ConstantProp.cpp revision 022103b3f33febb7e54b8fdf2c9bc461eea78cba 
1//===- ConstantProp.cpp - Code to perform Simple Constant Propogation -----===//
2//
3// This file implements constant propogation and merging:
4//
5// Specifically, this:
6//   * Converts instructions like "add int 1, 2" into 3
7//
8// Notice that:
9//   * This pass has a habit of making definitions be dead.  It is a good idea
10//     to to run a DIE pass sometime after running this pass.
11//
12//===----------------------------------------------------------------------===//
13
14#include "llvm/Transforms/Scalar.h"
15#include "llvm/Transforms/Utils/Local.h"
16#include "llvm/ConstantHandling.h"
17#include "llvm/Instruction.h"
18#include "llvm/Pass.h"
19#include "llvm/Support/InstIterator.h"
20#include <set>
21
22namespace {
23  struct ConstantPropogation : public FunctionPass {
24    const char *getPassName() const { return "Simple Constant Propogation"; }
25
26    inline bool runOnFunction(Function *F);
27
28    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
29      AU.preservesCFG();
30    }
31  };
32}
33
34Pass *createConstantPropogationPass() {
35  return new ConstantPropogation();
36}
37
38
39bool ConstantPropogation::runOnFunction(Function *F) {
40  // Initialize the worklist to all of the instructions ready to process...
41  std::set<Instruction*> WorkList(inst_begin(F), inst_end(F));
42  bool Changed = false;
43
44  while (!WorkList.empty()) {
45    Instruction *I = *WorkList.begin();
46    WorkList.erase(WorkList.begin());    // Get an element from the worklist...
47
48    if (!I->use_empty())                 // Don't muck with dead instructions...
49      if (Constant *C = ConstantFoldInstruction(I)) {
50        // Add all of the users of this instruction to the worklist, they might
51        // be constant propogatable now...
52        for (Value::use_iterator UI = I->use_begin(), UE = I->use_end();
53             UI != UE; ++UI)
54          WorkList.insert(cast<Instruction>(*UI));
55
56        // Replace all of the uses of a variable with uses of the constant.
57        I->replaceAllUsesWith(C);
58
59        // We made a change to the function...
60        Changed = true;
61      }
62  }
63  return Changed;
64}
65