adjustment at the end.
|
|
Threshold += Bonus;
|
|
|
|
// Sum up the cost of each instruction until we get to the terminator. Don't
|
|
// include the terminator because the copy won't include it.
|
|
unsigned Size = 0;
|
|
for (; &*I != StopAt; ++I) {
|
|
|
|
// Stop scanning the block if we've reached the threshold.
|
|
if (Size > Threshold)
|
|
return Size;
|
|
|
|
// Debugger intrinsics don't incur code size.
|
|
if (isa<DbgInfoIntrinsic>(I)) continue;
|
|
|
|
// Pseudo-probes don't incur code size.
|
|
if (isa<PseudoProbeInst>(I))
|
|
continue;
|
|
|
|
// If this is a pointer->pointer bitcast, it is free.
|
|
if (isa<BitCastInst>(I) && I->getType()->isPointerTy())
|
|
continue;
|
|
|
|
// Freeze instruction is free, too.
|
|
if (isa<FreezeInst>(I))
|
|
continue;
|
|
|
|
// Bail out if this instruction gives back a token type, it is not possible
|
|
// to duplicate it if it is used outside this BB.
|
|
if (I->getType()->isTokenTy() && I->isUsedOutsideOfBlock(BB))
|
|
return ~0U;
|
|
|
|
// All other instructions count for at least one unit.
|
|
++Size;
|
|
|
|
// Calls are more expensive. If they are non-intrinsic calls, we model them
|
|
// as having cost of 4. If they are a non-vector intrinsic, we model them
|
|
// as having cost of 2 total, and if they are a vector intrinsic, we model
|
|
// them as having cost 1.
|
|
if (const CallInst *CI = dyn_cast<CallInst>(I)) {
|
|
if (CI->cannotDuplicate() || CI->isConvergent())
|
|
// Blocks with NoDuplicate are modelled as having infinite cost, so they
|
|
// are never duplicated.
|
|
return ~0U;
|
|
else if (!isa<IntrinsicInst>(CI))
|
|
Size += 3;
|
|
else if (!CI->getType()->isVectorTy())
|
|
Size += 1;
|
|
}
|
|
}
|
|
|
|
return Size > Bonus ? Size - Bonus : 0;
|
|
}
|
|
|
|
/// findLoopHeaders - We do not want jump threading to turn proper loop
|
|
/// structures into irreducible loops. Doing this breaks up the loop nesting
|
|
/// hierarchy and pessimizes later transformations. To prevent this from
|
|
/// happening, we first have to find the loop headers. Here we approximate this
|
|
/// by finding targets of backedges in the CFG.
|
|
///
|
|
/// Note that there definitely are cases when we want to allow threading of
|
|
/// edges across a loop header. For example, threading a jump from outside the
|
|
/// loop (the preheader) to an exit block of the loop is definitely profitable.
|
|
/// It is also almost always profitable to thread backedges from within the loop
|
|
/// to exit blocks, and is often profitable to thread backedges to other blocks
|
|
/// within the loop (forming a nested loop). This simple analysis is not rich
|
|
/// enough to track all of these properties and keep it up-to-date as the CFG
|
|
/// mutates, so we don't allow any of these transformations.
|
|
void JumpThreadingPass::findLoopHeaders(Function &F) {
|
|
SmallVector<std::pair<const BasicBlock*,const BasicBlock*>, 32> Edges;
|
|
FindFunctionBackedges(F, Edges);
|
|
|
|
for (const auto &Edge : Edges)
|
|
LoopHeaders.insert(Edge.second);
|
|
}
|
|
|
|
/// getKnownConstant - Helper method to determine if we can thread over a
|
|
/// terminator with the given value as its condition, and if so what value to
|
|
/// use for that. What kind of value this is depends on whether we want an
|
|
/// integer or a block address, but an undef is always accepted.
|
|
/// Returns null if Val is null or not an appropriate constant.
|
|
static Constant *getKnownConstant(Value *Val, ConstantPreference Preference) {
|
|
if (!Val)
|
|
return nullptr;
|
|
|
|
// Undef is "known" enough.
|
|
if (UndefValue *U = dyn_cast<UndefValue>(Val))
|
|
return U;
|
|
|
|
if (Preference == WantBlockAddress)
|
|
return dyn_cast<BlockAddress>(Val->stripPointerCasts());
|
|
|
|
return dyn_cast<ConstantInt>(Val);
|
|
}
|
|
|
|
/// computeValueKnownInPredecessors - Given a basic block BB and a value V, see
|
|
/// if we can infer that the value is a known ConstantInt/BlockAddress or undef
|
|
/// in any of our predecessors. If so, return the known list of value and pred
|
|
/// BB in the result vector.
|
|
///
|
|
/// This returns true if there were any known values.
|
|
bool JumpThreadingPass::computeValueKnownInPredecessorsImpl(
|
|
Value *V, BasicBlock *BB, PredValueInfo &Result,
|
|
ConstantPreference Preference, DenseSet<Value *> &RecursionSet,
|
|
Instruction *CxtI) {
|
|
// This method walks up use-def chains recursively. Because of this, we could
|
|
// get into an infinite loop going around loops in the use-def chain. To
|
|
// prevent this, keep track of what (value, block) pairs we've already visited
|
|
// and terminate the search if we loop back to them
|
|
if (!RecursionSet.insert(V).second)
|
|
return false;
|
|
|
|
// If V is a constant, then it is known in all predecessors.
|
|
if (Constant *KC = getKnownConstant(V, Preference)) {
|
|
for (BasicBlock *Pred : predecessors(BB))
|
|
Result.emplace_back(KC, Pred);
|
|
|
|
return !Result.empty();
|
|
}
|
|
|
|
// If V is a non-instruction value, or an instruction in a different block,
|
|
// then it can't be derived from a PHI.
|
|
Instruction *I = dyn_cast<Instruction>(V);
|
|
if (!I || I->getParent() != BB) {
|
|
|
|
// Okay, if this is a live-in value, see if it has a known value at the end
|
|
// of any of our predecessors.
|
|
//
|
|
// FIXME: This should be an edge property, not a block end property.
|
|
/// TODO: Per PR2563, we could infer value range information about a
|
|
/// predecessor based on its terminator.
|
|
//
|
|
// FIXME: change this to use the more-rich 'getPredicateOnEdge' method if
|
|
// "I" is a non-local compare-with-a-constant instruction. This would be
|
|
// able to handle value inequalities better, for example if the compare is
|
|
// "X < 4" and "X < 3" is known true but "X < 4" itself is not available.
|
|
// Perhaps getConstantOnEdge should be smart enough to do this?
|
|
for (BasicBlock *P : predecessors(BB)) {
|
|
// If the value is known by LazyValueInfo to be a constant in a
|
|
// predecessor, use that information to try to thread this block.
|
|
Constant *PredCst = LVI->getConstantOnEdge(V, P, BB, CxtI);
|
|
if (Constant *KC = getKnownConstant(PredCst, Preference))
|
|
Result.emplace_back(KC, P);
|
|
}
|
|
|
|
return !Result.empty();
|
|
}
|
|
|
|
/// If I is a PHI node, then we know the incoming values for any constants.
|
|
if (PHINode *PN = dyn_cast<PHINode>(I)) {
|
|
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
|
|
Value *InVal = PN->getIncomingValue(i);
|
|
if (Constant *KC = getKnownConstant(InVal, Preference)) {
|
|
Result.emplace_back(KC, PN->getIncomingBlock(i));
|
|
} else {
|
|
Constant *CI = LVI->getConstantOnEdge(InVal,
|
|
PN->getIncomingBlock(i),
|
|
BB, CxtI);
|
|
if (Constant *KC = getKnownConstant(CI, Preference))
|
|
Result.emplace_back(KC, PN->getIncomingBlock(i));
|
|
}
|
|
}
|
|
|
|
return !Result.empty();
|
|
}
|
|
|
|
// Handle Cast instructions.
|
|
if (CastInst *CI = dyn_cast<CastInst>(I)) {
|
|
Value *Source = CI->getOperand(0);
|
|
computeValueKnownInPredecessorsImpl(Source, BB, Result, Preference,
|
|
RecursionSet, CxtI);
|
|
if (Result.empty())
|
|
return false;
|
|
|
|
// Convert the known values.
|
|
for (auto &R : Result)
|
|
R.first = ConstantExpr::getCast(CI->getOpcode(), R.first, CI->getType());
|
|
|
|
return true;
|
|
}
|
|
|
|
if (FreezeInst *FI = dyn_cast<FreezeInst>(I)) {
|
|
Value *Source = FI->getOperand(0);
|
|
computeValueKnownInPredecessorsImpl(Source, BB, Result, Preference,
|
|
RecursionSet, CxtI);
|
|
|
|
erase_if(Result, [](auto &Pair) {
|
|
return !isGuaranteedNotToBeUndefOrPoison(Pair.first);
|
|
});
|
|
|
|
return !Result.empty();
|
|
}
|
|
|
|
// Handle some boolean conditions.
|
|
if (I->getType()->getPrimitiveSizeInBits() == 1) {
|
|
using namespace PatternMatch;
|
|
|
|
assert(Preference == WantInteger && "One-bit non-integer type?");
|
|
// X | true -> true
|
|
// X & false -> false
|
|
Value *Op0, *Op1;
|
|
if (match(I, m_LogicalOr(m_Value(Op0), m_Value(Op1))) ||
|
|
match(I, m_LogicalAnd(m_Value(Op0), m_Value(Op1)))) {
|
|
PredValueInfoTy LHSVals, RHSVals;
|
|
|
|
computeValueKnownInPredecessorsImpl(Op0, BB, LHSVals, WantInteger,
|
|
RecursionSet, CxtI);
|
|
computeValueKnownInPredecessorsImpl(Op1, BB, RHSVals, WantInteger,
|
|
RecursionSet, CxtI);
|
|
|
|
if (LHSVals.empty() && RHSVals.empty())
|
|
return false;
|
|
|
|
ConstantInt *InterestingVal;
|
|
if (match(I, m_LogicalOr()))
|
|
InterestingVal = ConstantInt::getTrue(I->getContext());
|
|
else
|
|
InterestingVal = ConstantInt::getFalse(I->getContext());
|
|
|
|
SmallPtrSet<BasicBlock*, 4> LHSKnownBBs;
|
|
|
|
// Scan for the sentinel. If we find an undef, force it to the
|
|
// interesting value: x|undef -> true and x&undef -> false.
|
|
for (const auto &LHSVal : LHSVals)
|
|
if (LHSVal.first == InterestingVal || isa<UndefValue>(LHSVal.first)) {
|
|
Result.emplace_back(InterestingVal, LHSVal.second);
|
|
LHSKnownBBs.insert(LHSVal.second);
|
|
}
|
|
for (const auto &RHSVal : RHSVals)
|
|
if (RHSVal.first == InterestingVal || isa<UndefValue>(RHSVal.first)) {
|
|
// If we already inferred a value for this block on the LHS, don't
|
|
// re-add it.
|
|
if (!LHSKnownBBs.count(RHSVal.second))
|
|
Result.emplace_back(InterestingVal, RHSVal.second);
|
|
}
|
|
|
|
return !Result.empty();
|
|
}
|
|
|
|
// Handle the NOT form of XOR.
|
|
if (I->getOpcode() == Instruction::Xor &&
|
|
isa<ConstantInt>(I->getOperand(1)) &&
|
|
cast<ConstantInt>(I->getOperand(1))->isOne()) {
|
|
computeValueKnownInPredecessorsImpl(I->getOperand(0), BB, Result,
|
|
WantInteger, RecursionSet, CxtI);
|
|
if (Result.empty())
|
|
return false;
|
|
|
|
// Invert the known values.
|
|
for (auto &R : Result)
|
|
R.first = ConstantExpr::getNot(R.first);
|
|
|
|
return true;
|
|
}
|
|
|
|
// Try to simplify some other binary operator values.
|
|
} else if (BinaryOperator *BO = dyn_cast<BinaryOperator>(I)) {
|
|
assert(Preference != WantBlockAddress
|
|
&& "A binary operator creating a block address?");
|
|
if (ConstantInt *CI = dyn_cast<ConstantInt>(BO->getOperand(1))) {
|
|
PredValueInfoTy LHSVals;
|
|
computeValueKnownInPredecessorsImpl(BO->getOperand(0), BB, LHSVals,
|
|
WantInteger, RecursionSet, CxtI);
|
|
|
|
// Try to use constant folding to simplify the binary operator.
|
|
for (const auto &LHSVal : LHSVals) {
|
|
Constant *V = LHSVal.first;
|
|
Constant *Folded = ConstantExpr::get(BO->getOpcode(), V, CI);
|
|
|
|
if (Constant *KC = getKnownConstant(Folded, WantInteger))
|
|
Result.emplace_back(KC, LHSVal.second);
|
|
}
|
|
}
|
|
|
|
return !Result.empty();
|
|
}
|
|
|
|
// Handle compare with phi operand, where the PHI is defined in this block.
|
|
if (CmpInst *Cmp = dyn_cast<CmpInst>(I)) {
|
|
assert(Preference == WantInteger && "Compares only produce integers");
|
|
Type *CmpType = Cmp->getType();
|
|
Value *CmpLHS = Cmp->getOperand(0);
|
|
Value *CmpRHS = Cmp->getOperand(1);
|
|
CmpInst::Predicate Pred = Cmp->getPredicate();
|
|
|
|
PHINode *PN = dyn_cast<PHINode>(CmpLHS);
|
|
if (!PN)
|
|
PN = dyn_cast<PHINode>(CmpRHS);
|
|
if (PN && PN->getParent() == BB) {
|
|
const DataLayout &DL = PN->getModule()->getDataLayout();
|
|
// We can do this simplification if any comparisons fold to true or false.
|
|
// See if any do.
|
|
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
|
|
BasicBlock *PredBB = PN->getIncomingBlock(i);
|
|
Value *LHS, *RHS;
|
|
if (PN == CmpLHS) {
|
|
LHS = PN->getIncomingValue(i);
|
|
RHS = CmpRHS->DoPHITranslation(BB, PredBB);
|
|
} else {
|
|
LHS = CmpLHS->DoPHITranslation(BB, PredBB);
|
|
RHS = PN->getIncomingValue(i);
|
|
}
|
|
Value *Res = SimplifyCmpInst(Pred, LHS, RHS, {DL});
|
|
if (!Res) {
|
|
if (!isa<Constant>(RHS))
|
|
continue;
|
|
|
|
// getPredicateOnEdge call will make no sense if LHS is defined in BB.
|
|
auto LHSInst = dyn_cast<Instruction>(LHS);
|
|
if (LHSInst && LHSInst->getParent() == BB)
|
|
continue;
|
|
|
|
LazyValueInfo::Tristate
|
|
ResT = LVI->getPredicateOnEdge(Pred, LHS,
|
|
cast<Constant>(RHS), PredBB, BB,
|
|
CxtI ? CxtI : Cmp);
|
|
if (ResT == LazyValueInfo::Unknown)
|
|
continue;
|
|
Res = ConstantInt::get(Type::getInt1Ty(LHS->getContext()), ResT);
|
|
}
|
|
|
|
if (Constant *KC = getKnownConstant(Res, WantInteger))
|
|
Result.emplace_back(KC, PredBB);
|
|
}
|
|
|
|
return !Result.empty();
|
|
}
|
|
|
|
// If comparing a live-in value against a constant, see if we know the
|
|
// live-in value on any predecessors.
|
|
if (isa<Constant>(CmpRHS) && !CmpType->isVectorTy()) {
|
|
Constant *CmpConst = cast<Constant>(CmpRHS);
|
|
|
|
if (!isa<Instruction>(CmpLHS) ||
|
|
cast<Instruction>(CmpLHS)->getParent() != BB) {
|
|
for (BasicBlock *P : predecessors(BB)) {
|
|
// If the value is known by LazyValueInfo to be a constant in a
|
|
// predecessor, use that information to try to thread this block.
|
|
LazyValueInfo::Tristate Res =
|
|
LVI->getPredicateOnEdge(Pred, CmpLHS,
|
|
CmpConst, P, BB, CxtI ? CxtI : Cmp);
|
|
if (Res == LazyValueInfo::Unknown)
|
|
continue;
|
|
|
|
Constant *ResC = ConstantInt::get(CmpType, Res);
|
|
Result.emplace_back(ResC, P);
|
|
}
|
|
|
|
return !Result.empty();
|
|
}
|
|
|
|
// InstCombine can fold some forms of constant range checks into
|
|
// (icmp (add (x, C1)), C2). See if we have we have such a thing with
|
|
// x as a live-in.
|
|
{
|
|
using namespace PatternMatch;
|
|
|
|
Value *AddLHS;
|
|
ConstantInt *AddConst;
|
|
if (isa<ConstantInt>(CmpConst) &&
|
|
match(CmpLHS, m_Add(m_Value(AddLHS), m_ConstantInt(AddConst)))) {
|
|
if (!isa<Instruction>(AddLHS) ||
|
|
cast<Instruction>(AddLHS)->getParent() != BB) {
|
|
for (BasicBlock *P : predecessors(BB)) {
|
|
// If the value is known by LazyValueInfo to be a ConstantRange in
|
|
// a predecessor, use that information to try to thread this
|
|
// block.
|
|
ConstantRange CR = LVI->getConstantRangeOnEdge(
|
|
AddLHS, P, BB, CxtI ? CxtI : cast<Instruction>(CmpLHS));
|
|
// Propagate the range through the addition.
|
|
CR = CR.add(AddConst->getValue());
|
|
|
|
// Get the range where the compare returns true.
|
|
ConstantRange CmpRange = ConstantRange::makeExactICmpRegion(
|
|
Pred, cast<ConstantInt>(CmpConst)->getValue());
|
|
|
|
Constant *ResC;
|
|
if (CmpRange.contains(CR))
|
|
ResC = ConstantInt::getTrue(CmpType);
|
|
else if (CmpRange.inverse().contains(CR))
|
|
ResC = ConstantInt::getFalse(CmpType);
|
|
else
|
|
continue;
|
|
|
|
Result.emplace_back(ResC, P);
|
|
}
|
|
|
|
return !Result.empty();
|
|
}
|
|
}
|
|
}
|
|
|
|
// Try to find a constant value for the LHS of a comparison,
|
|
// and evaluate it statically if we can.
|
|
PredValueInfoTy LHSVals;
|
|
computeValueKnownInPredecessorsImpl(I->getOperand(0), BB, LHSVals,
|
|
WantInteger, RecursionSet, CxtI);
|
|
|
|
for (const auto &LHSVal : LHSVals) {
|
|
Constant *V = LHSVal.first;
|
|
Constant *Folded = ConstantExpr::getCompare(Pred, V, CmpConst);
|
|
if (Constant *KC = getKnownConstant(Folded, WantInteger))
|
|
Result.emplace_back(KC, LHSVal.second);
|
|
}
|
|
|
|
return !Result.empty();
|
|
}
|
|
}
|
|
|
|
if (SelectInst *SI = dyn_cast<SelectInst>(I)) {
|
|
// Handle select instructions where at least one operand is a known constant
|
|
// and we can figure out the condition value for any predecessor block.
|
|
Constant *TrueVal = getKnownConstant(SI->getTrueValue(), Preference);
|
|
Constant *FalseVal = getKnownConstant(SI->getFalseValue(), Preference);
|
|
PredValueInfoTy Conds;
|
|
if ((TrueVal || FalseVal) &&
|
|
computeValueKnownInPredecessorsImpl(SI->getCondition(), BB, Conds,
|
|
WantInteger, RecursionSet, CxtI)) {
|
|
for (auto &C : Conds) {
|
|
Constant *Cond = C.first;
|
|
|
|
// Figure out what value to use for the condition.
|
|
bool KnownCond;
|
|
if (ConstantInt *CI = dyn_cast<ConstantInt>(Cond)) {
|
|
// A known boolean.
|
|
KnownCond = CI->isOne();
|
|
} else {
|
|
assert(isa<UndefValue>(Cond) && "Unexpected condition value");
|
|
// Either operand will do, so be sure to pick the one that's a known
|
|
// constant.
|
|
// FIXME: Do this more cleverly if both values are known constants?
|
|
KnownCond = (TrueVal != nullptr);
|
|
}
|
|
|
|
// See if the select has a known constant value for this predecessor.
|
|
if (Constant *Val = KnownCond ? TrueVal : FalseVal)
|
|
Result.emplace_back(Val, C.second);
|
|
}
|
|
|
|
return !Result.empty();
|
|
}
|
|
}
|
|
|
|
// If all else fails, see if LVI can figure out a constant value for us.
|
|
assert(CxtI->getParent() == BB && "CxtI should be in BB");
|
|
Constant *CI = LVI->getConstant(V, CxtI);
|
|
if (Constant *KC = getKnownConstant(CI, Preference)) {
|
|
for (BasicBlock *Pred : predecessors(BB))
|
|
Result.emplace_back(KC, Pred);
|
|
}
|
|
|
|
return !Result.empty();
|
|
}
|
|
|
|
/// GetBestDestForBranchOnUndef - If we determine that the specified block ends
|
|
/// in an undefined jump, decide which block is best to revector to.
|
|
///
|
|
/// Since we can pick an arbitrary destination, we pick the successor with the
|
|
/// fewest predecessors. This should reduce the in-degree of the others.
|
|
static unsigned getBestDestForJumpOnUndef(BasicBlock *BB) {
|
|
Instruction *BBTerm = BB->getTerminator();
|
|
unsigned MinSucc = 0;
|
|
BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc);
|
|
// Compute the successor with the minimum number of predecessors.
|
|
unsigned MinNumPreds = pred_size(TestBB);
|
|
for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) {
|
|
TestBB = BBTerm->getSuccessor(i);
|
|
unsigned NumPreds = pred_size(TestBB);
|
|
if (NumPreds < MinNumPreds) {
|
|
MinSucc = i;
|
|
MinNumPreds = NumPreds;
|
|
}
|
|
}
|
|
|
|
return MinSucc;
|
|
}
|
|
|
|
static bool hasAddressTakenAndUsed(BasicBlock *BB) {
|
|
if (!BB->hasAddressTaken()) return false;
|
|
|
|
// If the block has its address taken, it may be a tree of dead constants
|
|
// hanging off of it. These shouldn't keep the block alive.
|
|
BlockAddress *BA = BlockAddress::get(BB);
|
|
BA->removeDeadConstantUsers();
|
|
return !BA->use_empty();
|
|
}
|
|
|
|
/// processBlock - If there are any predecessors whose control can be threaded
|
|
/// through to a successor, transform them now.
|
|
bool JumpThreadingPass::processBlock(BasicBlock *BB) {
|
|
// If the block is trivially dead, just return and let the caller nuke it.
|
|
// This simplifies other transformations.
|
|
if (DTU->isBBPendingDeletion(BB) ||
|
|
(pred_empty(BB) && BB != &BB->getParent()->getEntryBlock()))
|
|
return false;
|
|
|
|
// If this block has a single predecessor, and if that pred has a single
|
|
// successor, merge the blocks. This encourages recursive jump threading
|
|
// because now the condition in this block can be threaded through
|
|
// predecessors of our predecessor block.
|
|
if (maybeMergeBasicBlockIntoOnlyPred(BB))
|
|
return true;
|
|
|
|
if (tryToUnfoldSelectInCurrBB(BB))
|
|
return true;
|
|
|
|
// Look if we can propagate guards to predecessors.
|
|
if (HasGuards && processGuards(BB))
|
|
return true;
|
|
|
|
// What kind of constant we're looking for.
|
|
ConstantPreference Preference = WantInteger;
|
|
|
|
// Look to see if the terminator is a conditional branch, switch or indirect
|
|
// branch, if not we can't thread it.
|
|
Value *Condition;
|
|
Instruction *Terminator = BB->getTerminator();
|
|
if (BranchInst *BI = dyn_cast<BranchInst>(Terminator)) {
|
|
// Can't thread an unconditional jump.
|
|
if (BI->isUnconditional()) return false;
|
|
Condition = BI->getCondition();
|
|
} else if (SwitchInst *SI = dyn_cast<SwitchInst>(Terminator)) {
|
|
Condition = SI->getCondition();
|
|
} else if (IndirectBrInst *IB = dyn_cast<IndirectBrInst>(Terminator)) {
|
|
// Can't thread indirect branch with no successors.
|
|
if (IB->getNumSuccessors() == 0) return false;
|
|
Condition = IB->getAddress()->stripPointerCasts();
|
|
Preference = WantBlockAddress;
|
|
} else {
|
|
return false; // Must be an invoke or callbr.
|
|
}
|
|
|
|
// Keep track if we constant folded the condition in this invocation.
|
|
bool ConstantFolded = false;
|
|
|
|
// Run constant folding to see if we can reduce the condition to a simple
|
|
// constant.
|
|
if (Instruction *I = dyn_cast<Instruction>(Condition)) {
|
|
Value *SimpleVal =
|
|
ConstantFoldInstruction(I, BB->getModule()->getDataLayout(), TLI);
|
|
if (SimpleVal) {
|
|
I->replaceAllUsesWith(SimpleVal);
|
|
if (isInstructionTriviallyDead(I, TLI))
|
|
I->eraseFromParent();
|
|
Condition = SimpleVal;
|
|
ConstantFolded = true;
|
|
}
|
|
}
|
|
|
|
// If the terminator is branching on an undef or freeze undef, we can pick any
|
|
// of the successors to branch to. Let getBestDestForJumpOnUndef decide.
|
|
auto *FI = dyn_cast<FreezeInst>(Condition);
|
|
if (isa<UndefValue>(Condition) ||
|
|
(FI && isa<UndefValue>(FI->getOperand(0)) && FI->hasOneUse())) {
|
|
unsigned BestSucc = getBestDestForJumpOnUndef(BB);
|
|
std::vector<DominatorTree::UpdateType> Updates;
|
|
|
|
// Fold the branch/switch.
|
|
Instruction *BBTerm = BB->getTerminator();
|
|
Updates.reserve(BBTerm->getNumSuccessors());
|
|
for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) {
|
|
if (i == BestSucc) continue;
|
|
BasicBlock *Succ = BBTerm->getSuccessor(i);
|
|
Succ->removePredecessor(BB, true);
|
|
Updates.push_back({DominatorTree::Delete, BB, Succ});
|
|
}
|
|
|
|
LLVM_DEBUG(dbgs() << " In block '" << BB->getName()
|
|
<< "' folding undef terminator: " << *BBTerm << '\n');
|
|
BranchInst::Create(BBTerm->getSuccessor(BestSucc), BBTerm);
|
|
++NumFolds;
|
|
BBTerm->eraseFromParent();
|
|
DTU->applyUpdatesPermissive(Updates);
|
|
if (FI)
|
|
FI->eraseFromParent();
|
|
return true;
|
|
}
|
|
|
|
// If the terminator of this block is branching on a constant, simplify the
|
|
// terminator to an unconditional branch. This can occur due to threading in
|
|
// other blocks.
|
|
if (getKnownConstant(Condition, Preference)) {
|
|
LLVM_DEBUG(dbgs() << " In block '" << BB->getName()
|
|
<< "' folding terminator: " << *BB->getTerminator()
|
|
<< '\n');
|
|
++NumFolds;
|
|
ConstantFoldTerminator(BB, true, nullptr, DTU);
|
|
if (HasProfileData)
|
|
BPI->eraseBlock(BB);
|
|
return true;
|
|
}
|
|
|
|
Instruction *CondInst = dyn_cast<Instruction>(Condition);
|
|
|
|
// All the rest of our checks depend on the condition being an instruction.
|
|
if (!CondInst) {
|
|
// FIXME: Unify this with code below.
|
|
if (processThreadableEdges(Condition, BB, Preference, Terminator))
|
|
return true;
|
|
return ConstantFolded;
|
|
}
|
|
|
|
if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) {
|
|
// If we're branching on a conditional, LVI might be able to determine
|
|
// it's value at the branch instruction. We only handle comparisons
|
|
// against a constant at this time.
|
|
// TODO: This should be extended to handle switches as well.
|
|
BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
|
|
Constant *CondConst = dyn_cast<Constant>(CondCmp->getOperand(1));
|
|
if (CondBr && CondConst) {
|
|
// We should have returned as soon as we turn a conditional branch to
|
|
// unconditional. Because its no longer interesting as far as jump
|
|
// threading is concerned.
|
|
assert(CondBr->isConditional() && "Threading on unconditional terminator");
|
|
|
|
LazyValueInfo::Tristate Ret =
|
|
LVI->getPredicateAt(CondCmp->getPredicate(), CondCmp->getOperand(0),
|
|
CondConst, CondBr, /*UseBlockValue=*/false);
|
|
if (Ret != LazyValueInfo::Unknown) {
|
|
unsigned ToRemove = Ret == LazyValueInfo::True ? 1 : 0;
|
|
unsigned ToKeep = Ret == LazyValueInfo::True ? 0 : 1;
|
|
BasicBlock *ToRemoveSucc = CondBr->getSuccessor(ToRemove);
|
|
ToRemoveSucc->removePredecessor(BB, true);
|
|
BranchInst *UncondBr =
|
|
BranchInst::Create(CondBr->getSuccessor(ToKeep), CondBr);
|
|
UncondBr->setDebugLoc(CondBr->getDebugLoc());
|
|
++NumFolds;
|
|
CondBr->eraseFromParent();
|
|
if (CondCmp->use_empty())
|
|
CondCmp->eraseFromParent();
|
|
// We can safely replace *some* uses of the CondInst if it has
|
|
// exactly one value as returned by LVI. RAUW is incorrect in the
|
|
// presence of guards and assumes, that have the `Cond` as the use. This
|
|
// is because we use the guards/assume to reason about the `Cond` value
|
|
// at the end of block, but RAUW unconditionally replaces all uses
|
|
// including the guards/assumes themselves and the uses before the
|
|
// guard/assume.
|
|
else if (CondCmp->getParent() == BB) {
|
|
auto *CI = Ret == LazyValueInfo::True ?
|
|
ConstantInt::getTrue(CondCmp->getType()) :
|
|
ConstantInt::getFalse(CondCmp->getType());
|
|
replaceFoldableUses(CondCmp, CI);
|
|
}
|
|
DTU->applyUpdatesPermissive(
|
|
{{DominatorTree::Delete, BB, ToRemoveSucc}});
|
|
if (HasProfileData)
|
|
BPI->eraseBlock(BB);
|
|
return true;
|
|
}
|
|
|
|
// We did not manage to simplify this branch, try to see whether
|
|
// CondCmp depends on a known phi-select pattern.
|
|
if (tryToUnfoldSelect(CondCmp, BB))
|
|
return true;
|
|
}
|
|
}
|
|
|
|
if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator()))
|
|
if (tryToUnfoldSelect(SI, BB))
|
|
return true;
|
|
|
|
// Check for some cases that are worth simplifying. Right now we want to look
|
|
// for loads that are used by a switch or by the condition for the branch. If
|
|
// we see one, check to see if it's partially redundant. If so, insert a PHI
|
|
// which can then be used to thread the values.
|
|
Value *SimplifyValue = CondInst;
|
|
|
|
if (auto *FI = dyn_cast<FreezeInst>(SimplifyValue))
|
|
// Look into freeze's operand
|
|
SimplifyValue = FI->getOperand(0);
|
|
|
|
if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue))
|
|
if (isa<Constant>(CondCmp->getOperand(1)))
|
|
SimplifyValue = CondCmp->getOperand(0);
|
|
|
|
// TODO: There are other places where load PRE would be profitable, such as
|
|
// more complex comparisons.
|
|
if (LoadInst *LoadI = dyn_cast<LoadInst>(SimplifyValue))
|
|
if (simplifyPartiallyRedundantLoad(LoadI))
|
|
return true;
|
|
|
|
// Before threading, try to propagate profile data backwards:
|
|
if (PHINode *PN = dyn_cast<PHINode>(CondInst))
|
|
if (PN->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
|
|
updatePredecessorProfileMetadata(PN, BB);
|
|
|
|
// Handle a variety of cases where we are branching on something derived from
|
|
// a PHI node in the current block. If we can prove that any predecessors
|
|
// compute a predictable value based on a PHI node, thread those predecessors.
|
|
if (processThreadableEdges(CondInst, BB, Preference, Terminator))
|
|
return true;
|
|
|
|
// If this is an otherwise-unfoldable branch on a phi node or freeze(phi) in
|
|
// the current block, see if we can simplify.
|
|
PHINode *PN = dyn_cast<PHINode>(
|
|
isa<FreezeInst>(CondInst) ? cast<FreezeInst>(CondInst)->getOperand(0)
|
|
: CondInst);
|
|
|
|
if (PN && PN->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
|
|
return processBranchOnPHI(PN);
|
|
|
|
// If this is an otherwise-unfoldable branch on a XOR, see if we can simplify.
|
|
if (CondInst->getOpcode() == Instruction::Xor &&
|
|
CondInst->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
|
|
return processBranchOnXOR(cast<BinaryOperator>(CondInst));
|
|
|
|
// Search for a stronger dominating condition that can be used to simplify a
|
|
// conditional branch leaving BB.
|
|
if (processImpliedCondition(BB))
|
|
return true;
|
|
|
|
return false;
|
|
}
|
|
|
|
bool JumpThreadingPass::processImpliedCondition(BasicBlock *BB) {
|
|
auto *BI = dyn_cast<BranchInst>(BB->getTerminator());
|
|
if (!BI || !BI->isConditional())
|
|
return false;
|
|
|
|
Value *Cond = BI->getCondition();
|
|
BasicBlock *CurrentBB = BB;
|
|
BasicBlock *CurrentPred = BB->getSinglePredecessor();
|
|
unsigned Iter = 0;
|
|
|
|
auto &DL = BB->getModule()->getDataLayout();
|
|
|
|
while (CurrentPred && Iter++ < ImplicationSearchThreshold) {
|
|
auto *PBI = dyn_cast<BranchInst>(CurrentPred->getTerminator());
|
|
if (!PBI || !PBI->isConditional())
|
|
return false;
|
|
if (PBI->getSuccessor(0) != CurrentBB && PBI->getSuccessor(1) != CurrentBB)
|
|
return false;
|
|
|
|
bool CondIsTrue = PBI->getSuccessor(0) == CurrentBB;
|
|
Optional<bool> Implication =
|
|
isImpliedCondition(PBI->getCondition(), Cond, DL, CondIsTrue);
|
|
if (Implication) {
|
|
BasicBlock *KeepSucc = BI->getSuccessor(*Implication ? 0 : 1);
|
|
BasicBlock *RemoveSucc = BI->getSuccessor(*Implication ? 1 : 0);
|
|
RemoveSucc->removePredecessor(BB);
|
|
BranchInst *UncondBI = BranchInst::Create(KeepSucc, BI);
|
|
UncondBI->setDebugLoc(BI->getDebugLoc());
|
|
++NumFolds;
|
|
BI->eraseFromParent();
|
|
DTU->applyUpdatesPermissive({{DominatorTree::Delete, BB, RemoveSucc}});
|
|
if (HasProfileData)
|
|
BPI->eraseBlock(BB);
|
|
return true;
|
|
}
|
|
CurrentBB = CurrentPred;
|
|
CurrentPred = CurrentBB->getSinglePredecessor();
|
|
}
|
|
|
|
return false;
|
|
}
|
|
|
|
/// Return true if Op is an instruction defined in the given block.
|
|
static bool isOpDefinedInBlock(Value *Op, BasicBlock *BB) {
|
|
if (Instruction *OpInst = dyn_cast<Instruction>(Op))
|
|
if (OpInst->getParent() == BB)
|
|
return true;
|
|
return false;
|
|
}
|
|
|
|
/// simplifyPartiallyRedundantLoad - If LoadI is an obviously partially
|
|
/// redundant load instruction, eliminate it by replacing it with a PHI node.
|
|
/// This is an important optimization that encourages jump threading, and needs
|
|
/// to be run interlaced with other jump threading tasks.
|
|
bool JumpThreadingPass::simplifyPartiallyRedundantLoad(LoadInst *LoadI) {
|
|
// Don't hack volatile and ordered loads.
|
|
if (!LoadI->isUnordered()) return false;
|
|
|
|
// If the load is defined in a block with exactly one predecessor, it can't be
|
|
// partially redundant.
|
|
BasicBlock *LoadBB = LoadI->getParent();
|
|
if (LoadBB->getSinglePredecessor())
|
|
return false;
|
|
|
|
// If the load is defined in an EH pad, it can't be partially redundant,
|
|
// because the edges between the invoke and the EH pad cannot have other
|
|
// instructions between them.
|
|
if (LoadBB->isEHPad())
|
|
return false;
|
|
|
|
Value *LoadedPtr = LoadI->getOperand(0);
|
|
|
|
// If the loaded operand is defined in the LoadBB and its not a phi,
|
|
// it can't be available in predecessors.
|
|
if (isOpDefinedInBlock(LoadedPtr, LoadBB) && !isa<PHINode>(LoadedPtr))
|
|
return false;
|
|
|
|
// Scan a few instructions up from the load, to see if it is obviously live at
|
|
// the entry to its block.
|
|
BasicBlock::iterator BBIt(LoadI);
|
|
bool IsLoadCSE;
|
|
if (Value *AvailableVal = FindAvailableLoadedValue(
|
|
LoadI, LoadBB, BBIt, DefMaxInstsToScan, AA, &IsLoadCSE)) {
|
|
// If the value of the load is locally available within the block, just use
|
|
// it. This frequently occurs for reg2mem'd allocas.
|
|
|
|
if (IsLoadCSE) {
|
|
LoadInst *NLoadI = cast<LoadInst>(AvailableVal);
|
|
combineMetadataForCSE(NLoadI, LoadI, false);
|
|
};
|
|
|
|
// If the returned value is the load itself, replace with an undef. This can
|
|
// only happen in dead loops.
|
|
if (AvailableVal == LoadI)
|
|
AvailableVal = UndefValue::get(LoadI->getType());
|
|
if (AvailableVal->getType() != LoadI->getType())
|
|
AvailableVal = CastInst::CreateBitOrPointerCast(
|
|
AvailableVal, LoadI->getType(), "", LoadI);
|
|
LoadI->replaceAllUsesWith(AvailableVal);
|
|
LoadI->eraseFromParent();
|
|
return true;
|
|
}
|
|
|
|
// Otherwise, if we scanned the whole block and got to the top of the block,
|
|
// we know the block is locally transparent to the load. If not, something
|
|
// might clobber its value.
|
|
if (BBIt != LoadBB->begin())
|
|
return false;
|
|
|
|
// If all of the loads and stores that feed the value have the same AA tags,
|
|
// then we can propagate them onto any newly inserted loads.
|
|
AAMDNodes AATags;
|
|
LoadI->getAAMetadata(AATags);
|
|
|
|
SmallPtrSet<BasicBlock*, 8> PredsScanned;
|
|
|
|
using AvailablePredsTy = SmallVector<std::pair<BasicBlock *, Value *>, 8>;
|
|
|
|
AvailablePredsTy AvailablePreds;
|
|
BasicBlock *OneUnavailablePred = nullptr;
|
|
SmallVector<LoadInst*, 8> CSELoads;
|
|
|
|
// If we got here, the loaded value is transparent through to the start of the
|
|
// block. Check to see if it is available in any of the predecessor blocks.
|
|
for (BasicBlock *PredBB : predecessors(LoadBB)) {
|
|
// If we already scanned this predecessor, skip it.
|
|
if (!PredsScanned.insert(PredBB).second)
|
|
continue;
|
|
|
|
BBIt = PredBB->end();
|
|
unsigned NumScanedInst = 0;
|
|
Value *PredAvailable = nullptr;
|
|
// NOTE: We don't CSE load that is volatile or anything stronger than
|
|
// unordered, that should have been checked when we entered the function.
|
|
assert(LoadI->isUnordered() &&
|
|
"Attempting to CSE volatile or atomic loads");
|
|
// If this is a load on a phi pointer, phi-translate it and search
|
|
// for available load/store to the pointer in predecessors.
|
|
Type *AccessTy = LoadI->getType();
|
|
const auto &DL = LoadI->getModule()->getDataLayout();
|
|
MemoryLocation Loc(LoadedPtr->DoPHITranslation(LoadBB, PredBB),
|
|
LocationSize::precise(DL.getTypeStoreSize(AccessTy)),
|
|
AATags);
|
|
PredAvailable = findAvailablePtrLoadStore(Loc, AccessTy, LoadI->isAtomic(),
|
|
PredBB, BBIt, DefMaxInstsToScan,
|
|
AA, &IsLoadCSE, &NumScanedInst);
|
|
|
|
// If PredBB has a single predecessor, continue scanning through the
|
|
// single predecessor.
|
|
BasicBlock *SinglePredBB = PredBB;
|
|
while (!PredAvailable && SinglePredBB && BBIt == SinglePredBB->begin() &&
|
|
NumScanedInst < DefMaxInstsToScan) {
|
|
SinglePredBB = SinglePredBB->getSinglePredecessor();
|
|
if (SinglePredBB) {
|
|
BBIt = SinglePredBB->end();
|
|
PredAvailable = findAvailablePtrLoadStore(
|
|
Loc, AccessTy, LoadI->isAtomic(), SinglePredBB, BBIt,
|
|
(DefMaxInstsToScan - NumScanedInst), AA, &IsLoadCSE,
|
|
&NumScanedInst);
|
|
}
|
|
}
|
|
|
|
if (!PredAvailable) {
|
|
OneUnavailablePred = PredBB;
|
|
continue;
|
|
}
|
|
|
|
if (IsLoadCSE)
|
|
CSELoads.push_back(cast<LoadInst>(PredAvailable));
|
|
|
|
// If so, this load is partially redundant. Remember this info so that we
|
|
// can create a PHI node.
|
|
AvailablePreds.emplace_back(PredBB, PredAvailable);
|
|
}
|
|
|
|
// If the loaded value isn't available in any predecessor, it isn't partially
|
|
// redundant.
|
|
if (AvailablePreds.empty()) return false;
|
|
|
|
// Okay, the loaded value is available in at least one (and maybe all!)
|
|
// predecessors. If the value is unavailable in more than one unique
|
|
// predecessor, we want to insert a merge block for those common predecessors.
|
|
// This ensures that we only have to insert one reload, thus not increasing
|
|
// code size.
|
|
BasicBlock *UnavailablePred = nullptr;
|
|
|
|
// If the value is unavailable in one of predecessors, we will end up
|
|
// inserting a new instruction into them. It is only valid if all the
|
|
// instructions before LoadI are guaranteed to pass execution to its
|
|
// successor, or if LoadI is safe to speculate.
|
|
// TODO: If this logic becomes more complex, and we will perform PRE insertion
|
|
// farther than to a predecessor, we need to reuse the code from GVN's PRE.
|
|
// It requires domination tree analysis, so for this simple case it is an
|
|
// overkill.
|
|
if (PredsScanned.size() != AvailablePreds.size() &&
|
|
!isSafeToSpeculativelyExecute(LoadI))
|
|
for (auto I = LoadBB->begin(); &*I != LoadI; ++I)
|
|
if (!isGuaranteedToTransferExecutionToSuccessor(&*I))
|
|
return false;
|
|
|
|
// If there is exactly one predecessor where the value is unavailable, the
|
|
// already computed 'OneUnavailablePred' block is it. If it ends in an
|
|
// unconditional branch, we know that it isn't a critical edge.
|
|
if (PredsScanned.size() == AvailablePreds.size()+1 &&
|
|
OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) {
|
|
UnavailablePred = OneUnavailablePred;
|
|
} else if (PredsScanned.size() != AvailablePreds.size()) {
|
|
// Otherwise, we had multiple unavailable predecessors or we had a critical
|
|
// edge from the one.
|
|
SmallVector<BasicBlock*, 8> PredsToSplit;
|
|
SmallPtrSet<BasicBlock*, 8> AvailablePredSet;
|
|
|
|
for (const auto &AvailablePred : AvailablePreds)
|
|
AvailablePredSet.insert(AvailablePred.first);
|
|
|
|
// Add all the unavailable predecessors to the PredsToSplit list.
|
|
for (BasicBlock *P : predecessors(LoadBB)) {
|
|
// If the predecessor is an indirect goto, we can't split the edge.
|
|
// Same for CallBr.
|
|
if (isa<IndirectBrInst>(P->getTerminator()) ||
|
|
isa<CallBrInst>(P->getTerminator()))
|
|
return false;
|
|
|
|
if (!AvailablePredSet.count(P))
|
|
PredsToSplit.push_back(P);
|
|
}
|
|
|
|
// Split them out to their own block.
|
|
UnavailablePred = splitBlockPreds(LoadBB, PredsToSplit, "thread-pre-split");
|
|
}
|
|
|
|
// If the value isn't available in all predecessors, then there will be
|
|
// exactly one where it isn't available. Insert a load on that edge and add
|
|
// it to the AvailablePreds list.
|
|
if (UnavailablePred) {
|
|
assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 &&
|
|
"Can't handle critical edge here!");
|
|
LoadInst *NewVal = new LoadInst(
|
|
LoadI->getType(), LoadedPtr->DoPHITranslation(LoadBB, UnavailablePred),
|
|
LoadI->getName() + ".pr", false, LoadI->getAlign(),
|
|
LoadI->getOrdering(), LoadI->getSyncScopeID(),
|
|
UnavailablePred->getTerminator());
|
|
NewVal->setDebugLoc(LoadI->getDebugLoc());
|
|
if (AATags)
|
|
NewVal->setAAMetadata(AATags);
|
|
|
|
AvailablePreds.emplace_back(UnavailablePred, NewVal);
|
|
}
|
|
|
|
// Now we know that each predecessor of this block has a value in
|
|
// AvailablePreds, sort them for efficient access as we're walking the preds.
|
|
array_pod_sort(AvailablePreds.begin(), AvailablePreds.end());
|
|
|
|
// Create a PHI node at the start of the block for the PRE'd load value.
|
|
pred_iterator PB = pred_begin(LoadBB), PE = pred_end(LoadBB);
|
|
PHINode *PN = PHINode::Create(LoadI->getType(), std::distance(PB, PE), "",
|
|
&LoadBB->front());
|
|
PN->takeName(LoadI);
|
|
PN->setDebugLoc(LoadI->getDebugLoc());
|
|
|
|
// Insert new entries into the PHI for each predecessor. A single block may
|
|
// have multiple entries here.
|
|
for (pred_iterator PI = PB; PI != PE; ++PI) {
|
|
BasicBlock *P = *PI;
|
|
AvailablePredsTy::iterator I =
|
|
llvm::lower_bound(AvailablePreds, std::make_pair(P, (Value *)nullptr));
|
|
|
|
assert(I != AvailablePreds.end() && I->first == P &&
|
|
"Didn't find entry for predecessor!");
|
|
|
|
// If we have an available predecessor but it requires casting, insert the
|
|
// cast in the predecessor and use the cast. Note that we have to update the
|
|
// AvailablePreds vector as we go so that all of the PHI entries for this
|
|
// predecessor use the same bitcast.
|
|
Value *&PredV = I->second;
|
|
if (PredV->getType() != LoadI->getType())
|
|
PredV = CastInst::CreateBitOrPointerCast(PredV, LoadI->getType(), "",
|
|
P->getTerminator());
|
|
|
|
PN->addIncoming(PredV, I->first);
|
|
}
|
|
|
|
for (LoadInst *PredLoadI : CSELoads) {
|
|
combineMetadataForCSE(PredLoadI, LoadI, true);
|
|
}
|
|
|
|
LoadI->replaceAllUsesWith(PN);
|
|
LoadI->eraseFromParent();
|
|
|
|
return true;
|
|
}
|
|
|
|
/// findMostPopularDest - The specified list contains multiple possible
|
|
/// threadable destinations. Pick the one that occurs the most frequently in
|
|
/// the list.
|
|
static BasicBlock *
|
|
findMostPopularDest(BasicBlock *BB,
|
|
const SmallVectorImpl<std::pair<BasicBlock *,
|
|
BasicBlock *>> &PredToDestList) {
|
|
assert(!PredToDestList.empty());
|
|
|
|
// Determine popularity. If there are multiple possible destinations, we
|
|
// explicitly choose to ignore 'undef' destinations. We prefer to thread
|
|
// blocks with known and real destinations to threading undef. We'll handle
|
|
// them later if interesting.
|
|
MapVector<BasicBlock *, unsigned> DestPopularity;
|
|
|
|
// Populate DestPopularity with the successors in the order they appear in the
|
|
// successor list. This way, we ensure determinism by iterating it in the
|
|
// same order in std::max_element below. We map nullptr to 0 so that we can
|
|
// return nullptr when PredToDestList contains nullptr only.
|
|
DestPopularity[nullptr] = 0;
|
|
for (auto *SuccBB : successors(BB))
|
|
DestPopularity[SuccBB] = 0;
|
|
|
|
for (const auto &PredToDest : PredToDestList)
|
|
if (PredToDest.second)
|
|
DestPopularity[PredToDest.second]++;
|
|
|
|
// Find the most popular dest.
|
|
using VT = decltype(DestPopularity)::value_type;
|
|
auto MostPopular = std::max_element(
|
|
DestPopularity.begin(), DestPopularity.end(),
|
|
[](const VT &L, const VT &R) { return L.second < R.second; });
|
|
|
|
// Okay, we have finally picked the most popular destination.
|
|
return MostPopular->first;
|
|
}
|
|
|
|
// Try to evaluate the value of V when the control flows from PredPredBB to
|
|
// BB->getSinglePredecessor() and then on to BB.
|
|
Constant *JumpThreadingPass::evaluateOnPredecessorEdge(BasicBlock *BB,
|
|
BasicBlock *PredPredBB,
|
|
Value *V) {
|
|
BasicBlock *PredBB = BB->getSinglePredecessor();
|
|
assert(PredBB && "Expected a single predecessor");
|
|
|
|
if (Constant *Cst = dyn_cast<Constant>(V)) {
|
|
return Cst;
|
|
}
|
|
|
|
// Consult LVI if V is not an instruction in BB or PredBB.
|
|
Instruction *I = dyn_cast<Instruction>(V);
|
|
if (!I || (I->getParent() != BB && I->getParent() != PredBB)) {
|
|
return LVI->getConstantOnEdge(V, PredPredBB, PredBB, nullptr);
|
|
}
|
|
|
|
// Look into a PHI argument.
|
|
if (PHINode *PHI = dyn_cast<PHINode>(V)) {
|
|
if (PHI->getParent() == PredBB)
|
|
return dyn_cast<Constant>(PHI->getIncomingValueForBlock(PredPredBB));
|
|
return nullptr;
|
|
}
|
|
|
|
// If we have a CmpInst, try to fold it for each incoming edge into PredBB.
|
|
if (CmpInst *CondCmp = dyn_cast<CmpInst>(V)) {
|
|
if (CondCmp->getParent() == BB) {
|
|
Constant *Op0 =
|
|
evaluateOnPredecessorEdge(BB, PredPredBB, CondCmp->getOperand(0));
|
|
Constant *Op1 =
|
|
evaluateOnPredecessorEdge(BB, PredPredBB, CondCmp->getOperand(1));
|
|
if (Op0 && Op1) {
|
|
return ConstantExpr::getCompare(CondCmp->getPredicate(), Op0, Op1);
|
|
}
|
|
}
|
|
return nullptr;
|
|
}
|
|
|
|
return nullptr;
|
|
}
|
|
|
|
bool JumpThreadingPass::processThreadableEdges(Value *Cond, BasicBlock *BB,
|
|
ConstantPreference Preference,
|
|
Instruction *CxtI) {
|
|
// If threading this would thread across a loop header, don't even try to
|
|
// thread the edge.
|
|
if (LoopHeaders.count(BB))
|
|
return false;
|
|
|
|
PredValueInfoTy PredValues;
|
|
if (!computeValueKnownInPredecessors(Cond, BB, PredValues, Preference,
|
|
CxtI)) {
|
|
// We don't have known values in predecessors. See if we can thread through
|
|
// BB and its sole predecessor.
|
|
return maybethreadThroughTwoBasicBlocks(BB, Cond);
|
|
}
|
|
|
|
assert(!PredValues.empty() &&
|
|
"computeValueKnownInPredecessors returned true with no values");
|
|
|
|
LLVM_DEBUG(dbgs() << "IN BB: " << *BB;
|
|
for (const auto &PredValue : PredValues) {
|
|
dbgs() << " BB '" << BB->getName()
|
|
<< "': FOUND condition = " << *PredValue.first
|
|
<< " for pred '" << PredValue.second->getName() << "'.\n";
|
|
});
|
|
|
|
// Decide what we want to thread through. Convert our list of known values to
|
|
// a list of known destinations for each pred. This also discards duplicate
|
|
// predecessors and keeps track of the undefined inputs (which are represented
|
|
// as a null dest in the PredToDestList).
|
|
SmallPtrSet<BasicBlock*, 16> SeenPreds;
|
|
SmallVector<std::pair<BasicBlock*, BasicBlock*>, 16> PredToDestList;
|
|
|
|
BasicBlock *OnlyDest = nullptr;
|
|
BasicBlock *MultipleDestSentinel = (BasicBlock*)(intptr_t)~0ULL;
|
|
Constant *OnlyVal = nullptr;
|
|
Constant *MultipleVal = (Constant *)(intptr_t)~0ULL;
|
|
|
|
for (const auto &PredValue : PredValues) {
|
|
BasicBlock *Pred = PredValue.second;
|
|
if (!SeenPreds.insert(Pred).second)
|
|
continue; // Duplicate predecessor entry.
|
|
|
|
Constant *Val = PredValue.first;
|
|
|
|
BasicBlock *DestBB;
|
|
if (isa<UndefValue>(Val))
|
|
DestBB = nullptr;
|
|
else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
|
|
assert(isa<ConstantInt>(Val) && "Expecting a constant integer");
|
|
DestBB = BI->getSuccessor(cast<ConstantInt>(Val)->isZero());
|
|
} else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) {
|
|
assert(isa<ConstantInt>(Val) && "Expecting a constant integer");
|
|
DestBB = SI->findCaseValue(cast<ConstantInt>(Val))->getCaseSuccessor();
|
|
} else {
|
|
assert(isa<IndirectBrInst>(BB->getTerminator())
|
|
&& "Unexpected terminator");
|
|
assert(isa<BlockAddress>(Val) && "Expecting a constant blockaddress");
|
|
DestBB = cast<BlockAddress>(Val)->getBasicBlock();
|
|
}
|
|
|
|
// If we have exactly one destination, remember it for efficiency below.
|
|
if (PredToDestList.empty()) {
|
|
OnlyDest = DestBB;
|
|
OnlyVal = Val;
|
|
} else {
|
|
if (OnlyDest != DestBB)
|
|
OnlyDest = MultipleDestSentinel;
|
|
// It possible we have same destination, but different value, e.g. default
|
|
// case in switchinst.
|
|
if (Val != OnlyVal)
|
|
OnlyVal = MultipleVal;
|
|
}
|
|
|
|
// If the predecessor ends with an indirect goto, we can't change its
|
|
// destination. Same for CallBr.
|
|
if (isa<IndirectBrInst>(Pred->getTerminator()) ||
|
|
isa<CallBrInst>(Pred->getTerminator()))
|
|
continue;
|
|
|
|
PredToDestList.emplace_back(Pred, DestBB);
|
|
}
|
|
|
|
// If all edges were unthreadable, we fail.
|
|
if (PredToDestList.empty())
|
|
return false;
|
|
|
|
// If all the predecessors go to a single known successor, we want to fold,
|
|
// not thread. By doing so, we do not need to duplicate the current block and
|
|
// also miss potential opportunities in case we dont/cant duplicate.
|
|
if (OnlyDest && OnlyDest != MultipleDestSentinel) {
|
|
if (BB->hasNPredecessors(PredToDestList.size())) {
|
|
bool SeenFirstBranchToOnlyDest = false;
|
|
std::vector <DominatorTree::UpdateType> Updates;
|
|
Updates.reserve(BB->getTerminator()->getNumSuccessors() - 1);
|
|
for (BasicBlock *SuccBB : successors(BB)) {
|
|
if (SuccBB == OnlyDest && !SeenFirstBranchToOnlyDest) {
|
|
SeenFirstBranchToOnlyDest = true; // Don't modify the first branch.
|
|
} else {
|
|
SuccBB->removePredecessor(BB, true); // This is unreachable successor.
|
|
Updates.push_back({DominatorTree::Delete, BB, SuccBB});
|
|
}
|
|
}
|
|
|
|
// Finally update the terminator.
|
|
Instruction *Term = BB->getTerminator();
|
|
BranchInst::Create(OnlyDest, Term);
|
|
++NumFolds;
|
|
Term->eraseFromParent();
|
|
DTU->applyUpdatesPermissive(Updates);
|
|
if (HasProfileData)
|
|
BPI->eraseBlock(BB);
|
|
|
|
// If the condition is now dead due to the removal of the old terminator,
|
|
// erase it.
|
|
if (auto *CondInst = dyn_cast<Instruction>(Cond)) {
|
|
if (CondInst->use_empty() && !CondInst->mayHaveSideEffects())
|
|
CondInst->eraseFromParent();
|
|
// We can safely replace *some* uses of the CondInst if it has
|
|
// exactly one value as returned by LVI. RAUW is incorrect in the
|
|
// presence of guards and assumes, that have the `Cond` as the use. This
|
|
// is because we use the guards/assume to reason about the `Cond` value
|
|
// at the end of block, but RAUW unconditionally replaces all uses
|
|
// including the guards/assumes themselves and the uses before the
|
|
// guard/assume.
|
|
else if (OnlyVal && OnlyVal != MultipleVal &&
|
|
CondInst->getParent() == BB)
|
|
replaceFoldableUses(CondInst, OnlyVal);
|
|
}
|
|
return true;
|
|
}
|
|
}
|
|
|
|
// Determine which is the most common successor. If we have many inputs and
|
|
// this block is a switch, we want to start by threading the batch that goes
|
|
// to the most popular destination first. If we only know about one
|
|
// threadable destination (the common case) we can avoid this.
|
|
BasicBlock *MostPopularDest = OnlyDest;
|
|
|
|
if (MostPopularDest == MultipleDestSentinel) {
|
|
// Remove any loop headers from the Dest list, threadEdge conservatively
|
|
// won't process them, but we might have other destination that are eligible
|
|
// and we still want to process.
|
|
erase_if(PredToDestList,
|
|
[&](const std::pair<BasicBlock *, BasicBlock *> &PredToDest) {
|
|
return LoopHeaders.contains(PredToDest.second);
|
|
});
|
|
|
|
if (PredToDestList.empty())
|
|
return false;
|
|
|
|
MostPopularDest = findMostPopularDest(BB, PredToDestList);
|
|
}
|
|
|
|
// Now that we know what the most popular destination is, factor all
|
|
// predecessors that will jump to it into a single predecessor.
|
|
SmallVector<BasicBlock*, 16> PredsToFactor;
|
|
for (const auto &PredToDest : PredToDestList)
|
|
if (PredToDest.second == MostPopularDest) {
|
|
BasicBlock *Pred = PredToDest.first;
|
|
|
|
// This predecessor may be a switch or something else that has multiple
|
|
// edges to the block. Factor each of these edges by listing them
|
|
// according to # occurrences in PredsToFactor.
|
|
for (BasicBlock *Succ : successors(Pred))
|
|
if (Succ == BB)
|
|
PredsToFactor.push_back(Pred);
|
|
}
|
|
|
|
// If the threadable edges are branching on an undefined value, we get to pick
|
|
// the destination that these predecessors should get to.
|
|
if (!MostPopularDest)
|
|
MostPopularDest = BB->getTerminator()->
|
|
getSuccessor(getBestDestForJumpOnUndef(BB));
|
|
|
|
// Ok, try to thread it!
|
|
return tryThreadEdge(BB, PredsToFactor, MostPopularDest);
|
|
}
|
|
|
|
/// processBranchOnPHI - We have an otherwise unthreadable conditional branch on
|
|
/// a PHI node (or freeze PHI) in the current block. See if there are any
|
|
/// simplifications we can do based on inputs to the phi node.
|
|
bool JumpThreadingPass::processBranchOnPHI(PHINode *PN) {
|
|
BasicBlock *BB = PN->getParent();
|
|
|
|
// TODO: We could make use of this to do it once for blocks with common PHI
|
|
// values.
|
|
SmallVector<BasicBlock*, 1> PredBBs;
|
|
PredBBs.resize(1);
|
|
|
|
// If any of the predecessor blocks end in an unconditional branch, we can
|
|
// *duplicate* the conditional branch into that block in order to further
|
|
// encourage jump threading and to eliminate cases where we have branch on a
|
|
// phi of an icmp (branch on icmp is much better).
|
|
// This is still beneficial when a frozen phi is used as the branch condition
|
|
// because it allows CodeGenPrepare to further canonicalize br(freeze(icmp))
|
|
// to br(icmp(freeze ...)).
|
|
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
|
|
BasicBlock *PredBB = PN->getIncomingBlock(i);
|
|
if (BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator()))
|
|
if (PredBr->isUnconditional()) {
|
|
PredBBs[0] = PredBB;
|
|
// Try to duplicate BB into PredBB.
|
|
if (duplicateCondBranchOnPHIIntoPred(BB, PredBBs))
|
|
return true;
|
|
}
|
|
}
|
|
|
|
return false;
|
|
}
|
|
|
|
/// processBranchOnXOR - We have an otherwise unthreadable conditional branch on
|
|
/// a xor instruction in the current block. See if there are any
|
|
/// simplifications we can do based on inputs to the xor.
|
|
bool JumpThreadingPass::processBranchOnXOR(BinaryOperator *BO) {
|
|
BasicBlock *BB = BO->getParent();
|
|
|
|
// If either the LHS or RHS of the xor is a constant, don't do this
|
|
// optimization.
|
|
if (isa<ConstantInt>(BO->getOperand(0)) ||
|
|
isa<ConstantInt>(BO->getOperand(1)))
|
|
return false;
|
|
|
|
// If the first instruction in BB isn't a phi, we won't be able to infer
|
|
// anything special about any particular predecessor.
|
|
if (!isa<PHINode>(BB->front()))
|
|
return false;
|
|
|
|
// If this BB is a landing pad, we won't be able to split the edge into it.
|
|
if (BB->isEHPad())
|
|
return false;
|
|
|
|
// If we have a xor as the branch input to this block, and we know that the
|
|
// LHS or RHS of the xor in any predecessor is true/false, then we can clone
|
|
// the condition into the predecessor and fix that value to true, saving some
|
|
// logical ops on that path and encouraging other paths to simplify.
|
|
//
|
|
// This copies something like this:
|
|
//
|
|
// BB:
|
|
// %X = phi i1 [1], [%X']
|
|
// %Y = icmp eq i32 %A, %B
|
|
// %Z = xor i1 %X, %Y
|
|
// br i1 %Z, ...
|
|
//
|
|
// Into:
|
|
// BB':
|
|
// %Y = icmp ne i32 %A, %B
|
|
// br i1 %Y, ...
|
|
|
|
PredValueInfoTy XorOpValues;
|
|
bool isLHS = true;
|
|
if (!computeValueKnownInPredecessors(BO->getOperand(0), BB, XorOpValues,
|
|
WantInteger, BO)) {
|
|
assert(XorOpValues.empty());
|
|
if (!computeValueKnownInPredecessors(BO->getOperand(1), BB, XorOpValues,
|
|
WantInteger, BO))
|
|
return false;
|
|
isLHS = false;
|
|
}
|
|
|
|
assert(!XorOpValues.empty() &&
|
|
"computeValueKnownInPredecessors returned true with no values");
|
|
|
|
// Scan the information to see which is most popular: true or false. The
|
|
// predecessors can be of the set true, false, or undef.
|
|
unsigned NumTrue = 0, NumFalse = 0;
|
|
for (const auto &XorOpValue : XorOpValues) {
|
|
if (isa<UndefValue>(XorOpValue.first))
|
|
// Ignore undefs for the count.
|
|
continue;
|
|
if (cast<ConstantInt>(XorOpValue.first)->isZero())
|
|
++NumFalse;
|
|
else
|
|
++NumTrue;
|
|
}
|
|
|
|
// Determine which value to split on, true, false, or undef if neither.
|
|
ConstantInt *SplitVal = nullptr;
|
|
if (NumTrue > NumFalse)
|
|
SplitVal = ConstantInt::getTrue(BB->getContext());
|
|
else if (NumTrue != 0 || NumFalse != 0)
|
|
SplitVal = ConstantInt::getFalse(BB->getContext());
|
|
|
|
// Collect all of the blocks that this can be folded into so that we can
|
|
// factor this once and clone it once.
|
|
SmallVector<BasicBlock*, 8> BlocksToFoldInto;
|
|
for (const auto &XorOpValue : XorOpValues) {
|
|
if (XorOpValue.first != SplitVal && !isa<UndefValue>(XorOpValue.first))
|
|
continue;
|
|
|
|
BlocksToFoldInto.push_back(XorOpValue.second);
|
|
}
|
|
|
|
// If we inferred a value for all of the predecessors, then duplication won't
|
|
// help us. However, we can just replace the LHS or RHS with the constant.
|
|
if (BlocksToFoldInto.size() ==
|
|
cast<PHINode>(BB->front()).getNumIncomingValues()) {
|
|
if (!SplitVal) {
|
|
// If all preds provide undef, just nuke the xor, because it is undef too.
|
|
BO->replaceAllUsesWith(UndefValue::get(BO->getType()));
|
|
BO->eraseFromParent();
|
|
} else if (SplitVal->isZero()) {
|
|
// If all preds provide 0, replace the xor with the other input.
|
|
BO->replaceAllUsesWith(BO->getOperand(isLHS));
|
|
BO->eraseFromParent();
|
|
} else {
|
|
// If all preds provide 1, set the computed value to 1.
|
|
BO->setOperand(!isLHS, SplitVal);
|
|
}
|
|
|
|
return true;
|
|
}
|
|
|
|
// If any of predecessors end with an indirect goto, we can't change its
|
|
// destination. Same for CallBr.
|
|
if (any_of(BlocksToFoldInto, [](BasicBlock *Pred) {
|
|
return isa<IndirectBrInst>(Pred->getTerminator()) ||
|
|
isa<CallBrInst>(Pred->getTerminator());
|
|
}))
|
|
return false;
|
|
|
|
// Try to duplicate BB into PredBB.
|
|
return duplicateCondBranchOnPHIIntoPred(BB, BlocksToFoldInto);
|
|
}
|
|
|
|
/// addPHINodeEntriesForMappedBlock - We're adding 'NewPred' as a new
|
|
/// predecessor to the PHIBB block. If it has PHI nodes, add entries for
|
|
/// NewPred using the entries from OldPred (suitably mapped).
|
|
static void addPHINodeEntriesForMappedBlock(BasicBlock *PHIBB,
|
|
BasicBlock *OldPred,
|
|
BasicBlock *NewPred,
|
|
DenseMap<Instruction*, Value*> &ValueMap) {
|
|
for (PHINode &PN : PHIBB->phis()) {
|
|
// Ok, we have a PHI node. Figure out what the incoming value was for the
|
|
// DestBlock.
|
|
Value *IV = PN.getIncomingValueForBlock(OldPred);
|
|
|
|
// Remap the value if necessary.
|
|
if (Instruction *Inst = dyn_cast<Instruction>(IV)) {
|
|
DenseMap<Instruction*, Value*>::iterator I = ValueMap.find(Inst);
|
|
if (I != ValueMap.end())
|
|
IV = I->second;
|
|
}
|
|
|
|
PN.addIncoming(IV, NewPred);
|
|
}
|
|
}
|
|
|
|
/// Merge basic block BB into its sole predecessor if possible.
|
|
bool JumpThreadingPass::maybeMergeBasicBlockIntoOnlyPred(BasicBlock *BB) {
|
|
BasicBlock *SinglePred = BB->getSinglePredecessor();
|
|
if (!SinglePred)
|
|
return false;
|
|
|
|
const Instruction *TI = SinglePred->getTerminator();
|
|
if (TI->isExceptionalTerminator() || TI->getNumSuccessors() != 1 ||
|
|
SinglePred == BB || hasAddressTakenAndUsed(BB))
|
|
return false;
|
|
|
|
// If SinglePred was a loop header, BB becomes one.
|
|
if (LoopHeaders.erase(SinglePred))
|
|
LoopHeaders.insert(BB);
|
|
|
|
LVI->eraseBlock(SinglePred);
|
|
MergeBasicBlockIntoOnlyPred(BB, DTU);
|
|
|
|
// Now that BB is merged into SinglePred (i.e. SinglePred code followed by
|
|
// BB code within one basic block `BB`), we need to invalidate the LVI
|
|
// information associated with BB, because the LVI information need not be
|
|
// true for all of BB after the merge. For example,
|
|
// Before the merge, LVI info and code is as follows:
|
|
// SinglePred: <LVI info1 for %p val>
|
|
// %y = use of %p
|
|
// call @exit() // need not transfer execution to successor.
|
|
// assume(%p) // from this point on %p is true
|
|
// br label %BB
|
|
// BB: <LVI info2 for %p val, i.e. %p is true>
|
|
// %x = use of %p
|
|
// br label exit
|
|
//
|
|
// Note that this LVI info for blocks BB and SinglPred is correct for %p
|
|
// (info2 and info1 respectively). After the merge and the deletion of the
|
|
// LVI info1 for SinglePred. We have the following code:
|
|
// BB: <LVI info2 for %p val>
|
|
// %y = use of %p
|
|
// call @exit()
|
|
// assume(%p)
|
|
// %x = use of %p <-- LVI info2 is correct from here onwards.
|
|
// br label exit
|
|
// LVI info2 for BB is incorrect at the beginning of BB.
|
|
|
|
// Invalidate LVI information for BB if the LVI is not provably true for
|
|
// all of BB.
|
|
if (!isGuaranteedToTransferExecutionToSuccessor(BB))
|
|
LVI->eraseBlock(BB);
|
|
return true;
|
|
}
|
|
|
|
/// Update the SSA form. NewBB contains instructions that are copied from BB.
|
|
/// ValueMapping maps old values in BB to new ones in NewBB.
|
|
void JumpThreadingPass::updateSSA(
|
|
BasicBlock *BB, BasicBlock *NewBB,
|
|
DenseMap<Instruction *, Value *> &ValueMapping) {
|
|
// If there were values defined in BB that are used outside the block, then we
|
|
// now have to update all uses of the value to use either the original value,
|
|
// the cloned value, or some PHI derived value. This can require arbitrary
|
|
// PHI insertion, of which we are prepared to do, clean these up now.
|
|
SSAUpdater SSAUpdate;
|
|
SmallVector<Use *, 16> UsesToRename;
|
|
|
|
for (Instruction &I : *BB) {
|
|
// Scan all uses of this instruction to see if it is used outside of its
|
|
// block, and if so, record them in UsesToRename.
|
|
for (Use &U : I.uses()) {
|
|
Instruction *User = cast<Instruction>(U.getUser());
|
|
if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
|
|
if (UserPN->getIncomingBlock(U) == BB)
|
|
continue;
|
|
} else if (User->getParent() == BB)
|
|
continue;
|
|
|
|
UsesToRename.push_back(&U);
|
|
}
|
|
|
|
// If there are no uses outside the block, we're done with this instruction.
|
|
if (UsesToRename.empty())
|
|
continue;
|
|
LLVM_DEBUG(dbgs() << "JT: Renaming non-local uses of: " << I << "\n");
|
|
|
|
// We found a use of I outside of BB. Rename all uses of I that are outside
|
|
// its block to be uses of the appropriate PHI node etc. See ValuesInBlocks
|
|
// with the two values we know.
|
|
SSAUpdate.Initialize(I.getType(), I.getName());
|
|
SSAUpdate.AddAvailableValue(BB, &I);
|
|
SSAUpdate.AddAvailableValue(NewBB, ValueMapping[&I]);
|
|
|
|
while (!UsesToRename.empty())
|
|
SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
|
|
LLVM_DEBUG(dbgs() << "\n");
|
|
}
|
|
}
|
|
|
|
/// Clone instructions in range [BI, BE) to NewBB. For PHI nodes, we only clone
|
|
/// arguments that come from PredBB. Return the map from the variables in the
|
|
/// source basic block to the variables in the newly created basic block.
|
|
DenseMap<Instruction *, Value *>
|
|
JumpThreadingPass::cloneInstructions(BasicBlock::iterator BI,
|
|
BasicBlock::iterator BE, BasicBlock *NewBB,
|
|
BasicBlock *PredBB) {
|
|
// We are going to have to map operands from the source basic block to the new
|
|
// copy of the block 'NewBB'. If there are PHI nodes in the source basic
|
|
// block, evaluate them to account for entry from PredBB.
|
|
DenseMap<Instruction *, Value *> ValueMapping;
|
|
|
|
// Clone the phi nodes of the source basic block into NewBB. The resulting
|
|
// phi nodes are trivial since NewBB only has one predecessor, but SSAUpdater
|
|
// might need to rewrite the operand of the cloned phi.
|
|
for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI) {
|
|
PHINode *NewPN = PHINode::Create(PN->getType(), 1, PN->getName(), NewBB);
|
|
NewPN->addIncoming(PN->getIncomingValueForBlock(PredBB), PredBB);
|
|
ValueMapping[PN] = NewPN;
|
|
}
|
|
|
|
// Clone noalias scope declarations in the threaded block. When threading a
|
|
// loop exit, we would otherwise end up with two idential scope declarations
|
|
// visible at the same time.
|
|
SmallVector<MDNode *> NoAliasScopes;
|
|
DenseMap<MDNode *, MDNode *> ClonedScopes;
|
|
LLVMContext &Context = PredBB->getContext();
|
|
identifyNoAliasScopesToClone(BI, BE, NoAliasScopes);
|
|
cloneNoAliasScopes(NoAliasScopes, ClonedScopes, "thread", Context);
|
|
|
|
// Clone the non-phi instructions of the source basic block into NewBB,
|
|
// keeping track of the mapping and using it to remap operands in the cloned
|
|
// instructions.
|
|
for (; BI != BE; ++BI) {
|
|
Instruction *New = BI->clone();
|
|
New->setName(BI->getName());
|
|
NewBB->getInstList().push_back(New);
|
|
ValueMapping[&*BI] = New;
|
|
adaptNoAliasScopes(New, ClonedScopes, Context);
|
|
|
|
// Remap operands to patch up intra-block references.
|
|
for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
|
|
if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
|
|
DenseMap<Instruction *, Value *>::iterator I = ValueMapping.find(Inst);
|
|
if (I != ValueMapping.end())
|
|
New->setOperand(i, I->second);
|
|
}
|
|
}
|
|
|
|
return ValueMapping;
|
|
}
|
|
|
|
/// Attempt to thread through two successive basic blocks.
|
|
bool JumpThreadingPass::maybethreadThroughTwoBasicBlocks(BasicBlock *BB,
|
|
Value *Cond) {
|
|
// Consider:
|
|
//
|
|
// PredBB:
|
|
// %var = phi i32* [ null, %bb1 ], [ @a, %bb2 ]
|
|
// %tobool = icmp eq i32 %cond, 0
|
|
// br i1 %tobool, label %BB, label ...
|
|
//
|
|
// BB:
|
|
// %cmp = icmp eq i32* %var, null
|
|
// br i1 %cmp, label ..., label ...
|
|
//
|
|
// We don't know the value of %var at BB even if we know which incoming edge
|
|
// we take to BB. However, once we duplicate PredBB for each of its incoming
|
|
// edges (say, PredBB1 and PredBB2), we know the value of %var in each copy of
|
|
// PredBB. Then we can thread edges PredBB1->BB and PredBB2->BB through BB.
|
|
|
|
// Require that BB end with a Branch for simplicity.
|
|
BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
|
|
if (!CondBr)
|
|
return false;
|
|
|
|
// BB must have exactly one predecessor.
|
|
BasicBlock *PredBB = BB->getSinglePredecessor();
|
|
if (!PredBB)
|
|
return false;
|
|
|
|
// Require that PredBB end with a conditional Branch. If PredBB ends with an
|
|
// unconditional branch, we should be merging PredBB and BB instead. For
|
|
// simplicity, we don't deal with a switch.
|
|
BranchInst *PredBBBranch = dyn_cast<BranchInst>(PredBB->getTerminator());
|
|
if (!PredBBBranch || PredBBBranch->isUnconditional())
|
|
return false;
|
|
|
|
// If PredBB has exactly one incoming edge, we don't gain anything by copying
|
|
// PredBB.
|
|
if (PredBB->getSinglePredecessor())
|
|
return false;
|
|
|
|
// Don't thread through PredBB if it contains a successor edge to itself, in
|
|
// which case we would infinite loop. Suppose we are threading an edge from
|
|
// PredPredBB through PredBB and BB to SuccBB with PredBB containing a
|
|
// successor edge to itself. If we allowed jump threading in this case, we
|
|
// could duplicate PredBB and BB as, say, PredBB.thread and BB.thread. Since
|
|
// PredBB.thread has a successor edge to PredBB, we would immediately come up
|
|
// with another jump threading opportunity from PredBB.thread through PredBB
|
|
// and BB to SuccBB. This jump threading would repeatedly occur. That is, we
|
|
// would keep peeling one iteration from PredBB.
|
|
if (llvm::is_contained(successors(PredBB), PredBB))
|
|
return false;
|
|
|
|
// Don't thread across a loop header.
|
|
if (LoopHeaders.count(PredBB))
|
|
return false;
|
|
|
|
// Avoid complication with duplicating EH pads.
|
|
if (PredBB->isEHPad())
|
|
return false;
|
|
|
|
// Find a predecessor that we can thread. For simplicity, we only consider a
|
|
// successor edge out of BB to which we thread exactly one incoming edge into
|
|
// PredBB.
|
|
unsigned ZeroCount = 0;
|
|
unsigned OneCount = 0;
|
|
BasicBlock *ZeroPred = nullptr;
|
|
BasicBlock *OnePred = nullptr;
|
|
for (BasicBlock *P : predecessors(PredBB)) {
|
|
if (ConstantInt *CI = dyn_cast_or_null<ConstantInt>(
|
|
evaluateOnPredecessorEdge(BB, P, Cond))) {
|
|
if (CI->isZero()) {
|
|
ZeroCount++;
|
|
ZeroPred = P;
|
|
} else if (CI->isOne()) {
|
|
OneCount++;
|
|
OnePred = P;
|
|
}
|
|
}
|
|
}
|
|
|
|
// Disregard complicated cases where we have to thread multiple edges.
|
|
BasicBlock *PredPredBB;
|
|
if (ZeroCount == 1) {
|
|
PredPredBB = ZeroPred;
|
|
} else if (OneCount == 1) {
|
|
PredPredBB = OnePred;
|
|
} else {
|
|
return false;
|
|
}
|
|
|
|
BasicBlock *SuccBB = CondBr->getSuccessor(PredPredBB == ZeroPred);
|
|
|
|
// If threading to the same block as we come from, we would infinite loop.
|
|
if (SuccBB == BB) {
|
|
LLVM_DEBUG(dbgs() << " Not threading across BB '" << BB->getName()
|
|
<< "' - would thread to self!\n");
|
|
return false;
|
|
}
|
|
|
|
// If threading this would thread across a loop header, don't thread the edge.
|
|
// See the comments above findLoopHeaders for justifications and caveats.
|
|
if (LoopHeaders.count(BB) || LoopHeaders.count(SuccBB)) {
|
|
LLVM_DEBUG({
|
|
bool BBIsHeader = LoopHeaders.count(BB);
|
|
bool SuccIsHeader = LoopHeaders.count(SuccBB);
|
|
dbgs() << " Not threading across "
|
|
<< (BBIsHeader ? "loop header BB '" : "block BB '")
|
|
<< BB->getName() << "' to dest "
|
|
<< (SuccIsHeader ? "loop header BB '" : "block BB '")
|
|
<< SuccBB->getName()
|
|
<< "' - it might create an irreducible loop!\n";
|
|
});
|
|
return false;
|
|
}
|
|
|
|
// Compute the cost of duplicating BB and PredBB.
|
|
unsigned BBCost =
|
|
getJumpThreadDuplicationCost(BB, BB->getTerminator(), BBDupThreshold);
|
|
unsigned PredBBCost = getJumpThreadDuplicationCost(
|
|
PredBB, PredBB->getTerminator(), BBDupThreshold);
|
|
|
|
// Give up if costs are too high. We need to check BBCost and PredBBCost
|
|
// individually before checking their sum because getJumpThreadDuplicationCost
|
|
// return (unsigned)~0 for those basic blocks that cannot be duplicated.
|
|
if (BBCost > BBDupThreshold || PredBBCost > BBDupThreshold ||
|
|
BBCost + PredBBCost > BBDupThreshold) {
|
|
LLVM_DEBUG(dbgs() << " Not threading BB '" << BB->getName()
|
|
<< "' - Cost is too high: " << PredBBCost
|
|
<< " for PredBB, " << BBCost << "for BB\n");
|
|
return false;
|
|
}
|
|
|
|
// Now we are ready to duplicate PredBB.
|
|
threadThroughTwoBasicBlocks(PredPredBB, PredBB, BB, SuccBB);
|
|
return true;
|
|
}
|
|
|
|
void JumpThreadingPass::threadThroughTwoBasicBlocks(BasicBlock *PredPredBB,
|
|
BasicBlock *PredBB,
|
|
BasicBlock *BB,
|
|
BasicBlock *SuccBB) {
|
|
LLVM_DEBUG(dbgs() << " Threading through '" << PredBB->getName() << "' and '"
|
|
<< BB->getName() << "'\n");
|
|
|
|
BranchInst *CondBr = cast<BranchInst>(BB->getTerminator());
|
|
BranchInst *PredBBBranch = cast<BranchInst>(PredBB->getTerminator());
|
|
|
|
BasicBlock *NewBB =
|
|
BasicBlock::Create(PredBB->getContext(), PredBB->getName() + ".thread",
|
|
PredBB->getParent(), PredBB);
|
|
NewBB->moveAfter(PredBB);
|
|
|
|
// Set the block frequency of NewBB.
|
|
if (HasProfileData) {
|
|
auto NewBBFreq = BFI->getBlockFreq(PredPredBB) *
|
|
BPI->getEdgeProbability(PredPredBB, PredBB);
|
|
BFI->setBlockFreq(NewBB, NewBBFreq.getFrequency());
|
|
}
|
|
|
|
// We are going to have to map operands from the original BB block to the new
|
|
// copy of the block 'NewBB'. If there are PHI nodes in PredBB, evaluate them
|
|
// to account for entry from PredPredBB.
|
|
DenseMap<Instruction *, Value *> ValueMapping =
|
|
cloneInstructions(PredBB->begin(), PredBB->end(), NewBB, PredPredBB);
|
|
|
|
// Copy the edge probabilities from PredBB to NewBB.
|
|
if (HasProfileData)
|
|
BPI->copyEdgeProbabilities(PredBB, NewBB);
|
|
|
|
// Update the terminator of PredPredBB to jump to NewBB instead of PredBB.
|
|
// This eliminates predecessors from PredPredBB, which requires us to simplify
|
|
// any PHI nodes in PredBB.
|
|
Instruction *PredPredTerm = PredPredBB->getTerminator();
|
|
for (unsigned i = 0, e = PredPredTerm->getNumSuccessors(); i != e; ++i)
|
|
if (PredPredTerm->getSuccessor(i) == PredBB) {
|
|
PredBB->removePredecessor(PredPredBB, true);
|
|
PredPredTerm->setSuccessor(i, NewBB);
|
|
}
|
|
|
|
addPHINodeEntriesForMappedBlock(PredBBBranch->getSuccessor(0), PredBB, NewBB,
|
|
ValueMapping);
|
|
addPHINodeEntriesForMappedBlock(PredBBBranch->getSuccessor(1), PredBB, NewBB,
|
|
ValueMapping);
|
|
|
|
DTU->applyUpdatesPermissive(
|
|
{{DominatorTree::Insert, NewBB, CondBr->getSuccessor(0)},
|
|
{DominatorTree::Insert, NewBB, CondBr->getSuccessor(1)},
|
|
{DominatorTree::Insert, PredPredBB, NewBB},
|
|
{DominatorTree::Delete, PredPredBB, PredBB}});
|
|
|
|
updateSSA(PredBB, NewBB, ValueMapping);
|
|
|
|
// Clean up things like PHI nodes with single operands, dead instructions,
|
|
// etc.
|
|
SimplifyInstructionsInBlock(NewBB, TLI);
|
|
SimplifyInstructionsInBlock(PredBB, TLI);
|
|
|
|
SmallVector<BasicBlock *, 1> PredsToFactor;
|
|
PredsToFactor.push_back(NewBB);
|
|
threadEdge(BB, PredsToFactor, SuccBB);
|
|
}
|
|
|
|
/// tryThreadEdge - Thread an edge if it's safe and profitable to do so.
|
|
bool JumpThreadingPass::tryThreadEdge(
|
|
BasicBlock *BB, const SmallVectorImpl<BasicBlock *> &PredBBs,
|
|
BasicBlock *SuccBB) {
|
|
// If threading to the same block as we come from, we would infinite loop.
|
|
if (SuccBB == BB) {
|
|
LLVM_DEBUG(dbgs() << " Not threading across BB '" << BB->getName()
|
|
<< "' - would thread to self!\n");
|
|
return false;
|
|
}
|
|
|
|
// If threading this would thread across a loop header, don't thread the edge.
|
|
// See the comments above findLoopHeaders for justifications and caveats.
|
|
if (LoopHeaders.count(BB) || LoopHeaders.count(SuccBB)) {
|
|
LLVM_DEBUG({
|
|
bool BBIsHeader = LoopHeaders.count(BB);
|
|
bool SuccIsHeader = LoopHeaders.count(SuccBB);
|
|
dbgs() << " Not threading across "
|
|
<< (BBIsHeader ? "loop header BB '" : "block BB '") << BB->getName()
|
|
<< "' to dest " << (SuccIsHeader ? "loop header BB '" : "block BB '")
|
|
<< SuccBB->getName() << "' - it might create an irreducible loop!\n";
|
|
});
|
|
return false;
|
|
}
|
|
|
|
unsigned JumpThreadCost =
|
|
getJumpThreadDuplicationCost(BB, BB->getTerminator(), BBDupThreshold);
|
|
if (JumpThreadCost > BBDupThreshold) {
|
|
LLVM_DEBUG(dbgs() << " Not threading BB '" << BB->getName()
|
|
<< "' - Cost is too high: " << JumpThreadCost << "\n");
|
|
return false;
|
|
}
|
|
|
|
threadEdge(BB, PredBBs, SuccBB);
|
|
return true;
|
|
}
|
|
|
|
/// threadEdge - We have decided that it is safe and profitable to factor the
|
|
/// blocks in PredBBs to one predecessor, then thread an edge from it to SuccBB
|
|
/// across BB. Transform the IR to reflect this change.
|
|
void JumpThreadingPass::threadEdge(BasicBlock *BB,
|
|
const SmallVectorImpl<BasicBlock *> &PredBBs,
|
|
BasicBlock *SuccBB) {
|
|
assert(SuccBB != BB && "Don't create an infinite loop");
|
|
|
|
assert(!LoopHeaders.count(BB) && !LoopHeaders.count(SuccBB) &&
|
|
"Don't thread across loop headers");
|
|
|
|
// And finally, do it! Start by factoring the predecessors if needed.
|
|
BasicBlock *PredBB;
|
|
if (PredBBs.size() == 1)
|
|
PredBB = PredBBs[0];
|
|
else {
|
|
LLVM_DEBUG(dbgs() << " Factoring out " << PredBBs.size()
|
|
<< " common predecessors.\n");
|
|
PredBB = splitBlockPreds(BB, PredBBs, ".thr_comm");
|
|
}
|
|
|
|
// And finally, do it!
|
|
LLVM_DEBUG(dbgs() << " Threading edge from '" << PredBB->getName()
|
|
<< "' to '" << SuccBB->getName()
|
|
<< ", across block:\n " << *BB << "\n");
|
|
|
|
LVI->threadEdge(PredBB, BB, SuccBB);
|
|
|
|
BasicBlock *NewBB = BasicBlock::Create(BB->getContext(),
|
|
BB->getName()+".thread",
|
|
BB->getParent(), BB);
|
|
NewBB->moveAfter(PredBB);
|
|
|
|
// Set the block frequency of NewBB.
|
|
if (HasProfileData) {
|
|
auto NewBBFreq =
|
|
BFI->getBlockFreq(PredBB) * BPI->getEdgeProbability(PredBB, BB);
|
|
BFI->setBlockFreq(NewBB, NewBBFreq.getFrequency());
|
|
}
|
|
|
|
// Copy all the instructions from BB to NewBB except the terminator.
|
|
DenseMap<Instruction *, Value *> ValueMapping =
|
|
cloneInstructions(BB->begin(), std::prev(BB->end()), NewBB, PredBB);
|
|
|
|
// We didn't copy the terminator from BB over to NewBB, because there is now
|
|
// an unconditional jump to SuccBB. Insert the unconditional jump.
|
|
BranchInst *NewBI = BranchInst::Create(SuccBB, NewBB);
|
|
NewBI->setDebugLoc(BB->getTerminator()->getDebugLoc());
|
|
|
|
// Check to see if SuccBB has PHI nodes. If so, we need to add entries to the
|
|
// PHI nodes for NewBB now.
|
|
addPHINodeEntriesForMappedBlock(SuccBB, BB, NewBB, ValueMapping);
|
|
|
|
// Update the terminator of PredBB to jump to NewBB instead of BB. This
|
|
// eliminates predecessors from BB, which requires us to simplify any PHI
|
|
// nodes in BB.
|
|
Instruction *PredTerm = PredBB->getTerminator();
|
|
for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i)
|
|
if (PredTerm->getSuccessor(i) == BB) {
|
|
BB->removePredecessor(PredBB, true);
|
|
PredTerm->setSuccessor(i, NewBB);
|
|
}
|
|
|
|
// Enqueue required DT updates.
|
|
DTU->applyUpdatesPermissive({{DominatorTree::Insert, NewBB, SuccBB},
|
|
{DominatorTree::Insert, PredBB, NewBB},
|
|
{DominatorTree::Delete, PredBB, BB}});
|
|
|
|
updateSSA(BB, NewBB, ValueMapping);
|
|
|
|
// At this point, the IR is fully up to date and consistent. Do a quick scan
|
|
// over the new instructions and zap any that are constants or dead. This
|
|
// frequently happens because of phi translation.
|
|
SimplifyInstructionsInBlock(NewBB, TLI);
|
|
|
|
// Update the edge weight from BB to SuccBB, which should be less than before.
|
|
updateBlockFreqAndEdgeWeight(PredBB, BB, NewBB, SuccBB);
|
|
|
|
// Threaded an edge!
|
|
++NumThreads;
|
|
}
|
|
|
|
/// Create a new basic block that will be the predecessor of BB and successor of
|
|
/// all blocks in Preds. When profile data is available, update the frequency of
|
|
/// this new block.
|
|
BasicBlock *JumpThreadingPass::splitBlockPreds(BasicBlock *BB,
|
|
ArrayRef<BasicBlock *> Preds,
|
|
const char *Suffix) {
|
|
SmallVector<BasicBlock *, 2> NewBBs;
|
|
|
|
// Collect the frequencies of all predecessors of BB, which will be used to
|
|
// update the edge weight of the result of splitting predecessors.
|
|
DenseMap<BasicBlock *, BlockFrequency> FreqMap;
|
|
if (HasProfileData)
|
|
for (auto Pred : Preds)
|
|
FreqMap.insert(std::make_pair(
|
|
Pred, BFI->getBlockFreq(Pred) * BPI->getEdgeProbability(Pred, BB)));
|
|
|
|
// In the case when BB is a LandingPad block we create 2 new predecessors
|
|
// instead of just one.
|
|
if (BB->isLandingPad()) {
|
|
std::string NewName = std::string(Suffix) + ".split-lp";
|
|
SplitLandingPadPredecessors(BB, Preds, Suffix, NewName.c_str(), NewBBs);
|
|
} else {
|
|
NewBBs.push_back(SplitBlockPredecessors(BB, Preds, Suffix));
|
|
}
|
|
|
|
std::vector<DominatorTree::UpdateType> Updates;
|
|
Updates.reserve((2 * Preds.size()) + NewBBs.size());
|
|
for (auto NewBB : NewBBs) {
|
|
BlockFrequency NewBBFreq(0);
|
|
Updates.push_back({DominatorTree::Insert, NewBB, BB});
|
|
for (auto Pred : predecessors(NewBB)) {
|
|
Updates.push_back({DominatorTree::Delete, Pred, BB});
|
|
Updates.push_back({DominatorTree::Insert, Pred, NewBB});
|
|
if (HasProfileData) // Update frequencies between Pred -> NewBB.
|
|
NewBBFreq += FreqMap.lookup(Pred);
|
|
}
|
|
if (HasProfileData) // Apply the summed frequency to NewBB.
|
|
BFI->setBlockFreq(NewBB, NewBBFreq.getFrequency());
|
|
}
|
|
|
|
DTU->applyUpdatesPermissive(Updates);
|
|
return NewBBs[0];
|
|
}
|
|
|
|
bool JumpThreadingPass::doesBlockHaveProfileData(BasicBlock *BB) {
|
|
const Instruction *TI = BB->getTerminator();
|
|
assert(TI->getNumSuccessors() > 1 && "not a split");
|
|
|
|
MDNode *WeightsNode = TI->getMetadata(LLVMContext::MD_prof);
|
|
if (!WeightsNode)
|
|
return false;
|
|
|
|
MDString *MDName = cast<MDString>(WeightsNode->getOperand(0));
|
|
if (MDName->getString() != "branch_weights")
|
|
return false;
|
|
|
|
// Ensure there are weights for all of the successors. Note that the first
|
|
// operand to the metadata node is a name, not a weight.
|
|
return WeightsNode->getNumOperands() == TI->getNumSuccessors() + 1;
|
|
}
|
|
|
|
/// Update the block frequency of BB and branch weight and the metadata on the
|
|
/// edge BB->SuccBB. This is done by scaling the weight of BB->SuccBB by 1 -
|
|
/// Freq(PredBB->BB) / Freq(BB->SuccBB).
|
|
void JumpThreadingPass::updateBlockFreqAndEdgeWeight(BasicBlock *PredBB,
|
|
BasicBlock *BB,
|
|
BasicBlock *NewBB,
|
|
BasicBlock *SuccBB) {
|
|
if (!HasProfileData)
|
|
return;
|
|
|
|
assert(BFI && BPI && "BFI & BPI should have been created here");
|
|
|
|
// As the edge from PredBB to BB is deleted, we have to update the block
|
|
// frequency of BB.
|
|
auto BBOrigFreq = BFI->getBlockFreq(BB);
|
|
auto NewBBFreq = BFI->getBlockFreq(NewBB);
|
|
auto BB2SuccBBFreq = BBOrigFreq * BPI->getEdgeProbability(BB, SuccBB);
|
|
auto BBNewFreq = BBOrigFreq - NewBBFreq;
|
|
BFI->setBlockFreq(BB, BBNewFreq.getFrequency());
|
|
|
|
// Collect updated outgoing edges' frequencies from BB and use them to update
|
|
// edge probabilities.
|
|
SmallVector<uint64_t, 4> BBSuccFreq;
|
|
for (BasicBlock *Succ : successors(BB)) {
|
|
auto SuccFreq = (Succ == SuccBB)
|
|
? BB2SuccBBFreq - NewBBFreq
|
|
: BBOrigFreq * BPI->getEdgeProbability(BB, Succ);
|
|
BBSuccFreq.push_back(SuccFreq.getFrequency());
|
|
}
|
|
|
|
uint64_t MaxBBSuccFreq =
|
|
*std::max_element(BBSuccFreq.begin(), BBSuccFreq.end());
|
|
|
|
SmallVector<BranchProbability, 4> BBSuccProbs;
|
|
if (MaxBBSuccFreq == 0)
|
|
BBSuccProbs.assign(BBSuccFreq.size(),
|
|
{1, static_cast<unsigned>(BBSuccFreq.size())});
|
|
else {
|
|
for (uint64_t Freq : BBSuccFreq)
|
|
BBSuccProbs.push_back(
|
|
BranchProbability::getBranchProbability(Freq, MaxBBSuccFreq));
|
|
// Normalize edge probabilities so that they sum up to one.
|
|
BranchProbability::normalizeProbabilities(BBSuccProbs.begin(),
|
|
BBSuccProbs.end());
|
|
}
|
|
|
|
// Update edge probabilities in BPI.
|
|
BPI->setEdgeProbability(BB, BBSuccProbs);
|
|
|
|
// Update the profile metadata as well.
|
|
//
|
|
// Don't do this if the profile of the transformed blocks was statically
|
|
// estimated. (This could occur despite the function having an entry
|
|
// frequency in completely cold parts of the CFG.)
|
|
//
|
|
// In this case we don't want to suggest to subsequent passes that the
|
|
// calculated weights are fully consistent. Consider this graph:
|
|
//
|
|
// check_1
|
|
// 50% / |
|
|
// eq_1 | 50%
|
|
// \ |
|
|
// check_2
|
|
// 50% / |
|
|
// eq_2 | 50%
|
|
// \ |
|
|
// check_3
|
|
// 50% / |
|
|
// eq_3 | 50%
|
|
// \ |
|
|
//
|
|
// Assuming the blocks check_* all compare the same value against 1, 2 and 3,
|
|
// the overall probabilities are inconsistent; the total probability that the
|
|
// value is either 1, 2 or 3 is 150%.
|
|
//
|
|
// As a consequence if we thread eq_1 -> check_2 to check_3, check_2->check_3
|
|
// becomes 0%. This is even worse if the edge whose probability becomes 0% is
|
|
// the loop exit edge. Then based solely on static estimation we would assume
|
|
// the loop was extremely hot.
|
|
//
|
|
// FIXME this locally as well so that BPI and BFI are consistent as well. We
|
|
// shouldn't make edges extremely likely or unlikely based solely on static
|
|
// estimation.
|
|
if (BBSuccProbs.size() >= 2 && doesBlockHaveProfileData(BB)) {
|
|
SmallVector<uint32_t, 4> Weights;
|
|
for (auto Prob : BBSuccProbs)
|
|
Weights.push_back(Prob.getNumerator());
|
|
|
|
auto TI = BB->getTerminator();
|
|
TI->setMetadata(
|
|
LLVMContext::MD_prof,
|
|
MDBuilder(TI->getParent()->getContext()).createBranchWeights(Weights));
|
|
}
|
|
}
|
|
|
|
/// duplicateCondBranchOnPHIIntoPred - PredBB contains an unconditional branch
|
|
/// to BB which contains an i1 PHI node and a conditional branch on that PHI.
|
|
/// If we can duplicate the contents of BB up into PredBB do so now, this
|
|
/// improves the odds that the branch will be on an analyzable instruction like
|
|
/// a compare.
|
|
bool JumpThreadingPass::duplicateCondBranchOnPHIIntoPred(
|
|
BasicBlock *BB, const SmallVectorImpl<BasicBlock *> &PredBBs) {
|
|
assert(!PredBBs.empty() && "Can't handle an empty set");
|
|
|
|
// If BB is a loop header, then duplicating this block outside the loop would
|
|
// cause us to transform this into an irreducible loop, don't do this.
|
|
// See the comments above findLoopHeaders for justifications and caveats.
|
|
if (LoopHeaders.count(BB)) {
|
|
LLVM_DEBUG(dbgs() << " Not duplicating loop header '" << BB->getName()
|
|
<< "' into predecessor block '" << PredBBs[0]->getName()
|
|
<< "' - it might create an irreducible loop!\n");
|
|
return false;
|
|
}
|
|
|
|
unsigned DuplicationCost =
|
|
getJumpThreadDuplicationCost(BB, BB->getTerminator(), BBDupThreshold);
|
|
if (DuplicationCost > BBDupThreshold) {
|
|
LLVM_DEBUG(dbgs() << " Not duplicating BB '" << BB->getName()
|
|
<< "' - Cost is too high: " << DuplicationCost << "\n");
|
|
return false;
|
|
}
|
|
|
|
// And finally, do it! Start by factoring the predecessors if needed.
|
|
std::vector<DominatorTree::UpdateType> Updates;
|
|
BasicBlock *PredBB;
|
|
if (PredBBs.size() == 1)
|
|
PredBB = PredBBs[0];
|
|
else {
|
|
LLVM_DEBUG(dbgs() << " Factoring out " << PredBBs.size()
|
|
<< " common predecessors.\n");
|
|
PredBB = splitBlockPreds(BB, PredBBs, ".thr_comm");
|
|
}
|
|
Updates.push_back({DominatorTree::Delete, PredBB, BB});
|
|
|
|
// Okay, we decided to do this! Clone all the instructions in BB onto the end
|
|
// of PredBB.
|
|
LLVM_DEBUG(dbgs() << " Duplicating block '" << BB->getName()
|
|
<< "' into end of '" << PredBB->getName()
|
|
<< "' to eliminate branch on phi. Cost: "
|
|
<< DuplicationCost << " block is:" << *BB << "\n");
|
|
|
|
// Unless PredBB ends with an unconditional branch, split the edge so that we
|
|
// can just clone the bits from BB into the end of the new PredBB.
|
|
BranchInst *OldPredBranch = dyn_cast<BranchInst>(PredBB->getTerminator());
|
|
|
|
if (!OldPredBranch || !OldPredBranch->isUnconditional()) {
|
|
BasicBlock *OldPredBB = PredBB;
|
|
PredBB = SplitEdge(OldPredBB, BB);
|
|
Updates.push_back({DominatorTree::Insert, OldPredBB, PredBB});
|
|
Updates.push_back({DominatorTree::Insert, PredBB, BB});
|
|
Updates.push_back({DominatorTree::Delete, OldPredBB, BB});
|
|
OldPredBranch = cast<BranchInst>(PredBB->getTerminator());
|
|
}
|
|
|
|
// We are going to have to map operands from the original BB block into the
|
|
// PredBB block. Evaluate PHI nodes in BB.
|
|
DenseMap<Instruction*, Value*> ValueMapping;
|
|
|
|
BasicBlock::iterator BI = BB->begin();
|
|
for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
|
|
ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
|
|
// Clone the non-phi instructions of BB into PredBB, keeping track of the
|
|
// mapping and using it to remap operands in the cloned instructions.
|
|
for (; BI != BB->end(); ++BI) {
|
|
Instruction *New = BI->clone();
|
|
|
|
// Remap operands to patch up intra-block references.
|
|
for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
|
|
if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
|
|
DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
|
|
if (I != ValueMapping.end())
|
|
New->setOperand(i, I->second);
|
|
}
|
|
|
|
// If this instruction can be simplified after the operands are updated,
|
|
// just use the simplified value instead. This frequently happens due to
|
|
// phi translation.
|
|
if (Value *IV = SimplifyInstruction(
|
|
New,
|
|
{BB->getModule()->getDataLayout(), TLI, nullptr, nullptr, New})) {
|
|
ValueMapping[&*BI] = IV;
|
|
if (!New->mayHaveSideEffects()) {
|
|
New->deleteValue();
|
|
New = nullptr;
|
|
}
|
|
} else {
|
|
ValueMapping[&*BI] = New;
|
|
}
|
|
if (New) {
|
|
// Otherwise, insert the new instruction into the block.
|
|
New->setName(BI->getName());
|
|
PredBB->getInstList().insert(OldPredBranch->getIterator(), New);
|
|
// Update Dominance from simplified New instruction operands.
|
|
for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
|
|
if (BasicBlock *SuccBB = dyn_cast<BasicBlock>(New->getOperand(i)))
|
|
Updates.push_back({DominatorTree::Insert, PredBB, SuccBB});
|
|
}
|
|
}
|
|
|
|
// Check to see if the targets of the branch had PHI nodes. If so, we need to
|
|
// add entries to the PHI nodes for branch from PredBB now.
|
|
BranchInst *BBBranch = cast<BranchInst>(BB->getTerminator());
|
|
addPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(0), BB, PredBB,
|
|
ValueMapping);
|
|
addPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(1), BB, PredBB,
|
|
ValueMapping);
|
|
|
|
updateSSA(BB, PredBB, ValueMapping);
|
|
|
|
// PredBB no longer jumps to BB, remove entries in the PHI node for the edge
|
|
// that we nuked.
|
|
BB->removePredecessor(PredBB, true);
|
|
|
|
// Remove the unconditional branch at the end of the PredBB block.
|
|
OldPredBranch->eraseFromParent();
|
|
if (HasProfileData)
|
|
BPI->copyEdgeProbabilities(BB, PredBB);
|
|
DTU->applyUpdatesPermissive(Updates);
|
|
|
|
++NumDupes;
|
|
return true;
|
|
}
|
|
|
|
// Pred is a predecessor of BB with an unconditional branch to BB. SI is
|
|
// a Select instruction in Pred. BB has other predecessors and SI is used in
|
|
// a PHI node in BB. SI has no other use.
|
|
// A new basic block, NewBB, is created and SI is converted to compare and
|
|
// conditional branch. SI is erased from parent.
|
|
void JumpThreadingPass::unfoldSelectInstr(BasicBlock *Pred, BasicBlock *BB,
|
|
SelectInst *SI, PHINode *SIUse,
|
|
unsigned Idx) {
|
|
// Expand the select.
|
|
//
|
|
// Pred --
|
|
// | v
|
|
// | NewBB
|
|
// | |
|
|
// |-----
|
|
// v
|
|
// BB
|
|
BranchInst *PredTerm = cast<BranchInst>(Pred->getTerminator());
|
|
BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), "select.unfold",
|
|
BB->getParent(), BB);
|
|
// Move the unconditional branch to NewBB.
|
|
PredTerm->removeFromParent();
|
|
NewBB->getInstList().insert(NewBB->end(), PredTerm);
|
|
// Create a conditional branch and update PHI nodes.
|
|
auto *BI = BranchInst::Create(NewBB, BB, SI->getCondition(), Pred);
|
|
BI->applyMergedLocation(PredTerm->getDebugLoc(), SI->getDebugLoc());
|
|
SIUse->setIncomingValue(Idx, SI->getFalseValue());
|
|
SIUse->addIncoming(SI->getTrueValue(), NewBB);
|
|
|
|
// The select is now dead.
|
|
SI->eraseFromParent();
|
|
DTU->applyUpdatesPermissive({{DominatorTree::Insert, NewBB, BB},
|
|
{DominatorTree::Insert, Pred, NewBB}});
|
|
|
|
// Update any other PHI nodes in BB.
|
|
for (BasicBlock::iterator BI = BB->begin();
|
|
PHINode *Phi = dyn_cast<PHINode>(BI); ++BI)
|
|
if (Phi != SIUse)
|
|
Phi->addIncoming(Phi->getIncomingValueForBlock(Pred), NewBB);
|
|
}
|
|
|
|
bool JumpThreadingPass::tryToUnfoldSelect(SwitchInst *SI, BasicBlock *BB) {
|
|
PHINode *CondPHI = dyn_cast<PHINode>(SI->getCondition());
|
|
|
|
if (!CondPHI || CondPHI->getParent() != BB)
|
|
return false;
|
|
|
|
for (unsigned I = 0, E = CondPHI->getNumIncomingValues(); I != E; ++I) {
|
|
BasicBlock *Pred = CondPHI->getIncomingBlock(I);
|
|
SelectInst *PredSI = dyn_cast<SelectInst>(CondPHI->getIncomingValue(I));
|
|
|
|
// The second and third condition can be potentially relaxed. Currently
|
|
// the conditions help to simplify the code and allow us to reuse existing
|
|
// code, developed for tryToUnfoldSelect(CmpInst *, BasicBlock *)
|
|
if (!PredSI || PredSI->getParent() != Pred || !PredSI->hasOneUse())
|
|
continue;
|
|
|
|
BranchInst *PredTerm = dyn_cast<BranchInst>(Pred->getTerminator());
|
|
if (!PredTerm || !PredTerm->isUnconditional())
|
|
continue;
|
|
|
|
unfoldSelectInstr(Pred, BB, PredSI, CondPHI, I);
|
|
return true;
|
|
}
|
|
return false;
|
|
}
|
|
|
|
/// tryToUnfoldSelect - Look for blocks of the form
|
|
/// bb1:
|
|
/// %a = select
|
|
/// br bb2
|
|
///
|
|
/// bb2:
|
|
/// %p = phi [%a, %bb1] ...
|
|
/// %c = icmp %p
|
|
/// br i1 %c
|
|
///
|
|
/// And expand the select into a branch structure if one of its arms allows %c
|
|
/// to be folded. This later enables threading from bb1 over bb2.
|
|
bool JumpThreadingPass::tryToUnfoldSelect(CmpInst *CondCmp, BasicBlock *BB) {
|
|
BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
|
|
PHINode *CondLHS = dyn_cast<PHINode>(CondCmp->getOperand(0));
|
|
Constant *CondRHS = cast<Constant>(CondCmp->getOperand(1));
|
|
|
|
if (!CondBr || !CondBr->isConditional() || !CondLHS ||
|
|
CondLHS->getParent() != BB)
|
|
return false;
|
|
|
|
for (unsigned I = 0, E = CondLHS->getNumIncomingValues(); I != E; ++I) {
|
|
BasicBlock *Pred = CondLHS->getIncomingBlock(I);
|
|
SelectInst *SI = dyn_cast<SelectInst>(CondLHS->getIncomingValue(I));
|
|
|
|
// Look if one of the incoming values is a select in the corresponding
|
|
// predecessor.
|
|
if (!SI || SI->getParent() != Pred || !SI->hasOneUse())
|
|
continue;
|
|
|
|
BranchInst *PredTerm = dyn_cast<BranchInst>(Pred->getTerminator());
|
|
if (!PredTerm || !PredTerm->isUnconditional())
|
|
continue;
|
|
|
|
// Now check if one of the select values would allow us to constant fold the
|
|
// terminator in BB. We don't do the transform if both sides fold, those
|
|
// cases will be threaded in any case.
|
|
LazyValueInfo::Tristate LHSFolds =
|
|
LVI->getPredicateOnEdge(CondCmp->getPredicate(), SI->getOperand(1),
|
|
CondRHS, Pred, BB, CondCmp);
|
|
LazyValueInfo::Tristate RHSFolds =
|
|
LVI->getPredicateOnEdge(CondCmp->getPredicate(), SI->getOperand(2),
|
|
CondRHS, Pred, BB, CondCmp);
|
|
if ((LHSFolds != LazyValueInfo::Unknown ||
|
|
RHSFolds != LazyValueInfo::Unknown) &&
|
|
LHSFolds != RHSFolds) {
|
|
unfoldSelectInstr(Pred, BB, SI, CondLHS, I);
|
|
return true;
|
|
}
|
|
}
|
|
return false;
|
|
}
|
|
|
|
/// tryToUnfoldSelectInCurrBB - Look for PHI/Select or PHI/CMP/Select in the
|
|
/// same BB in the form
|
|
/// bb:
|
|
/// %p = phi [false, %bb1], [true, %bb2], [false, %bb3], [true, %bb4], ...
|
|
/// %s = select %p, trueval, falseval
|
|
///
|
|
/// or
|
|
///
|
|
/// bb:
|
|
/// %p = phi [0, %bb1], [1, %bb2], [0, %bb3], [1, %bb4], ...
|
|
/// %c = cmp %p, 0
|
|
/// %s = select %c, trueval, falseval
|
|
///
|
|
/// And expand the select into a branch structure. This later enables
|
|
/// jump-threading over bb in this pass.
|
|
///
|
|
/// Using the similar approach of SimplifyCFG::FoldCondBranchOnPHI(), unfold
|
|
/// select if the associated PHI has at least one constant. If the unfolded
|
|
/// select is not jump-threaded, it will be folded again in the later
|
|
/// optimizations.
|
|
bool JumpThreadingPass::tryToUnfoldSelectInCurrBB(BasicBlock *BB) {
|
|
// This transform would reduce the quality of msan diagnostics.
|
|
// Disable this transform under MemorySanitizer.
|
|
if (BB->getParent()->hasFnAttribute(Attribute::SanitizeMemory))
|
|
return false;
|
|
|
|
// If threading this would thread across a loop header, don't thread the edge.
|
|
// See the comments above findLoopHeaders for justifications and caveats.
|
|
if (LoopHeaders.count(BB))
|
|
return false;
|
|
|
|
for (BasicBlock::iterator BI = BB->begin();
|
|
PHINode *PN = dyn_cast<PHINode>(BI); ++BI) {
|
|
// Look for a Phi having at least one constant incoming value.
|
|
if (llvm::all_of(PN->incoming_values(),
|
|
[](Value *V) { return !isa<ConstantInt>(V); }))
|
|
continue;
|
|
|
|
auto isUnfoldCandidate = [BB](SelectInst *SI, Value *V) {
|
|
using namespace PatternMatch;
|
|
|
|
// Check if SI is in BB and use V as condition.
|
|
if (SI->getParent() != BB)
|
|
return false;
|
|
Value *Cond = SI->getCondition();
|
|
bool IsAndOr = match(SI, m_CombineOr(m_LogicalAnd(), m_LogicalOr()));
|
|
return Cond && Cond == V && Cond->getType()->isIntegerTy(1) && !IsAndOr;
|
|
};
|
|
|
|
SelectInst *SI = nullptr;
|
|
for (Use &U : PN->uses()) {
|
|
if (ICmpInst *Cmp = dyn_cast<ICmpInst>(U.getUser())) {
|
|
// Look for a ICmp in BB that compares PN with a constant and is the
|
|
// condition of a Select.
|
|
if (Cmp->getParent() == BB && Cmp->hasOneUse() &&
|
|
isa<ConstantInt>(Cmp->getOperand(1 - U.getOperandNo())))
|
|
if (SelectInst *SelectI = dyn_cast<SelectInst>(Cmp->user_back()))
|
|
if (isUnfoldCandidate(SelectI, Cmp->use_begin()->get())) {
|
|
SI = SelectI;
|
|
break;
|
|
}
|
|
} else if (SelectInst *SelectI = dyn_cast<SelectInst>(U.getUser())) {
|
|
// Look for a Select in BB that uses PN as condition.
|
|
if (isUnfoldCandidate(SelectI, U.get())) {
|
|
SI = SelectI;
|
|
break;
|
|
}
|
|
}
|
|
}
|
|
|
|
if (!SI)
|
|
continue;
|
|
// Expand the select.
|
|
Value *Cond = SI->getCondition();
|
|
if (InsertFreezeWhenUnfoldingSelect &&
|
|
!isGuaranteedNotToBeUndefOrPoison(Cond, nullptr, SI,
|
|
&DTU->getDomTree()))
|
|
Cond = new FreezeInst(Cond, "cond.fr", SI);
|
|
Instruction *Term = SplitBlockAndInsertIfThen(Cond, SI, false);
|
|
BasicBlock *SplitBB = SI->getParent();
|
|
BasicBlock *NewBB = Term->getParent();
|
|
PHINode *NewPN = PHINode::Create(SI->getType(), 2, "", SI);
|
|
NewPN->addIncoming(SI->getTrueValue(), Term->getParent());
|
|
NewPN->addIncoming(SI->getFalseValue(), BB);
|
|
SI->replaceAllUsesWith(NewPN);
|
|
SI->eraseFromParent();
|
|
// NewBB and SplitBB are newly created blocks which require insertion.
|
|
std::vector<DominatorTree::UpdateType> Updates;
|
|
Updates.reserve((2 * SplitBB->getTerminator()->getNumSuccessors()) + 3);
|
|
Updates.push_back({DominatorTree::Insert, BB, SplitBB});
|
|
Updates.push_back({DominatorTree::Insert, BB, NewBB});
|
|
Updates.push_back({DominatorTree::Insert, NewBB, SplitBB});
|
|
// BB's successors were moved to SplitBB, update DTU accordingly.
|
|
for (auto *Succ : successors(SplitBB)) {
|
|
Updates.push_back({DominatorTree::Delete, BB, Succ});
|
|
Updates.push_back({DominatorTree::Insert, SplitBB, Succ});
|
|
}
|
|
DTU->applyUpdatesPermissive(Updates);
|
|
return true;
|
|
}
|
|
return false;
|
|
}
|
|
|
|
/// Try to propagate a guard from the current BB into one of its predecessors
|
|
/// in case if another branch of execution implies that the condition of this
|
|
/// guard is always true. Currently we only process the simplest case that
|
|
/// looks like:
|
|
///
|
|
/// Start:
|
|
/// %cond = ...
|
|
/// br i1 %cond, label %T1, label %F1
|
|
/// T1:
|
|
/// br label %Merge
|
|
/// F1:
|
|
/// br label %Merge
|
|
/// Merge:
|
|
/// %condGuard = ...
|
|
/// call void(i1, ...) @llvm.experimental.guard( i1 %condGuard )[ "deopt"() ]
|
|
///
|
|
/// And cond either implies condGuard or !condGuard. In this case all the
|
|
/// instructions before the guard can be duplicated in both branches, and the
|
|
/// guard is then threaded to one of them.
|
|
bool JumpThreadingPass::processGuards(BasicBlock *BB) {
|
|
using namespace PatternMatch;
|
|
|
|
// We only want to deal with two predecessors.
|
|
BasicBlock *Pred1, *Pred2;
|
|
auto PI = pred_begin(BB), PE = pred_end(BB);
|
|
if (PI == PE)
|
|
return false;
|
|
Pred1 = *PI++;
|
|
if (PI == PE)
|
|
return false;
|
|
Pred2 = *PI++;
|
|
if (PI != PE)
|
|
return false;
|
|
if (Pred1 == Pred2)
|
|
return false;
|
|
|
|
// Try to thread one of the guards of the block.
|
|
// TODO: Look up deeper than to immediate predecessor?
|
|
auto *Parent = Pred1->getSinglePredecessor();
|
|
if (!Parent || Parent != Pred2->getSinglePredecessor())
|
|
return false;
|
|
|
|
if (auto *BI = dyn_cast<BranchInst>(Parent->getTerminator()))
|
|
for (auto &I : *BB)
|
|
if (isGuard(&I) && threadGuard(BB, cast<IntrinsicInst>(&I), BI))
|
|
return true;
|
|
|
|
return false;
|
|
}
|
|
|
|
/// Try to propagate the guard from BB which is the lower block of a diamond
|
|
/// to one of its branches, in case if diamond's condition implies guard's
|
|
/// condition.
|
|
bool JumpThreadingPass::threadGuard(BasicBlock *BB, IntrinsicInst *Guard,
|
|
BranchInst *BI) {
|
|
assert(BI->getNumSuccessors() == 2 && "Wrong number of successors?");
|
|
assert(BI->isConditional() && "Unconditional branch has 2 successors?");
|
|
Value *GuardCond = Guard->getArgOperand(0);
|
|
Value *BranchCond = BI->getCondition();
|
|
BasicBlock *TrueDest = BI->getSuccessor(0);
|
|
BasicBlock *FalseDest = BI->getSuccessor(1);
|
|
|
|
auto &DL = BB->getModule()->getDataLayout();
|
|
bool TrueDestIsSafe = false;
|
|
bool FalseDestIsSafe = false;
|
|
|
|
// True dest is safe if BranchCond => GuardCond.
|
|
auto Impl = isImpliedCondition(BranchCond, GuardCond, DL);
|
|
if (Impl && *Impl)
|
|
TrueDestIsSafe = true;
|
|
else {
|
|
// False dest is safe if !BranchCond => GuardCond.
|
|
Impl = isImpliedCondition(BranchCond, GuardCond, DL, /* LHSIsTrue */ false);
|
|
if (Impl && *Impl)
|
|
FalseDestIsSafe = true;
|
|
}
|
|
|
|
if (!TrueDestIsSafe && !FalseDestIsSafe)
|
|
return false;
|
|
|
|
BasicBlock *PredUnguardedBlock = TrueDestIsSafe ? TrueDest : FalseDest;
|
|
BasicBlock *PredGuardedBlock = FalseDestIsSafe ? TrueDest : FalseDest;
|
|
|
|
ValueToValueMapTy UnguardedMapping, GuardedMapping;
|
|
Instruction *AfterGuard = Guard->getNextNode();
|
|
unsigned Cost = getJumpThreadDuplicationCost(BB, AfterGuard, BBDupThreshold);
|
|
if (Cost > BBDupThreshold)
|
|
return false;
|
|
// Duplicate all instructions before the guard and the guard itself to the
|
|
// branch where implication is not proved.
|
|
BasicBlock *GuardedBlock = DuplicateInstructionsInSplitBetween(
|
|
BB, PredGuardedBlock, AfterGuard, GuardedMapping, *DTU);
|
|
assert(GuardedBlock && "Could not create the guarded block?");
|
|
// Duplicate all instructions before the guard in the unguarded branch.
|
|
// Since we have successfully duplicated the guarded block and this block
|
|
// has fewer instructions, we expect it to succeed.
|
|
BasicBlock *UnguardedBlock = DuplicateInstructionsInSplitBetween(
|
|
BB, PredUnguardedBlock, Guard, UnguardedMapping, *DTU);
|
|
assert(UnguardedBlock && "Could not create the unguarded block?");
|
|
LLVM_DEBUG(dbgs() << "Moved guard " << *Guard << " to block "
|
|
<< GuardedBlock->getName() << "\n");
|
|
// Some instructions before the guard may still have uses. For them, we need
|
|
// to create Phi nodes merging their copies in both guarded and unguarded
|
|
// branches. Those instructions that have no uses can be just removed.
|
|
SmallVector<Instruction *, 4> ToRemove;
|
|
for (auto BI = BB->begin(); &*BI != AfterGuard; ++BI)
|
|
if (!isa<PHINode>(&*BI))
|
|
ToRemove.push_back(&*BI);
|
|
|
|
Instruction *InsertionPoint = &*BB->getFirstInsertionPt();
|
|
assert(InsertionPoint && "Empty block?");
|
|
// Substitute with Phis & remove.
|
|
for (auto *Inst : reverse(ToRemove)) {
|
|
if (!Inst->use_empty()) {
|
|
PHINode *NewPN = PHINode::Create(Inst->getType(), 2);
|
|
NewPN->addIncoming(UnguardedMapping[Inst], UnguardedBlock);
|
|
NewPN->addIncoming(GuardedMapping[Inst], GuardedBlock);
|
|
NewPN->insertBefore(InsertionPoint);
|
|
Inst->replaceAllUsesWith(NewPN);
|
|
}
|
|
Inst->eraseFromParent();
|
|
}
|
|
return true;
|
|
}
|