mirror of
https://github.com/c64scene-ar/llvm-6502.git
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Carlo Alberto Ferraris. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@157736 91177308-0d34-0410-b5e6-96231b3b80d8
269 lines
9.5 KiB
C++
269 lines
9.5 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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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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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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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(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 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 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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/// 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, 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), *cur = LI->getLoopFor(Inst->getParent());
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if (succ != 0 && 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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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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// 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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// 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 == 0; ++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 == 0; ++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 == 0)
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return false;
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DEBUG(dbgs() << "Sink" << *Inst << " (";
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WriteAsOperand(dbgs(), Inst->getParent(), false);
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dbgs() << " -> ";
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WriteAsOperand(dbgs(), SuccToSinkTo, 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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