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/InstIterator.h" 26 #include "llvm/IR/Instruction.h" 27 #include "llvm/Pass.h" 28 #include "llvm/Analysis/TargetLibraryInfo.h" 29 #include <set> 30 using namespace llvm; 31 32 #define DEBUG_TYPE "constprop" 33 34 STATISTIC(NumInstKilled, "Number of instructions killed"); 35 36 namespace { 37 struct ConstantPropagation : public FunctionPass { 38 static char ID; // Pass identification, replacement for typeid 39 ConstantPropagation() : FunctionPass(ID) { 40 initializeConstantPropagationPass(*PassRegistry::getPassRegistry()); 41 } 42 43 bool runOnFunction(Function &F) override; 44 45 void getAnalysisUsage(AnalysisUsage &AU) const override { 46 AU.setPreservesCFG(); 47 AU.addRequired<TargetLibraryInfoWrapperPass>(); 48 } 49 }; 50 } 51 52 char ConstantPropagation::ID = 0; 53 INITIALIZE_PASS_BEGIN(ConstantPropagation, "constprop", 54 "Simple constant propagation", false, false) 55 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass) 56 INITIALIZE_PASS_END(ConstantPropagation, "constprop", 57 "Simple constant propagation", false, false) 58 59 FunctionPass *llvm::createConstantPropagationPass() { 60 return new ConstantPropagation(); 61 } 62 63 bool ConstantPropagation::runOnFunction(Function &F) { 64 if (skipFunction(F)) 65 return false; 66 67 // Initialize the worklist to all of the instructions ready to process... 68 std::set<Instruction*> WorkList; 69 for (Instruction &I: instructions(&F)) 70 WorkList.insert(&I); 71 72 bool Changed = false; 73 const DataLayout &DL = F.getParent()->getDataLayout(); 74 TargetLibraryInfo *TLI = 75 &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(); 76 77 while (!WorkList.empty()) { 78 Instruction *I = *WorkList.begin(); 79 WorkList.erase(WorkList.begin()); // Get an element from the worklist... 80 81 if (!I->use_empty()) // Don't muck with dead instructions... 82 if (Constant *C = ConstantFoldInstruction(I, DL, TLI)) { 83 // Add all of the users of this instruction to the worklist, they might 84 // be constant propagatable now... 85 for (User *U : I->users()) 86 WorkList.insert(cast<Instruction>(U)); 87 88 // Replace all of the uses of a variable with uses of the constant. 89 I->replaceAllUsesWith(C); 90 91 // Remove the dead instruction. 92 WorkList.erase(I); 93 I->eraseFromParent(); 94 95 // We made a change to the function... 96 Changed = true; 97 ++NumInstKilled; 98 } 99 } 100 return Changed; 101 } 102
