ConstantProp.cpp revision 198090
1301238Sdelphij//===- ConstantProp.cpp - Code to perform Simple Constant Propagation -----===// 2301238Sdelphij// 3301238Sdelphij// The LLVM Compiler Infrastructure 4301238Sdelphij// 5301238Sdelphij// This file is distributed under the University of Illinois Open Source 6301238Sdelphij// License. See LICENSE.TXT for details. 7301238Sdelphij// 8301238Sdelphij//===----------------------------------------------------------------------===// 9301238Sdelphij// 10301238Sdelphij// This file implements constant propagation and merging: 11301238Sdelphij// 12301238Sdelphij// Specifically, this: 13301238Sdelphij// * Converts instructions like "add int 1, 2" into 3 14301238Sdelphij// 15301238Sdelphij// Notice that: 16301238Sdelphij// * This pass has a habit of making definitions be dead. It is a good idea 17301238Sdelphij// to run a DIE pass sometime after running this pass. 18301238Sdelphij// 19301238Sdelphij//===----------------------------------------------------------------------===// 20301238Sdelphij 21301238Sdelphij#define DEBUG_TYPE "constprop" 22301238Sdelphij#include "llvm/Transforms/Scalar.h" 23301238Sdelphij#include "llvm/Analysis/ConstantFolding.h" 24301238Sdelphij#include "llvm/Constant.h" 25301238Sdelphij#include "llvm/Instruction.h" 26301238Sdelphij#include "llvm/Pass.h" 27301238Sdelphij#include "llvm/Support/InstIterator.h" 28301238Sdelphij#include "llvm/ADT/Statistic.h" 29301238Sdelphij#include <set> 30301238Sdelphijusing namespace llvm; 31301238Sdelphij 32301238SdelphijSTATISTIC(NumInstKilled, "Number of instructions killed"); 33301238Sdelphij 34301238Sdelphijnamespace { 35301238Sdelphij struct ConstantPropagation : public FunctionPass { 36301238Sdelphij static char ID; // Pass identification, replacement for typeid 37310419Sdelphij ConstantPropagation() : FunctionPass(&ID) {} 38310419Sdelphij 39301238Sdelphij bool runOnFunction(Function &F); 40301238Sdelphij 41301238Sdelphij virtual void getAnalysisUsage(AnalysisUsage &AU) const { 42310419Sdelphij AU.setPreservesCFG(); 43310419Sdelphij } 44301238Sdelphij }; 45301238Sdelphij} 46310419Sdelphij 47301238Sdelphijchar ConstantPropagation::ID = 0; 48301238Sdelphijstatic RegisterPass<ConstantPropagation> 49301238SdelphijX("constprop", "Simple constant propagation"); 50301238Sdelphij 51301238SdelphijFunctionPass *llvm::createConstantPropagationPass() { 52301238Sdelphij return new ConstantPropagation(); 53310419Sdelphij} 54301238Sdelphij 55301238Sdelphij 56301238Sdelphijbool ConstantPropagation::runOnFunction(Function &F) { 57301238Sdelphij // Initialize the worklist to all of the instructions ready to process... 58301238Sdelphij std::set<Instruction*> WorkList; 59301238Sdelphij for(inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i) { 60301238Sdelphij WorkList.insert(&*i); 61301238Sdelphij } 62301238Sdelphij bool Changed = false; 63301238Sdelphij 64301238Sdelphij while (!WorkList.empty()) { 65301238Sdelphij Instruction *I = *WorkList.begin(); 66301238Sdelphij WorkList.erase(WorkList.begin()); // Get an element from the worklist... 67301238Sdelphij 68301238Sdelphij if (!I->use_empty()) // Don't muck with dead instructions... 69301238Sdelphij if (Constant *C = ConstantFoldInstruction(I, F.getContext())) { 70301238Sdelphij // Add all of the users of this instruction to the worklist, they might 71301238Sdelphij // be constant propagatable now... 72301238Sdelphij for (Value::use_iterator UI = I->use_begin(), UE = I->use_end(); 73301238Sdelphij UI != UE; ++UI) 74310419Sdelphij WorkList.insert(cast<Instruction>(*UI)); 75301238Sdelphij 76301238Sdelphij // Replace all of the uses of a variable with uses of the constant. 77301238Sdelphij I->replaceAllUsesWith(C); 78301238Sdelphij 79301238Sdelphij // Remove the dead instruction. 80301238Sdelphij WorkList.erase(I); 81301238Sdelphij I->eraseFromParent(); 82301238Sdelphij 83301238Sdelphij // We made a change to the function... 84301238Sdelphij Changed = true; 85 ++NumInstKilled; 86 } 87 } 88 return Changed; 89} 90