ConstantProp.cpp revision 9ccaf53ada99c63737547c0235baeb8454b04e80 
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/Analysis/ConstantFolding.h"
24#include "llvm/Constant.h"
25#include "llvm/Instruction.h"
26#include "llvm/Pass.h"
27#include "llvm/Support/InstIterator.h"
28#include "llvm/ADT/Statistic.h"
29#include <set>
30using namespace llvm;
31
32STATISTIC(NumInstKilled, "Number of instructions killed");
33
34namespace {
35  struct ConstantPropagation : public FunctionPass {
36    static char ID; // Pass identification, replacement for typeid
37    ConstantPropagation() : FunctionPass(ID) {}
38
39    bool runOnFunction(Function &F);
40
41    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
42      AU.setPreservesCFG();
43    }
44  };
45}
46
47char ConstantPropagation::ID = 0;
48INITIALIZE_PASS(ConstantPropagation, "constprop",
49                "Simple constant propagation", false, false);
50
51FunctionPass *llvm::createConstantPropagationPass() {
52  return new ConstantPropagation();
53}
54
55
56bool ConstantPropagation::runOnFunction(Function &F) {
57  // Initialize the worklist to all of the instructions ready to process...
58  std::set<Instruction*> WorkList;
59  for(inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i) {
60      WorkList.insert(&*i);
61  }
62  bool Changed = false;
63
64  while (!WorkList.empty()) {
65    Instruction *I = *WorkList.begin();
66    WorkList.erase(WorkList.begin());    // Get an element from the worklist...
67
68    if (!I->use_empty())                 // Don't muck with dead instructions...
69      if (Constant *C = ConstantFoldInstruction(I)) {
70        // Add all of the users of this instruction to the worklist, they might
71        // be constant propagatable now...
72        for (Value::use_iterator UI = I->use_begin(), UE = I->use_end();
73             UI != UE; ++UI)
74          WorkList.insert(cast<Instruction>(*UI));
75
76        // Replace all of the uses of a variable with uses of the constant.
77        I->replaceAllUsesWith(C);
78
79        // Remove the dead instruction.
80        WorkList.erase(I);
81        I->eraseFromParent();
82
83        // We made a change to the function...
84        Changed = true;
85        ++NumInstKilled;
86      }
87  }
88  return Changed;
89}
90