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/ADT/Statistic.h" 24 #include "llvm/Analysis/ConstantFolding.h" 25 #include "llvm/IR/Constant.h" 26 #include "llvm/IR/DataLayout.h" 27 #include "llvm/IR/Instruction.h" 28 #include "llvm/Pass.h" 29 #include "llvm/Support/InstIterator.h" 30 #include "llvm/Target/TargetLibraryInfo.h" 31 #include <set> 32 using namespace llvm; 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); 44 45 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 46 AU.setPreservesCFG(); 47 AU.addRequired<TargetLibraryInfo>(); 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(TargetLibraryInfo) 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 // Initialize the worklist to all of the instructions ready to process... 65 std::set<Instruction*> WorkList; 66 for(inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i) { 67 WorkList.insert(&*i); 68 } 69 bool Changed = false; 70 DataLayout *TD = getAnalysisIfAvailable<DataLayout>(); 71 TargetLibraryInfo *TLI = &getAnalysis<TargetLibraryInfo>(); 72 73 while (!WorkList.empty()) { 74 Instruction *I = *WorkList.begin(); 75 WorkList.erase(WorkList.begin()); // Get an element from the worklist... 76 77 if (!I->use_empty()) // Don't muck with dead instructions... 78 if (Constant *C = ConstantFoldInstruction(I, TD, TLI)) { 79 // Add all of the users of this instruction to the worklist, they might 80 // be constant propagatable now... 81 for (Value::use_iterator UI = I->use_begin(), UE = I->use_end(); 82 UI != UE; ++UI) 83 WorkList.insert(cast<Instruction>(*UI)); 84 85 // Replace all of the uses of a variable with uses of the constant. 86 I->replaceAllUsesWith(C); 87 88 // Remove the dead instruction. 89 WorkList.erase(I); 90 I->eraseFromParent(); 91 92 // We made a change to the function... 93 Changed = true; 94 ++NumInstKilled; 95 } 96 } 97 return Changed; 98 } 99
