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> 31 using namespace llvm; 32 33 #define DEBUG_TYPE "constprop" 34 35 STATISTIC(NumInstKilled, "Number of instructions killed"); 36 37 namespace { 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 53 char ConstantPropagation::ID = 0; 54 INITIALIZE_PASS_BEGIN(ConstantPropagation, "constprop", 55 "Simple constant propagation", false, false) 56 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo) 57 INITIALIZE_PASS_END(ConstantPropagation, "constprop", 58 "Simple constant propagation", false, false) 59 60 FunctionPass *llvm::createConstantPropagationPass() { 61 return new ConstantPropagation(); 62 } 63 64 bool 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
