ConstantProp.cpp revision e922c0201916e0b980ab3cfe91e1413e68d55647
1//===- ConstantProp.cpp - Code to perform Simple Constant Propagation -----===// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file is distributed under the University of Illinois Open Source 6// 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 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 static char ID; // Pass identification, replacement for typeid 38 ConstantPropagation() : FunctionPass(&ID) {} 39 40 bool runOnFunction(Function &F); 41 42 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 43 AU.setPreservesCFG(); 44 } 45 }; 46} 47 48char ConstantPropagation::ID = 0; 49static RegisterPass<ConstantPropagation> 50X("constprop", "Simple constant propagation"); 51 52FunctionPass *llvm::createConstantPropagationPass() { 53 return new ConstantPropagation(); 54} 55 56 57bool ConstantPropagation::runOnFunction(Function &F) { 58 // Initialize the worklist to all of the instructions ready to process... 59 std::set<Instruction*> WorkList; 60 for(inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i) { 61 WorkList.insert(&*i); 62 } 63 bool Changed = false; 64 65 while (!WorkList.empty()) { 66 Instruction *I = *WorkList.begin(); 67 WorkList.erase(WorkList.begin()); // Get an element from the worklist... 68 69 if (!I->use_empty()) // Don't muck with dead instructions... 70 if (Constant *C = ConstantFoldInstruction(I, F.getContext())) { 71 // Add all of the users of this instruction to the worklist, they might 72 // be constant propagatable now... 73 for (Value::use_iterator UI = I->use_begin(), UE = I->use_end(); 74 UI != UE; ++UI) 75 WorkList.insert(cast<Instruction>(*UI)); 76 77 // Replace all of the uses of a variable with uses of the constant. 78 I->replaceAllUsesWith(C); 79 80 // Remove the dead instruction. 81 WorkList.erase(I); 82 I->eraseFromParent(); 83 84 // We made a change to the function... 85 Changed = true; 86 ++NumInstKilled; 87 } 88 } 89 return Changed; 90} 91