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Diffstat (limited to 'contrib/llvm/lib/Transforms/Scalar/Sink.cpp')
| -rw-r--r-- | contrib/llvm/lib/Transforms/Scalar/Sink.cpp | 266 |
1 files changed, 266 insertions, 0 deletions
diff --git a/contrib/llvm/lib/Transforms/Scalar/Sink.cpp b/contrib/llvm/lib/Transforms/Scalar/Sink.cpp new file mode 100644 index 000000000000..95d3dedfb62d --- /dev/null +++ b/contrib/llvm/lib/Transforms/Scalar/Sink.cpp @@ -0,0 +1,266 @@ +//===-- Sink.cpp - Code Sinking -------------------------------------------===// +// +// The LLVM Compiler Infrastructure +// +// This file is distributed under the University of Illinois Open Source +// License. See LICENSE.TXT for details. +// +//===----------------------------------------------------------------------===// +// +// This pass moves instructions into successor blocks, when possible, so that +// they aren't executed on paths where their results aren't needed. +// +//===----------------------------------------------------------------------===// + +#define DEBUG_TYPE "sink" +#include "llvm/Transforms/Scalar.h" +#include "llvm/IntrinsicInst.h" +#include "llvm/Analysis/Dominators.h" +#include "llvm/Analysis/LoopInfo.h" +#include "llvm/Analysis/AliasAnalysis.h" +#include "llvm/Assembly/Writer.h" +#include "llvm/ADT/Statistic.h" +#include "llvm/Support/CFG.h" +#include "llvm/Support/Debug.h" +#include "llvm/Support/raw_ostream.h" +using namespace llvm; + +STATISTIC(NumSunk, "Number of instructions sunk"); + +namespace { + class Sinking : public FunctionPass { + DominatorTree *DT; + LoopInfo *LI; + AliasAnalysis *AA; + + public: + static char ID; // Pass identification + Sinking() : FunctionPass(ID) {} + + virtual bool runOnFunction(Function &F); + + virtual void getAnalysisUsage(AnalysisUsage &AU) const { + AU.setPreservesCFG(); + FunctionPass::getAnalysisUsage(AU); + AU.addRequired<AliasAnalysis>(); + AU.addRequired<DominatorTree>(); + AU.addRequired<LoopInfo>(); + AU.addPreserved<DominatorTree>(); + AU.addPreserved<LoopInfo>(); + } + private: + bool ProcessBlock(BasicBlock &BB); + bool SinkInstruction(Instruction *I, SmallPtrSet<Instruction *, 8> &Stores); + bool AllUsesDominatedByBlock(Instruction *Inst, BasicBlock *BB) const; + }; +} // end anonymous namespace + +char Sinking::ID = 0; +INITIALIZE_PASS(Sinking, "sink", "Code sinking", false, false); + +FunctionPass *llvm::createSinkingPass() { return new Sinking(); } + +/// AllUsesDominatedByBlock - Return true if all uses of the specified value +/// occur in blocks dominated by the specified block. +bool Sinking::AllUsesDominatedByBlock(Instruction *Inst, + BasicBlock *BB) const { + // Ignoring debug uses is necessary so debug info doesn't affect the code. + // This may leave a referencing dbg_value in the original block, before + // the definition of the vreg. Dwarf generator handles this although the + // user might not get the right info at runtime. + for (Value::use_iterator I = Inst->use_begin(), + E = Inst->use_end(); I != E; ++I) { + // Determine the block of the use. + Instruction *UseInst = cast<Instruction>(*I); + BasicBlock *UseBlock = UseInst->getParent(); + if (PHINode *PN = dyn_cast<PHINode>(UseInst)) { + // PHI nodes use the operand in the predecessor block, not the block with + // the PHI. + unsigned Num = PHINode::getIncomingValueNumForOperand(I.getOperandNo()); + UseBlock = PN->getIncomingBlock(Num); + } + // Check that it dominates. + if (!DT->dominates(BB, UseBlock)) + return false; + } + return true; +} + +bool Sinking::runOnFunction(Function &F) { + DT = &getAnalysis<DominatorTree>(); + LI = &getAnalysis<LoopInfo>(); + AA = &getAnalysis<AliasAnalysis>(); + + bool EverMadeChange = false; + + while (1) { + bool MadeChange = false; + + // Process all basic blocks. + for (Function::iterator I = F.begin(), E = F.end(); + I != E; ++I) + MadeChange |= ProcessBlock(*I); + + // If this iteration over the code changed anything, keep iterating. + if (!MadeChange) break; + EverMadeChange = true; + } + return EverMadeChange; +} + +bool Sinking::ProcessBlock(BasicBlock &BB) { + // Can't sink anything out of a block that has less than two successors. + if (BB.getTerminator()->getNumSuccessors() <= 1 || BB.empty()) return false; + + // Don't bother sinking code out of unreachable blocks. In addition to being + // unprofitable, it can also lead to infinite looping, because in an unreachable + // loop there may be nowhere to stop. + if (!DT->isReachableFromEntry(&BB)) return false; + + bool MadeChange = false; + + // Walk the basic block bottom-up. Remember if we saw a store. + BasicBlock::iterator I = BB.end(); + --I; + bool ProcessedBegin = false; + SmallPtrSet<Instruction *, 8> Stores; + do { + Instruction *Inst = I; // The instruction to sink. + + // Predecrement I (if it's not begin) so that it isn't invalidated by + // sinking. + ProcessedBegin = I == BB.begin(); + if (!ProcessedBegin) + --I; + + if (isa<DbgInfoIntrinsic>(Inst)) + continue; + + if (SinkInstruction(Inst, Stores)) + ++NumSunk, MadeChange = true; + + // If we just processed the first instruction in the block, we're done. + } while (!ProcessedBegin); + + return MadeChange; +} + +static bool isSafeToMove(Instruction *Inst, AliasAnalysis *AA, + SmallPtrSet<Instruction *, 8> &Stores) { + if (LoadInst *L = dyn_cast<LoadInst>(Inst)) { + if (L->isVolatile()) return false; + + Value *Ptr = L->getPointerOperand(); + unsigned Size = AA->getTypeStoreSize(L->getType()); + for (SmallPtrSet<Instruction *, 8>::iterator I = Stores.begin(), + E = Stores.end(); I != E; ++I) + if (AA->getModRefInfo(*I, Ptr, Size) & AliasAnalysis::Mod) + return false; + } + + if (Inst->mayWriteToMemory()) { + Stores.insert(Inst); + return false; + } + + return Inst->isSafeToSpeculativelyExecute(); +} + +/// SinkInstruction - Determine whether it is safe to sink the specified machine +/// instruction out of its current block into a successor. +bool Sinking::SinkInstruction(Instruction *Inst, + SmallPtrSet<Instruction *, 8> &Stores) { + // Check if it's safe to move the instruction. + if (!isSafeToMove(Inst, AA, Stores)) + return false; + + // FIXME: This should include support for sinking instructions within the + // block they are currently in to shorten the live ranges. We often get + // instructions sunk into the top of a large block, but it would be better to + // also sink them down before their first use in the block. This xform has to + // be careful not to *increase* register pressure though, e.g. sinking + // "x = y + z" down if it kills y and z would increase the live ranges of y + // and z and only shrink the live range of x. + + // Loop over all the operands of the specified instruction. If there is + // anything we can't handle, bail out. + BasicBlock *ParentBlock = Inst->getParent(); + + // SuccToSinkTo - This is the successor to sink this instruction to, once we + // decide. + BasicBlock *SuccToSinkTo = 0; + + // FIXME: This picks a successor to sink into based on having one + // successor that dominates all the uses. However, there are cases where + // sinking can happen but where the sink point isn't a successor. For + // example: + // x = computation + // if () {} else {} + // use x + // the instruction could be sunk over the whole diamond for the + // if/then/else (or loop, etc), allowing it to be sunk into other blocks + // after that. + + // Instructions can only be sunk if all their uses are in blocks + // dominated by one of the successors. + // Look at all the successors and decide which one + // we should sink to. + for (succ_iterator SI = succ_begin(ParentBlock), + E = succ_end(ParentBlock); SI != E; ++SI) { + if (AllUsesDominatedByBlock(Inst, *SI)) { + SuccToSinkTo = *SI; + break; + } + } + + // If we couldn't find a block to sink to, ignore this instruction. + if (SuccToSinkTo == 0) + return false; + + // It is not possible to sink an instruction into its own block. This can + // happen with loops. + if (Inst->getParent() == SuccToSinkTo) + return false; + + DEBUG(dbgs() << "Sink instr " << *Inst); + DEBUG(dbgs() << "to block "; + WriteAsOperand(dbgs(), SuccToSinkTo, false)); + + // If the block has multiple predecessors, this would introduce computation on + // a path that it doesn't already exist. We could split the critical edge, + // but for now we just punt. + // FIXME: Split critical edges if not backedges. + if (SuccToSinkTo->getUniquePredecessor() != ParentBlock) { + // We cannot sink a load across a critical edge - there may be stores in + // other code paths. + if (!Inst->isSafeToSpeculativelyExecute()) { + DEBUG(dbgs() << " *** PUNTING: Wont sink load along critical edge.\n"); + return false; + } + + // We don't want to sink across a critical edge if we don't dominate the + // successor. We could be introducing calculations to new code paths. + if (!DT->dominates(ParentBlock, SuccToSinkTo)) { + DEBUG(dbgs() << " *** PUNTING: Critical edge found\n"); + return false; + } + + // Don't sink instructions into a loop. + if (LI->isLoopHeader(SuccToSinkTo)) { + DEBUG(dbgs() << " *** PUNTING: Loop header found\n"); + return false; + } + + // Otherwise we are OK with sinking along a critical edge. + DEBUG(dbgs() << "Sinking along critical edge.\n"); + } + + // Determine where to insert into. Skip phi nodes. + BasicBlock::iterator InsertPos = SuccToSinkTo->begin(); + while (InsertPos != SuccToSinkTo->end() && isa<PHINode>(InsertPos)) + ++InsertPos; + + // Move the instruction. + Inst->moveBefore(InsertPos); + return true; +} |