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309 lines
11 KiB
C++
309 lines
11 KiB
C++
//===- SimplifyCFGPass.cpp - CFG Simplification Pass ----------------------===//
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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 dead code elimination and basic block merging, along
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// with a collection of other peephole control flow optimizations. For example:
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//
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// * Removes basic blocks with no predecessors.
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// * Merges a basic block into its predecessor if there is only one and the
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// predecessor only has one successor.
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// * Eliminates PHI nodes for basic blocks with a single predecessor.
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// * Eliminates a basic block that only contains an unconditional branch.
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// * Changes invoke instructions to nounwind functions to be calls.
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// * Change things like "if (x) if (y)" into "if (x&y)".
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// * etc..
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//
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//===----------------------------------------------------------------------===//
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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/AssumptionCache.h"
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#include "llvm/Analysis/CFG.h"
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#include "llvm/Analysis/GlobalsModRef.h"
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#include "llvm/Analysis/TargetTransformInfo.h"
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#include "llvm/IR/Attributes.h"
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#include "llvm/IR/CFG.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/Instructions.h"
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#include "llvm/IR/IntrinsicInst.h"
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#include "llvm/IR/Module.h"
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#include "llvm/InitializePasses.h"
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#include "llvm/Pass.h"
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#include "llvm/Support/CommandLine.h"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Transforms/Scalar/SimplifyCFG.h"
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#include "llvm/Transforms/Utils/Local.h"
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#include "llvm/Transforms/Utils/SimplifyCFGOptions.h"
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#include <utility>
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using namespace llvm;
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#define DEBUG_TYPE "simplifycfg"
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static cl::opt<unsigned> UserBonusInstThreshold(
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"bonus-inst-threshold", cl::Hidden, cl::init(1),
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cl::desc("Control the number of bonus instructions (default = 1)"));
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static cl::opt<bool> UserKeepLoops(
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"keep-loops", cl::Hidden, cl::init(true),
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cl::desc("Preserve canonical loop structure (default = true)"));
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static cl::opt<bool> UserSwitchToLookup(
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"switch-to-lookup", cl::Hidden, cl::init(false),
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cl::desc("Convert switches to lookup tables (default = false)"));
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static cl::opt<bool> UserForwardSwitchCond(
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"forward-switch-cond", cl::Hidden, cl::init(false),
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cl::desc("Forward switch condition to phi ops (default = false)"));
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static cl::opt<bool> UserHoistCommonInsts(
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"hoist-common-insts", cl::Hidden, cl::init(false),
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cl::desc("hoist common instructions (default = false)"));
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static cl::opt<bool> UserSinkCommonInsts(
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"sink-common-insts", cl::Hidden, cl::init(false),
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cl::desc("Sink common instructions (default = false)"));
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STATISTIC(NumSimpl, "Number of blocks simplified");
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/// If we have more than one empty (other than phi node) return blocks,
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/// merge them together to promote recursive block merging.
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static bool mergeEmptyReturnBlocks(Function &F) {
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bool Changed = false;
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BasicBlock *RetBlock = nullptr;
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// Scan all the blocks in the function, looking for empty return blocks.
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for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
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BasicBlock &BB = *BBI++;
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// Only look at return blocks.
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ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
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if (!Ret) continue;
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// Only look at the block if it is empty or the only other thing in it is a
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// single PHI node that is the operand to the return.
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if (Ret != &BB.front()) {
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// Check for something else in the block.
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BasicBlock::iterator I(Ret);
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--I;
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// Skip over debug info.
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while (isa<DbgInfoIntrinsic>(I) && I != BB.begin())
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--I;
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if (!isa<DbgInfoIntrinsic>(I) &&
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(!isa<PHINode>(I) || I != BB.begin() || Ret->getNumOperands() == 0 ||
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Ret->getOperand(0) != &*I))
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continue;
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}
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// If this is the first returning block, remember it and keep going.
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if (!RetBlock) {
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RetBlock = &BB;
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continue;
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}
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// Skip merging if this would result in a CallBr instruction with a
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// duplicate destination. FIXME: See note in CodeGenPrepare.cpp.
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bool SkipCallBr = false;
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for (pred_iterator PI = pred_begin(&BB), E = pred_end(&BB);
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PI != E && !SkipCallBr; ++PI) {
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if (auto *CBI = dyn_cast<CallBrInst>((*PI)->getTerminator()))
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for (unsigned i = 0, e = CBI->getNumSuccessors(); i != e; ++i)
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if (RetBlock == CBI->getSuccessor(i)) {
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SkipCallBr = true;
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break;
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}
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}
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if (SkipCallBr)
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continue;
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// Otherwise, we found a duplicate return block. Merge the two.
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Changed = true;
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// Case when there is no input to the return or when the returned values
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// agree is trivial. Note that they can't agree if there are phis in the
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// blocks.
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if (Ret->getNumOperands() == 0 ||
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Ret->getOperand(0) ==
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cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) {
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BB.replaceAllUsesWith(RetBlock);
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BB.eraseFromParent();
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continue;
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}
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// If the canonical return block has no PHI node, create one now.
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PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin());
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if (!RetBlockPHI) {
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Value *InVal = cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0);
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pred_iterator PB = pred_begin(RetBlock), PE = pred_end(RetBlock);
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RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(),
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std::distance(PB, PE), "merge",
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&RetBlock->front());
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for (pred_iterator PI = PB; PI != PE; ++PI)
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RetBlockPHI->addIncoming(InVal, *PI);
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RetBlock->getTerminator()->setOperand(0, RetBlockPHI);
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}
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// Turn BB into a block that just unconditionally branches to the return
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// block. This handles the case when the two return blocks have a common
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// predecessor but that return different things.
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RetBlockPHI->addIncoming(Ret->getOperand(0), &BB);
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BB.getTerminator()->eraseFromParent();
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BranchInst::Create(RetBlock, &BB);
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}
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return Changed;
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}
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/// Call SimplifyCFG on all the blocks in the function,
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/// iterating until no more changes are made.
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static bool iterativelySimplifyCFG(Function &F, const TargetTransformInfo &TTI,
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const SimplifyCFGOptions &Options) {
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bool Changed = false;
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bool LocalChange = true;
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SmallVector<std::pair<const BasicBlock *, const BasicBlock *>, 32> Edges;
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FindFunctionBackedges(F, Edges);
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SmallPtrSet<BasicBlock *, 16> LoopHeaders;
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for (unsigned i = 0, e = Edges.size(); i != e; ++i)
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LoopHeaders.insert(const_cast<BasicBlock *>(Edges[i].second));
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while (LocalChange) {
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LocalChange = false;
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// Loop over all of the basic blocks and remove them if they are unneeded.
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for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) {
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if (simplifyCFG(&*BBIt++, TTI, Options, &LoopHeaders)) {
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LocalChange = true;
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++NumSimpl;
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}
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}
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Changed |= LocalChange;
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}
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return Changed;
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}
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static bool simplifyFunctionCFG(Function &F, const TargetTransformInfo &TTI,
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const SimplifyCFGOptions &Options) {
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bool EverChanged = removeUnreachableBlocks(F);
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EverChanged |= mergeEmptyReturnBlocks(F);
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EverChanged |= iterativelySimplifyCFG(F, TTI, Options);
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// If neither pass changed anything, we're done.
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if (!EverChanged) return false;
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// iterativelySimplifyCFG can (rarely) make some loops dead. If this happens,
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// removeUnreachableBlocks is needed to nuke them, which means we should
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// iterate between the two optimizations. We structure the code like this to
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// avoid rerunning iterativelySimplifyCFG if the second pass of
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// removeUnreachableBlocks doesn't do anything.
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if (!removeUnreachableBlocks(F))
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return true;
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do {
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EverChanged = iterativelySimplifyCFG(F, TTI, Options);
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EverChanged |= removeUnreachableBlocks(F);
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} while (EverChanged);
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return true;
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}
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// Command-line settings override compile-time settings.
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static void applyCommandLineOverridesToOptions(SimplifyCFGOptions &Options) {
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if (UserBonusInstThreshold.getNumOccurrences())
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Options.BonusInstThreshold = UserBonusInstThreshold;
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if (UserForwardSwitchCond.getNumOccurrences())
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Options.ForwardSwitchCondToPhi = UserForwardSwitchCond;
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if (UserSwitchToLookup.getNumOccurrences())
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Options.ConvertSwitchToLookupTable = UserSwitchToLookup;
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if (UserKeepLoops.getNumOccurrences())
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Options.NeedCanonicalLoop = UserKeepLoops;
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if (UserHoistCommonInsts.getNumOccurrences())
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Options.HoistCommonInsts = UserHoistCommonInsts;
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if (UserSinkCommonInsts.getNumOccurrences())
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Options.SinkCommonInsts = UserSinkCommonInsts;
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}
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SimplifyCFGPass::SimplifyCFGPass() : Options() {
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applyCommandLineOverridesToOptions(Options);
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}
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SimplifyCFGPass::SimplifyCFGPass(const SimplifyCFGOptions &Opts)
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: Options(Opts) {
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applyCommandLineOverridesToOptions(Options);
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}
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PreservedAnalyses SimplifyCFGPass::run(Function &F,
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FunctionAnalysisManager &AM) {
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auto &TTI = AM.getResult<TargetIRAnalysis>(F);
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Options.AC = &AM.getResult<AssumptionAnalysis>(F);
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if (!simplifyFunctionCFG(F, TTI, Options))
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return PreservedAnalyses::all();
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PreservedAnalyses PA;
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PA.preserve<GlobalsAA>();
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return PA;
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}
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namespace {
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struct CFGSimplifyPass : public FunctionPass {
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static char ID;
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SimplifyCFGOptions Options;
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std::function<bool(const Function &)> PredicateFtor;
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CFGSimplifyPass(SimplifyCFGOptions Options_ = SimplifyCFGOptions(),
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std::function<bool(const Function &)> Ftor = nullptr)
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: FunctionPass(ID), Options(Options_), PredicateFtor(std::move(Ftor)) {
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initializeCFGSimplifyPassPass(*PassRegistry::getPassRegistry());
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// Check for command-line overrides of options for debug/customization.
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applyCommandLineOverridesToOptions(Options);
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}
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bool runOnFunction(Function &F) override {
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if (skipFunction(F) || (PredicateFtor && !PredicateFtor(F)))
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return false;
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Options.AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
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if (F.hasFnAttribute(Attribute::OptForFuzzing)) {
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Options.setSimplifyCondBranch(false)
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.setFoldTwoEntryPHINode(false);
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} else {
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Options.setSimplifyCondBranch(true)
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.setFoldTwoEntryPHINode(true);
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}
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auto &TTI = getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
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return simplifyFunctionCFG(F, TTI, Options);
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}
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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AU.addRequired<AssumptionCacheTracker>();
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AU.addRequired<TargetTransformInfoWrapperPass>();
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AU.addPreserved<GlobalsAAWrapperPass>();
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}
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};
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}
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char CFGSimplifyPass::ID = 0;
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INITIALIZE_PASS_BEGIN(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
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false)
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INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
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INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
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INITIALIZE_PASS_END(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
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false)
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// Public interface to the CFGSimplification pass
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FunctionPass *
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llvm::createCFGSimplificationPass(SimplifyCFGOptions Options,
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std::function<bool(const Function &)> Ftor) {
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return new CFGSimplifyPass(Options, std::move(Ftor));
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}
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