ConstantProp.cpp revision 081c34b725980f995be9080eaec24cd3dfaaf065 
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      initializeConstantPropagationPass(*PassRegistry::getPassRegistry());
39    }
40
41    bool runOnFunction(Function &F);
42
43    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
44      AU.setPreservesCFG();
45    }
46  };
47}
48
49char ConstantPropagation::ID = 0;
50INITIALIZE_PASS(ConstantPropagation, "constprop",
51                "Simple constant propagation", false, false)
52
53FunctionPass *llvm::createConstantPropagationPass() {
54  return new ConstantPropagation();
55}
56
57
58bool ConstantPropagation::runOnFunction(Function &F) {
59  // Initialize the worklist to all of the instructions ready to process...
60  std::set<Instruction*> WorkList;
61  for(inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i) {
62      WorkList.insert(&*i);
63  }
64  bool Changed = false;
65
66  while (!WorkList.empty()) {
67    Instruction *I = *WorkList.begin();
68    WorkList.erase(WorkList.begin());    // Get an element from the worklist...
69
70    if (!I->use_empty())                 // Don't muck with dead instructions...
71      if (Constant *C = ConstantFoldInstruction(I)) {
72        // Add all of the users of this instruction to the worklist, they might
73        // be constant propagatable now...
74        for (Value::use_iterator UI = I->use_begin(), UE = I->use_end();
75             UI != UE; ++UI)
76          WorkList.insert(cast<Instruction>(*UI));
77
78        // Replace all of the uses of a variable with uses of the constant.
79        I->replaceAllUsesWith(C);
80
81        // Remove the dead instruction.
82        WorkList.erase(I);
83        I->eraseFromParent();
84
85        // We made a change to the function...
86        Changed = true;
87        ++NumInstKilled;
88      }
89  }
90  return Changed;
91}
92