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//===-- Sink.cpp - Code Sinking -------------------------------------------===//
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// The LLVM Compiler Infrastructure
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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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// 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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#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/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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STATISTIC(NumSunk, "Number of instructions sunk");
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class Sinking : public FunctionPass {
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static char ID; // Pass identification
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Sinking() : FunctionPass(ID) {}
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virtual bool runOnFunction(Function &F);
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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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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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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} // end anonymous namespace
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INITIALIZE_PASS(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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unsigned Num = PHINode::getIncomingValueNumForOperand(I.getOperandNo());
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UseBlock = PN->getIncomingBlock(Num);
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// Check that it dominates.
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if (!DT->dominates(BB, UseBlock))
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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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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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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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return EverMadeChange;
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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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bool ProcessedBegin = false;
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SmallPtrSet<Instruction *, 8> Stores;
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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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ProcessedBegin = I == BB.begin();
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if (isa<DbgInfoIntrinsic>(Inst))
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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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static bool isSafeToMove(Instruction *Inst, AliasAnalysis *AA,
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SmallPtrSet<Instruction *, 8> &Stores) {
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if (LoadInst *L = dyn_cast<LoadInst>(Inst)) {
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if (L->isVolatile()) return false;
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Value *Ptr = L->getPointerOperand();
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unsigned Size = AA->getTypeStoreSize(L->getType());
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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, Ptr, Size) & AliasAnalysis::Mod)
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if (Inst->mayWriteToMemory()) {
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return Inst->isSafeToSpeculativelyExecute();
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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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// 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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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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// 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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// 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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// If we couldn't find a block to sink to, ignore this instruction.
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if (SuccToSinkTo == 0)
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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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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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if (!Inst->isSafeToSpeculativelyExecute()) {
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DEBUG(dbgs() << " *** PUNTING: Wont sink load along critical edge.\n");
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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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// 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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// 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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// 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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// Move the instruction.
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Inst->moveBefore(InsertPos);