lib/Transforms/Scalar/ConstantProp.cpp

   1 //===- ConstantProp.cpp - Code to perform Simple Constant Propagation -----===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file implements constant propagation and merging:
  10 //
  11 // Specifically, this:
  12 //   * Converts instructions like "add int 1, 2" into 3
  13 //
  14 // Notice that:
  15 //   * This pass has a habit of making definitions be dead.  It is a good idea
  16 //     to run a DIE pass sometime after running this pass.
  17 //
  18 //===----------------------------------------------------------------------===//
  19
  20 #include "llvm/ADT/SmallPtrSet.h"
  21 #include "llvm/ADT/SmallVector.h"
  22 #include "llvm/ADT/Statistic.h"
  23 #include "llvm/Analysis/ConstantFolding.h"
  24 #include "llvm/Analysis/TargetLibraryInfo.h"
  25 #include "llvm/IR/Constant.h"
  26 #include "llvm/IR/InstIterator.h"
  27 #include "llvm/IR/Instruction.h"
  28 #include "llvm/Pass.h"
  29 #include "llvm/Support/DebugCounter.h"
  30 #include "llvm/Transforms/Scalar.h"
  31 #include "llvm/Transforms/Utils/Local.h"
  32 using namespace llvm;
  33
  34 #define DEBUG_TYPE "constprop"
  35
  36 STATISTIC(NumInstKilled, "Number of instructions killed");
  37 DEBUG_COUNTER(CPCounter, "constprop-transform",
  38               "Controls which instructions are killed");
  39
  40 namespace {
  41   struct ConstantPropagation : public FunctionPass {
  42     static char ID; // Pass identification, replacement for typeid
  43     ConstantPropagation() : FunctionPass(ID) {
  44       initializeConstantPropagationPass(*PassRegistry::getPassRegistry());
  45     }
  46
  47     bool runOnFunction(Function &F) override;
  48
  49     void getAnalysisUsage(AnalysisUsage &AU) const override {
  50       AU.setPreservesCFG();
  51       AU.addRequired<TargetLibraryInfoWrapperPass>();
  52     }
  53   };
  54 }
  55
  56 char ConstantPropagation::ID = 0;
  57 INITIALIZE_PASS_BEGIN(ConstantPropagation, "constprop",
  58                 "Simple constant propagation", false, false)
  59 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
  60 INITIALIZE_PASS_END(ConstantPropagation, "constprop",
  61                 "Simple constant propagation", false, false)
  62
  63 FunctionPass *llvm::createConstantPropagationPass() {
  64   return new ConstantPropagation();
  65 }
  66
  67 bool ConstantPropagation::runOnFunction(Function &F) {
  68   if (skipFunction(F))
  69     return false;
  70
  71   // Initialize the worklist to all of the instructions ready to process...
  72   SmallPtrSet<Instruction *, 16> WorkList;
  73   // The SmallVector of WorkList ensures that we do iteration at stable order.
  74   // We use two containers rather than one SetVector, since remove is
  75   // linear-time, and we don't care enough to remove from Vec.
  76   SmallVector<Instruction *, 16> WorkListVec;
  77   for (Instruction &I : instructions(&F)) {
  78     WorkList.insert(&I);
  79     WorkListVec.push_back(&I);
  80   }
  81
  82   bool Changed = false;
  83   const DataLayout &DL = F.getParent()->getDataLayout();
  84   TargetLibraryInfo *TLI =
  85       &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
  86
  87   while (!WorkList.empty()) {
  88     SmallVector<Instruction*, 16> NewWorkListVec;
  89     for (auto *I : WorkListVec) {
  90       WorkList.erase(I); // Remove element from the worklist...
  91
  92       if (!I->use_empty()) // Don't muck with dead instructions...
  93         if (Constant *C = ConstantFoldInstruction(I, DL, TLI)) {
  94           if (!DebugCounter::shouldExecute(CPCounter))
  95             continue;
  96
  97           // Add all of the users of this instruction to the worklist, they might
  98           // be constant propagatable now...
  99           for (User *U : I->users()) {
 100             // If user not in the set, then add it to the vector.
 101             if (WorkList.insert(cast<Instruction>(U)).second)
 102               NewWorkListVec.push_back(cast<Instruction>(U));
 103           }
 104
 105           // Replace all of the uses of a variable with uses of the constant.
 106           I->replaceAllUsesWith(C);
 107
 108           if (isInstructionTriviallyDead(I, TLI)) {
 109             I->eraseFromParent();
 110             ++NumInstKilled;
 111           }
 112
 113           // We made a change to the function...
 114           Changed = true;
 115         }
 116     }
 117     WorkListVec = std::move(NewWorkListVec);
 118   }
 119   return Changed;
 120 }