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#include "llvm/Transforms/Scalar.h" 22#include "llvm/ADT/Statistic.h" 23#include "llvm/Analysis/ConstantFolding.h" 24#include "llvm/IR/Constant.h" 25#include "llvm/IR/DataLayout.h" 26#include "llvm/IR/InstIterator.h" 27#include "llvm/IR/Instruction.h" 28#include "llvm/Pass.h" 29#include "llvm/Target/TargetLibraryInfo.h" 30#include <set> 31using namespace llvm; 32 33#define DEBUG_TYPE "constprop" 34 35STATISTIC(NumInstKilled, "Number of instructions killed"); 36 37namespace { 38 struct ConstantPropagation : public FunctionPass { 39 static char ID; // Pass identification, replacement for typeid 40 ConstantPropagation() : FunctionPass(ID) { 41 initializeConstantPropagationPass(*PassRegistry::getPassRegistry()); 42 } 43 44 bool runOnFunction(Function &F) override; 45 46 void getAnalysisUsage(AnalysisUsage &AU) const override { 47 AU.setPreservesCFG(); 48 AU.addRequired<TargetLibraryInfo>(); 49 } 50 }; 51} 52 53char ConstantPropagation::ID = 0; 54INITIALIZE_PASS_BEGIN(ConstantPropagation, "constprop", 55 "Simple constant propagation", false, false) 56INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo) 57INITIALIZE_PASS_END(ConstantPropagation, "constprop", 58 "Simple constant propagation", false, false) 59 60FunctionPass *llvm::createConstantPropagationPass() { 61 return new ConstantPropagation(); 62} 63 64bool ConstantPropagation::runOnFunction(Function &F) { 65 // Initialize the worklist to all of the instructions ready to process... 66 std::set<Instruction*> WorkList; 67 for(inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i) { 68 WorkList.insert(&*i); 69 } 70 bool Changed = false; 71 DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>(); 72 const DataLayout *DL = DLP ? &DLP->getDataLayout() : nullptr; 73 TargetLibraryInfo *TLI = &getAnalysis<TargetLibraryInfo>(); 74 75 while (!WorkList.empty()) { 76 Instruction *I = *WorkList.begin(); 77 WorkList.erase(WorkList.begin()); // Get an element from the worklist... 78 79 if (!I->use_empty()) // Don't muck with dead instructions... 80 if (Constant *C = ConstantFoldInstruction(I, DL, TLI)) { 81 // Add all of the users of this instruction to the worklist, they might 82 // be constant propagatable now... 83 for (User *U : I->users()) 84 WorkList.insert(cast<Instruction>(U)); 85 86 // Replace all of the uses of a variable with uses of the constant. 87 I->replaceAllUsesWith(C); 88 89 // Remove the dead instruction. 90 WorkList.erase(I); 91 I->eraseFromParent(); 92 93 // We made a change to the function... 94 Changed = true; 95 ++NumInstKilled; 96 } 97 } 98 return Changed; 99} 100