1 //===- ConstantProp.cpp - Code to perform Simple Constant Propagation -----===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements constant propagation and merging:
12 // Specifically, this:
13 // * Converts instructions like "add int 1, 2" into 3
16 // * This pass has a habit of making definitions be dead. It is a good idea
17 // to to run a DIE pass sometime after running this pass.
19 //===----------------------------------------------------------------------===//
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"
33 STATISTIC(NumInstKilled, "Number of instructions killed");
36 struct VISIBILITY_HIDDEN ConstantPropagation : public FunctionPass {
37 bool runOnFunction(Function &F);
39 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
44 RegisterPass<ConstantPropagation> X("constprop",
45 "Simple constant propagation");
48 FunctionPass *llvm::createConstantPropagationPass() {
49 return new ConstantPropagation();
53 bool ConstantPropagation::runOnFunction(Function &F) {
54 // Initialize the worklist to all of the instructions ready to process...
55 std::set<Instruction*> WorkList;
56 for(inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i) {
61 while (!WorkList.empty()) {
62 Instruction *I = *WorkList.begin();
63 WorkList.erase(WorkList.begin()); // Get an element from the worklist...
65 if (!I->use_empty()) // Don't muck with dead instructions...
66 if (Constant *C = ConstantFoldInstruction(I)) {
67 // Add all of the users of this instruction to the worklist, they might
68 // be constant propagatable now...
69 for (Value::use_iterator UI = I->use_begin(), UE = I->use_end();
71 WorkList.insert(cast<Instruction>(*UI));
73 // Replace all of the uses of a variable with uses of the constant.
74 I->replaceAllUsesWith(C);
76 // Remove the dead instruction.
78 I->getParent()->getInstList().erase(I);
80 // We made a change to the function...