lib/Transforms/Scalar/ConstantProp.cpp

   1 //===- ConstantProp.cpp - Code to perform Simple Constant Propagation -----===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   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.
   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 to run a DIE pass sometime after running this pass.
  18 //
  19 //===----------------------------------------------------------------------===//
  20
  21 #include "llvm/Transforms/Scalar.h"
  22 #include "llvm/Transforms/Utils/Local.h"
  23 #include "llvm/Constant.h"
  24 #include "llvm/Instruction.h"
  25 #include "llvm/Pass.h"
  26 #include "llvm/Support/InstIterator.h"
  27 #include "llvm/ADT/Statistic.h"
  28 #include <set>
  29 using namespace llvm;
  30
  31 namespace {
  32   Statistic<> NumInstKilled("constprop", "Number of instructions killed");
  33
  34   struct ConstantPropagation : public FunctionPass {
  35     bool runOnFunction(Function &F);
  36
  37     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
  38       AU.setPreservesCFG();
  39     }
  40   };
  41
  42   RegisterOpt<ConstantPropagation> X("constprop","Simple constant propagation");
  43 }
  44
  45 FunctionPass *llvm::createConstantPropagationPass() {
  46   return new ConstantPropagation();
  47 }
  48
  49
  50 bool ConstantPropagation::runOnFunction(Function &F) {
  51   // Initialize the worklist to all of the instructions ready to process...
  52   std::set<Instruction*> WorkList;
  53   for(inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i) {
  54       WorkList.insert(&*i);
  55   }
  56   bool Changed = false;
  57
  58   while (!WorkList.empty()) {
  59     Instruction *I = *WorkList.begin();
  60     WorkList.erase(WorkList.begin());    // Get an element from the worklist...
  61
  62     if (!I->use_empty())                 // Don't muck with dead instructions...
  63       if (Constant *C = ConstantFoldInstruction(I)) {
  64         // Add all of the users of this instruction to the worklist, they might
  65         // be constant propagatable now...
  66         for (Value::use_iterator UI = I->use_begin(), UE = I->use_end();
  67              UI != UE; ++UI)
  68           WorkList.insert(cast<Instruction>(*UI));
  69
  70         // Replace all of the uses of a variable with uses of the constant.
  71         I->replaceAllUsesWith(C);
  72
  73         // Remove the dead instruction.
  74         WorkList.erase(I);
  75         I->getParent()->getInstList().erase(I);
  76
  77         // We made a change to the function...
  78         Changed = true;
  79         ++NumInstKilled;
  80       }
  81   }
  82   return Changed;
  83 }