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121 lines
4.1 KiB
C++
121 lines
4.1 KiB
C++
//===- ConstantProp.cpp - Code to perform Simple Constant Propagation -----===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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//
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// This file implements constant propagation and merging:
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//
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// Specifically, this:
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// * Converts instructions like "add int 1, 2" into 3
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//
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// Notice that:
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// * This pass has a habit of making definitions be dead. It is a good idea
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// to run a DIE pass sometime after running this pass.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/ADT/SmallPtrSet.h"
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#include "llvm/ADT/SmallVector.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Analysis/ConstantFolding.h"
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#include "llvm/Analysis/TargetLibraryInfo.h"
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#include "llvm/IR/Constant.h"
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#include "llvm/IR/InstIterator.h"
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#include "llvm/IR/Instruction.h"
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#include "llvm/Pass.h"
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#include "llvm/Support/DebugCounter.h"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Transforms/Utils/Local.h"
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using namespace llvm;
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#define DEBUG_TYPE "constprop"
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STATISTIC(NumInstKilled, "Number of instructions killed");
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DEBUG_COUNTER(CPCounter, "constprop-transform",
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"Controls which instructions are killed");
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namespace {
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struct ConstantPropagation : public FunctionPass {
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static char ID; // Pass identification, replacement for typeid
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ConstantPropagation() : FunctionPass(ID) {
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initializeConstantPropagationPass(*PassRegistry::getPassRegistry());
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}
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bool runOnFunction(Function &F) override;
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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AU.setPreservesCFG();
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AU.addRequired<TargetLibraryInfoWrapperPass>();
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}
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};
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}
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char ConstantPropagation::ID = 0;
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INITIALIZE_PASS_BEGIN(ConstantPropagation, "constprop",
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"Simple constant propagation", false, false)
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INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
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INITIALIZE_PASS_END(ConstantPropagation, "constprop",
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"Simple constant propagation", false, false)
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FunctionPass *llvm::createConstantPropagationPass() {
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return new ConstantPropagation();
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}
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bool ConstantPropagation::runOnFunction(Function &F) {
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if (skipFunction(F))
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return false;
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// Initialize the worklist to all of the instructions ready to process...
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SmallPtrSet<Instruction *, 16> WorkList;
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// The SmallVector of WorkList ensures that we do iteration at stable order.
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// We use two containers rather than one SetVector, since remove is
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// linear-time, and we don't care enough to remove from Vec.
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SmallVector<Instruction *, 16> WorkListVec;
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for (Instruction &I : instructions(&F)) {
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WorkList.insert(&I);
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WorkListVec.push_back(&I);
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}
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bool Changed = false;
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const DataLayout &DL = F.getParent()->getDataLayout();
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TargetLibraryInfo *TLI =
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&getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
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while (!WorkList.empty()) {
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SmallVector<Instruction*, 16> NewWorkListVec;
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for (auto *I : WorkListVec) {
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WorkList.erase(I); // Remove element from the worklist...
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if (!I->use_empty()) // Don't muck with dead instructions...
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if (Constant *C = ConstantFoldInstruction(I, DL, TLI)) {
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if (!DebugCounter::shouldExecute(CPCounter))
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continue;
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// Add all of the users of this instruction to the worklist, they might
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// be constant propagatable now...
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for (User *U : I->users()) {
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// If user not in the set, then add it to the vector.
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if (WorkList.insert(cast<Instruction>(U)).second)
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NewWorkListVec.push_back(cast<Instruction>(U));
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}
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// Replace all of the uses of a variable with uses of the constant.
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I->replaceAllUsesWith(C);
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if (isInstructionTriviallyDead(I, TLI)) {
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I->eraseFromParent();
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++NumInstKilled;
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}
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// We made a change to the function...
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Changed = true;
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}
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}
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WorkListVec = std::move(NewWorkListVec);
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}
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return Changed;
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}
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