ConstantProp.cpp revision 9133fe28954d498fc4de13064c7d65bd811de02c
1//===- ConstantProp.cpp - Code to perform Simple Constant Propagation -----===// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file was developed by the LLVM research group and is distributed under 6// the University of Illinois Open Source License. See LICENSE.TXT for details. 7// 8//===----------------------------------------------------------------------===// 9// 10// This file implements constant propagation and merging: 11// 12// Specifically, this: 13// * Converts instructions like "add int 1, 2" into 3 14// 15// Notice that: 16// * This pass has a habit of making definitions be dead. It is a good idea 17// to to run a DIE pass sometime after running this pass. 18// 19//===----------------------------------------------------------------------===// 20 21#define DEBUG_TYPE "constprop" 22#include "llvm/Transforms/Scalar.h" 23#include "llvm/Analysis/ConstantFolding.h" 24#include "llvm/Constant.h" 25#include "llvm/Instruction.h" 26#include "llvm/Pass.h" 27#include "llvm/Support/Compiler.h" 28#include "llvm/Support/InstIterator.h" 29#include "llvm/ADT/Statistic.h" 30#include <set> 31using namespace llvm; 32 33STATISTIC(NumInstKilled, "Number of instructions killed"); 34 35namespace { 36 struct VISIBILITY_HIDDEN ConstantPropagation : public FunctionPass { 37 bool runOnFunction(Function &F); 38 39 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 40 AU.setPreservesCFG(); 41 } 42 }; 43 44 RegisterPass<ConstantPropagation> X("constprop", 45 "Simple constant propagation"); 46} 47 48FunctionPass *llvm::createConstantPropagationPass() { 49 return new ConstantPropagation(); 50} 51 52 53bool ConstantPropagation::runOnFunction(Function &F) { 54 // Initialize the worklist to all of the instructions ready to process... 55 std::set<Instruction*> WorkList; 56 for(inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i) { 57 WorkList.insert(&*i); 58 } 59 bool Changed = false; 60 61 while (!WorkList.empty()) { 62 Instruction *I = *WorkList.begin(); 63 WorkList.erase(WorkList.begin()); // Get an element from the worklist... 64 65 if (!I->use_empty()) // Don't muck with dead instructions... 66 if (Constant *C = ConstantFoldInstruction(I)) { 67 // Add all of the users of this instruction to the worklist, they might 68 // be constant propagatable now... 69 for (Value::use_iterator UI = I->use_begin(), UE = I->use_end(); 70 UI != UE; ++UI) 71 WorkList.insert(cast<Instruction>(*UI)); 72 73 // Replace all of the uses of a variable with uses of the constant. 74 I->replaceAllUsesWith(C); 75 76 // Remove the dead instruction. 77 WorkList.erase(I); 78 I->getParent()->getInstList().erase(I); 79 80 // We made a change to the function... 81 Changed = true; 82 ++NumInstKilled; 83 } 84 } 85 return Changed; 86} 87