lib/Transforms/Scalar/ConstantProp.cpp

   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>
  30 using namespace llvm;
  31
  32 STATISTIC(NumInstKilled, "Number of instructions killed");
  33
  34 namespace {
  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
  47 char ConstantPropagation::ID = 0;
  48 static RegisterPass<ConstantPropagation>
  49 X("constprop", "Simple constant propagation");
  50
  51 FunctionPass *llvm::createConstantPropagationPass() {
  52   return new ConstantPropagation();
  53 }
  54
  55
  56 bool 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, F.getContext())) {
  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 }