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https://github.com/RPCS3/llvm-mirror.git
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1add31cc93
into Analysis as a standalone function, since there's no need for it to be in VMCore. Also, update it to use isKnownNonZero and other goodies available in Analysis, making it more precise, enabling more aggressive optimization. llvm-svn: 146610
275 lines
9.4 KiB
C++
275 lines
9.4 KiB
C++
//===-- Sink.cpp - Code Sinking -------------------------------------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This pass moves instructions into successor blocks, when possible, so that
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// they aren't executed on paths where their results aren't needed.
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//
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "sink"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/IntrinsicInst.h"
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#include "llvm/Analysis/Dominators.h"
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#include "llvm/Analysis/LoopInfo.h"
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#include "llvm/Analysis/AliasAnalysis.h"
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#include "llvm/Analysis/ValueTracking.h"
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#include "llvm/Assembly/Writer.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Support/CFG.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/raw_ostream.h"
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using namespace llvm;
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STATISTIC(NumSunk, "Number of instructions sunk");
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namespace {
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class Sinking : public FunctionPass {
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DominatorTree *DT;
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LoopInfo *LI;
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AliasAnalysis *AA;
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public:
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static char ID; // Pass identification
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Sinking() : FunctionPass(ID) {
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initializeSinkingPass(*PassRegistry::getPassRegistry());
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}
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virtual bool runOnFunction(Function &F);
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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AU.setPreservesCFG();
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FunctionPass::getAnalysisUsage(AU);
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AU.addRequired<AliasAnalysis>();
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AU.addRequired<DominatorTree>();
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AU.addRequired<LoopInfo>();
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AU.addPreserved<DominatorTree>();
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AU.addPreserved<LoopInfo>();
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}
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private:
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bool ProcessBlock(BasicBlock &BB);
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bool SinkInstruction(Instruction *I, SmallPtrSet<Instruction *, 8> &Stores);
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bool AllUsesDominatedByBlock(Instruction *Inst, BasicBlock *BB) const;
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};
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} // end anonymous namespace
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char Sinking::ID = 0;
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INITIALIZE_PASS_BEGIN(Sinking, "sink", "Code sinking", false, false)
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INITIALIZE_PASS_DEPENDENCY(LoopInfo)
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INITIALIZE_PASS_DEPENDENCY(DominatorTree)
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INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
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INITIALIZE_PASS_END(Sinking, "sink", "Code sinking", false, false)
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FunctionPass *llvm::createSinkingPass() { return new Sinking(); }
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/// AllUsesDominatedByBlock - Return true if all uses of the specified value
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/// occur in blocks dominated by the specified block.
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bool Sinking::AllUsesDominatedByBlock(Instruction *Inst,
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BasicBlock *BB) const {
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// Ignoring debug uses is necessary so debug info doesn't affect the code.
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// This may leave a referencing dbg_value in the original block, before
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// the definition of the vreg. Dwarf generator handles this although the
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// user might not get the right info at runtime.
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for (Value::use_iterator I = Inst->use_begin(),
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E = Inst->use_end(); I != E; ++I) {
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// Determine the block of the use.
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Instruction *UseInst = cast<Instruction>(*I);
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BasicBlock *UseBlock = UseInst->getParent();
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if (PHINode *PN = dyn_cast<PHINode>(UseInst)) {
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// PHI nodes use the operand in the predecessor block, not the block with
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// the PHI.
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unsigned Num = PHINode::getIncomingValueNumForOperand(I.getOperandNo());
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UseBlock = PN->getIncomingBlock(Num);
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}
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// Check that it dominates.
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if (!DT->dominates(BB, UseBlock))
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return false;
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}
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return true;
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}
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bool Sinking::runOnFunction(Function &F) {
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DT = &getAnalysis<DominatorTree>();
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LI = &getAnalysis<LoopInfo>();
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AA = &getAnalysis<AliasAnalysis>();
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bool EverMadeChange = false;
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while (1) {
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bool MadeChange = false;
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// Process all basic blocks.
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for (Function::iterator I = F.begin(), E = F.end();
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I != E; ++I)
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MadeChange |= ProcessBlock(*I);
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// If this iteration over the code changed anything, keep iterating.
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if (!MadeChange) break;
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EverMadeChange = true;
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}
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return EverMadeChange;
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}
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bool Sinking::ProcessBlock(BasicBlock &BB) {
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// Can't sink anything out of a block that has less than two successors.
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if (BB.getTerminator()->getNumSuccessors() <= 1 || BB.empty()) return false;
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// Don't bother sinking code out of unreachable blocks. In addition to being
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// unprofitable, it can also lead to infinite looping, because in an unreachable
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// loop there may be nowhere to stop.
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if (!DT->isReachableFromEntry(&BB)) return false;
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bool MadeChange = false;
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// Walk the basic block bottom-up. Remember if we saw a store.
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BasicBlock::iterator I = BB.end();
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--I;
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bool ProcessedBegin = false;
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SmallPtrSet<Instruction *, 8> Stores;
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do {
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Instruction *Inst = I; // The instruction to sink.
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// Predecrement I (if it's not begin) so that it isn't invalidated by
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// sinking.
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ProcessedBegin = I == BB.begin();
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if (!ProcessedBegin)
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--I;
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if (isa<DbgInfoIntrinsic>(Inst))
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continue;
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if (SinkInstruction(Inst, Stores))
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++NumSunk, MadeChange = true;
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// If we just processed the first instruction in the block, we're done.
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} while (!ProcessedBegin);
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return MadeChange;
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}
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static bool isSafeToMove(Instruction *Inst, AliasAnalysis *AA,
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SmallPtrSet<Instruction *, 8> &Stores) {
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if (Inst->mayWriteToMemory()) {
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Stores.insert(Inst);
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return false;
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}
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if (LoadInst *L = dyn_cast<LoadInst>(Inst)) {
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AliasAnalysis::Location Loc = AA->getLocation(L);
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for (SmallPtrSet<Instruction *, 8>::iterator I = Stores.begin(),
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E = Stores.end(); I != E; ++I)
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if (AA->getModRefInfo(*I, Loc) & AliasAnalysis::Mod)
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return false;
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}
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if (isa<TerminatorInst>(Inst) || isa<PHINode>(Inst))
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return false;
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return true;
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}
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/// SinkInstruction - Determine whether it is safe to sink the specified machine
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/// instruction out of its current block into a successor.
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bool Sinking::SinkInstruction(Instruction *Inst,
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SmallPtrSet<Instruction *, 8> &Stores) {
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// Check if it's safe to move the instruction.
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if (!isSafeToMove(Inst, AA, Stores))
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return false;
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// FIXME: This should include support for sinking instructions within the
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// block they are currently in to shorten the live ranges. We often get
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// instructions sunk into the top of a large block, but it would be better to
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// also sink them down before their first use in the block. This xform has to
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// be careful not to *increase* register pressure though, e.g. sinking
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// "x = y + z" down if it kills y and z would increase the live ranges of y
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// and z and only shrink the live range of x.
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// Loop over all the operands of the specified instruction. If there is
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// anything we can't handle, bail out.
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BasicBlock *ParentBlock = Inst->getParent();
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// SuccToSinkTo - This is the successor to sink this instruction to, once we
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// decide.
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BasicBlock *SuccToSinkTo = 0;
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// FIXME: This picks a successor to sink into based on having one
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// successor that dominates all the uses. However, there are cases where
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// sinking can happen but where the sink point isn't a successor. For
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// example:
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// x = computation
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// if () {} else {}
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// use x
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// the instruction could be sunk over the whole diamond for the
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// if/then/else (or loop, etc), allowing it to be sunk into other blocks
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// after that.
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// Instructions can only be sunk if all their uses are in blocks
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// dominated by one of the successors.
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// Look at all the successors and decide which one
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// we should sink to.
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for (succ_iterator SI = succ_begin(ParentBlock),
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E = succ_end(ParentBlock); SI != E; ++SI) {
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if (AllUsesDominatedByBlock(Inst, *SI)) {
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SuccToSinkTo = *SI;
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break;
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}
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}
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// If we couldn't find a block to sink to, ignore this instruction.
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if (SuccToSinkTo == 0)
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return false;
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// It is not possible to sink an instruction into its own block. This can
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// happen with loops.
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if (Inst->getParent() == SuccToSinkTo)
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return false;
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DEBUG(dbgs() << "Sink instr " << *Inst);
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DEBUG(dbgs() << "to block ";
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WriteAsOperand(dbgs(), SuccToSinkTo, false));
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// If the block has multiple predecessors, this would introduce computation on
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// a path that it doesn't already exist. We could split the critical edge,
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// but for now we just punt.
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// FIXME: Split critical edges if not backedges.
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if (SuccToSinkTo->getUniquePredecessor() != ParentBlock) {
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// We cannot sink a load across a critical edge - there may be stores in
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// other code paths.
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if (!isSafeToSpeculativelyExecute(Inst)) {
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DEBUG(dbgs() << " *** PUNTING: Wont sink load along critical edge.\n");
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return false;
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}
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// We don't want to sink across a critical edge if we don't dominate the
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// successor. We could be introducing calculations to new code paths.
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if (!DT->dominates(ParentBlock, SuccToSinkTo)) {
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DEBUG(dbgs() << " *** PUNTING: Critical edge found\n");
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return false;
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}
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// Don't sink instructions into a loop.
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if (LI->isLoopHeader(SuccToSinkTo)) {
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DEBUG(dbgs() << " *** PUNTING: Loop header found\n");
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return false;
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}
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// Otherwise we are OK with sinking along a critical edge.
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DEBUG(dbgs() << "Sinking along critical edge.\n");
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}
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// Determine where to insert into. Skip phi nodes.
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BasicBlock::iterator InsertPos = SuccToSinkTo->begin();
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while (InsertPos != SuccToSinkTo->end() && isa<PHINode>(InsertPos))
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++InsertPos;
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// Move the instruction.
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Inst->moveBefore(InsertPos);
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return true;
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}
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