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52ab0bc417
preparation for de-coupling the AA implementations. In order to do this, they had to become fake-scoped using the traditional LLVM pattern of a leading initialism. These can't be actual scoped enumerations because they're bitfields and thus inherently we use them as integers. I've also renamed the behavior enums that are specific to reasoning about the mod/ref behavior of functions when called. This makes it more clear that they have a very narrow domain of applicability. I think there is a significantly cleaner API for all of this, but I don't want to try to do really substantive changes for now, I just want to refactor the things away from analysis groups so I'm preserving the exact original design and just cleaning up the names, style, and lifting out of the class. Differential Revision: http://reviews.llvm.org/D10564 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242963 91177308-0d34-0410-b5e6-96231b3b80d8
284 lines
9.9 KiB
C++
284 lines
9.9 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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#include "llvm/Transforms/Scalar.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/LoopInfo.h"
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#include "llvm/Analysis/ValueTracking.h"
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#include "llvm/IR/CFG.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/IntrinsicInst.h"
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#include "llvm/IR/Module.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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#define DEBUG_TYPE "sink"
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STATISTIC(NumSunk, "Number of instructions sunk");
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STATISTIC(NumSinkIter, "Number of sinking iterations");
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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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bool runOnFunction(Function &F) override;
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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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<DominatorTreeWrapperPass>();
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AU.addRequired<LoopInfoWrapperPass>();
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AU.addPreserved<DominatorTreeWrapperPass>();
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AU.addPreserved<LoopInfoWrapperPass>();
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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, SmallPtrSetImpl<Instruction*> &Stores);
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bool AllUsesDominatedByBlock(Instruction *Inst, BasicBlock *BB) const;
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bool IsAcceptableTarget(Instruction *Inst, BasicBlock *SuccToSinkTo) 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(LoopInfoWrapperPass)
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INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
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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 (Use &U : Inst->uses()) {
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// Determine the block of the use.
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Instruction *UseInst = cast<Instruction>(U.getUser());
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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(U.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<DominatorTreeWrapperPass>().getDomTree();
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LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
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AA = &getAnalysis<AliasAnalysis>();
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bool MadeChange, EverMadeChange = false;
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do {
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MadeChange = false;
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DEBUG(dbgs() << "Sinking iteration " << NumSinkIter << "\n");
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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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EverMadeChange |= MadeChange;
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NumSinkIter++;
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} while (MadeChange);
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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
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// unreachable 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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SmallPtrSetImpl<Instruction *> &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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MemoryLocation Loc = MemoryLocation::get(L);
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for (Instruction *S : Stores)
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if (AA->getModRefInfo(S, Loc) & MRI_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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// Convergent operations can only be moved to control equivalent blocks.
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if (auto CS = CallSite(Inst)) {
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if (CS.hasFnAttr(Attribute::Convergent))
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return false;
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}
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return true;
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}
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/// IsAcceptableTarget - Return true if it is possible to sink the instruction
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/// in the specified basic block.
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bool Sinking::IsAcceptableTarget(Instruction *Inst,
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BasicBlock *SuccToSinkTo) const {
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assert(Inst && "Instruction to be sunk is null");
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assert(SuccToSinkTo && "Candidate sink target is null");
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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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// If the block has multiple predecessors, this would introduce computation
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// on different code paths. We could split the critical edge, but for now we
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// just punt.
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// FIXME: Split critical edges if not backedges.
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if (SuccToSinkTo->getUniquePredecessor() != Inst->getParent()) {
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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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return false;
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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(Inst->getParent(), SuccToSinkTo))
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return false;
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// Don't sink instructions into a loop.
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Loop *succ = LI->getLoopFor(SuccToSinkTo);
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Loop *cur = LI->getLoopFor(Inst->getParent());
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if (succ != nullptr && succ != cur)
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return false;
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}
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// Finally, check that all the uses of the instruction are actually
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// dominated by the candidate
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return AllUsesDominatedByBlock(Inst, SuccToSinkTo);
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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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SmallPtrSetImpl<Instruction *> &Stores) {
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// Don't sink static alloca instructions. CodeGen assumes allocas outside the
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// entry block are dynamically sized stack objects.
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if (AllocaInst *AI = dyn_cast<AllocaInst>(Inst))
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if (AI->isStaticAlloca())
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return false;
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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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// SuccToSinkTo - This is the successor to sink this instruction to, once we
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// decide.
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BasicBlock *SuccToSinkTo = nullptr;
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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 postdominators and see if we can sink it in one.
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DomTreeNode *DTN = DT->getNode(Inst->getParent());
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for (DomTreeNode::iterator I = DTN->begin(), E = DTN->end();
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I != E && SuccToSinkTo == nullptr; ++I) {
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BasicBlock *Candidate = (*I)->getBlock();
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if ((*I)->getIDom()->getBlock() == Inst->getParent() &&
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IsAcceptableTarget(Inst, Candidate))
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SuccToSinkTo = Candidate;
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}
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// If no suitable postdominator was found, look at all the successors and
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// decide which one we should sink to, if any.
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for (succ_iterator I = succ_begin(Inst->getParent()),
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E = succ_end(Inst->getParent()); I != E && !SuccToSinkTo; ++I) {
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if (IsAcceptableTarget(Inst, *I))
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SuccToSinkTo = *I;
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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)
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return false;
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DEBUG(dbgs() << "Sink" << *Inst << " (";
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Inst->getParent()->printAsOperand(dbgs(), false);
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dbgs() << " -> ";
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SuccToSinkTo->printAsOperand(dbgs(), false);
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dbgs() << ")\n");
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// Move the instruction.
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Inst->moveBefore(SuccToSinkTo->getFirstInsertionPt());
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return true;
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}
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