diff --git a/include/llvm/Transforms/Scalar.h b/include/llvm/Transforms/Scalar.h index 4fea3f73be1..59306dceaca 100644 --- a/include/llvm/Transforms/Scalar.h +++ b/include/llvm/Transforms/Scalar.h @@ -330,6 +330,13 @@ FunctionPass *createGVNPREPass(); // FunctionPass *createFastDeadStoreEliminationPass(); +//===----------------------------------------------------------------------===// +// +// FastDeadStoreElimination - This pass deletes loads that are dominated by +// must-aliased loads and are not stored to between the loads. +// +FunctionPass *createFastDeadLoadEliminationPass(); + //===----------------------------------------------------------------------===// // // CodeGenPrepare - This pass prepares a function for instruction selection. diff --git a/lib/Transforms/Scalar/FastDLE.cpp b/lib/Transforms/Scalar/FastDLE.cpp new file mode 100644 index 00000000000..001f834b5f0 --- /dev/null +++ b/lib/Transforms/Scalar/FastDLE.cpp @@ -0,0 +1,128 @@ +//===- FastDLE.cpp - Fast Dead Load Elimination ---------------------------===// +// +// The LLVM Compiler Infrastructure +// +// This file was developed by Owen Anderson and is distributed under +// the University of Illinois Open Source License. See LICENSE.TXT for details. +// +//===----------------------------------------------------------------------===// +// +// This file implements a trivial dead load elimination that only considers +// basic-block local redundant load. +// +// FIXME: This should eventually be extended to be a post-dominator tree +// traversal. Doing so would be pretty trivial. +// +//===----------------------------------------------------------------------===// + +#define DEBUG_TYPE "fdle" +#include "llvm/Transforms/Scalar.h" +#include "llvm/Function.h" +#include "llvm/Instructions.h" +#include "llvm/Pass.h" +#include "llvm/ADT/SmallPtrSet.h" +#include "llvm/ADT/Statistic.h" +#include "llvm/Analysis/MemoryDependenceAnalysis.h" +#include "llvm/Transforms/Utils/Local.h" +#include "llvm/Support/Compiler.h" +using namespace llvm; + +STATISTIC(NumFastLoads, "Number of loads deleted"); + +namespace { + struct VISIBILITY_HIDDEN FDLE : public FunctionPass { + static char ID; // Pass identification, replacement for typeid + FDLE() : FunctionPass((intptr_t)&ID) {} + + virtual bool runOnFunction(Function &F) { + bool Changed = false; + for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) + Changed |= runOnBasicBlock(*I); + return Changed; + } + + bool runOnBasicBlock(BasicBlock &BB); + + // getAnalysisUsage - We require post dominance frontiers (aka Control + // Dependence Graph) + virtual void getAnalysisUsage(AnalysisUsage &AU) const { + AU.setPreservesCFG(); + AU.addRequired(); + AU.addPreserved(); + } + }; + char FDLE::ID = 0; + RegisterPass X("fdle", "Fast Dead Load Elimination"); +} + +FunctionPass *llvm::createFastDeadLoadEliminationPass() { return new FDLE(); } + +bool FDLE::runOnBasicBlock(BasicBlock &BB) { + MemoryDependenceAnalysis& MD = getAnalysis(); + + // Record the last-seen load from this pointer + DenseMap lastLoad; + + bool MadeChange = false; + + // Do a top-down walk on the BB + for (BasicBlock::iterator BBI = BB.begin(), BBE = BB.end(); BBI != BBE; ++BBI) { + // If we find a store or a free... + if (LoadInst* L = dyn_cast(BBI)) { + Value* pointer = L->getPointerOperand(); + LoadInst*& last = lastLoad[pointer]; + + // ... to a pointer that has been loaded from before... + Instruction* dep = MD.getDependency(BBI); + bool deletedLoad = false; + + while (dep != MemoryDependenceAnalysis::None && + dep != MemoryDependenceAnalysis::NonLocal && + (isa(dep) || isa(dep))) { + // ... that depends on a store ... + if (StoreInst* S = dyn_cast(dep)) { + if (S->getPointerOperand() == pointer) { + // Remove it! + MD.removeInstruction(BBI); + + BBI--; + L->replaceAllUsesWith(S->getOperand(0)); + L->eraseFromParent(); + NumFastLoads++; + deletedLoad = true; + MadeChange = true; + } + + // Whether we removed it or not, we can't + // go any further + break; + } else if (!last) { + // If we don't depend on a store, and we haven't + // been loaded before, bail. + break; + } else if (dep == last) { + // Remove it! + MD.removeInstruction(BBI); + + BBI--; + L->replaceAllUsesWith(last); + L->eraseFromParent(); + deletedLoad = true; + NumFastLoads++; + MadeChange = true; + + break; + } else { + dep = MD.getDependency(BBI, dep); + } + } + + if (!deletedLoad) + last = L; + } + } + + return MadeChange; +} + +