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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@52606 91177308-0d34-0410-b5e6-96231b3b80d8
206 lines
7.4 KiB
C++
206 lines
7.4 KiB
C++
//===-- GCSE.cpp - SSA-based Global Common Subexpression Elimination ------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This pass is designed to be a very quick global transformation that
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// eliminates global common subexpressions from a function. It does this by
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// using an existing value numbering analysis pass to identify the common
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// subexpressions, eliminating them when possible.
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//
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// This pass is deprecated by the Global Value Numbering pass (which does a
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// better job with its own value numbering).
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//
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "gcse"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Instructions.h"
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#include "llvm/Function.h"
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#include "llvm/Type.h"
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#include "llvm/Analysis/ConstantFolding.h"
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#include "llvm/Analysis/Dominators.h"
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#include "llvm/Analysis/ValueNumbering.h"
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#include "llvm/ADT/DepthFirstIterator.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Support/Compiler.h"
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#include <algorithm>
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using namespace llvm;
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STATISTIC(NumInstRemoved, "Number of instructions removed");
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STATISTIC(NumLoadRemoved, "Number of loads removed");
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STATISTIC(NumCallRemoved, "Number of calls removed");
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STATISTIC(NumNonInsts , "Number of instructions removed due "
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"to non-instruction values");
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STATISTIC(NumArgsRepl , "Number of function arguments replaced "
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"with constant values");
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namespace {
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struct VISIBILITY_HIDDEN GCSE : public FunctionPass {
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static char ID; // Pass identification, replacement for typeid
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GCSE() : FunctionPass((intptr_t)&ID) {}
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virtual bool runOnFunction(Function &F);
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private:
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void ReplaceInstructionWith(Instruction *I, Value *V);
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// This transformation requires dominator and immediate dominator info
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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AU.setPreservesCFG();
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AU.addRequired<DominatorTree>();
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AU.addRequired<ValueNumbering>();
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}
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};
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}
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char GCSE::ID = 0;
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static RegisterPass<GCSE>
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X("gcse", "Global Common Subexpression Elimination");
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// createGCSEPass - The public interface to this file...
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FunctionPass *llvm::createGCSEPass() { return new GCSE(); }
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// GCSE::runOnFunction - This is the main transformation entry point for a
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// function.
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//
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bool GCSE::runOnFunction(Function &F) {
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bool Changed = false;
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// Get pointers to the analysis results that we will be using...
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DominatorTree &DT = getAnalysis<DominatorTree>();
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ValueNumbering &VN = getAnalysis<ValueNumbering>();
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std::vector<Value*> EqualValues;
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// Check for value numbers of arguments. If the value numbering
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// implementation can prove that an incoming argument is a constant or global
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// value address, substitute it, making the argument dead.
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for (Function::arg_iterator AI = F.arg_begin(), E = F.arg_end(); AI != E;++AI)
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if (!AI->use_empty()) {
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VN.getEqualNumberNodes(AI, EqualValues);
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if (!EqualValues.empty()) {
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for (unsigned i = 0, e = EqualValues.size(); i != e; ++i)
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if (isa<Constant>(EqualValues[i])) {
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AI->replaceAllUsesWith(EqualValues[i]);
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++NumArgsRepl;
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Changed = true;
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break;
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}
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EqualValues.clear();
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}
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}
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// Traverse the CFG of the function in dominator order, so that we see each
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// instruction after we see its operands.
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for (df_iterator<DomTreeNode*> DI = df_begin(DT.getRootNode()),
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E = df_end(DT.getRootNode()); DI != E; ++DI) {
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BasicBlock *BB = DI->getBlock();
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// Remember which instructions we've seen in this basic block as we scan.
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std::set<Instruction*> BlockInsts;
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for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) {
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Instruction *Inst = I++;
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if (Constant *C = ConstantFoldInstruction(Inst)) {
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ReplaceInstructionWith(Inst, C);
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} else if (Inst->getType() != Type::VoidTy) {
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// If this instruction computes a value, try to fold together common
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// instructions that compute it.
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//
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VN.getEqualNumberNodes(Inst, EqualValues);
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// If this instruction computes a value that is already computed
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// elsewhere, try to recycle the old value.
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if (!EqualValues.empty()) {
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if (Inst == &*BB->begin())
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I = BB->end();
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else {
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I = Inst; --I;
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}
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// First check to see if we were able to value number this instruction
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// to a non-instruction value. If so, prefer that value over other
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// instructions which may compute the same thing.
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for (unsigned i = 0, e = EqualValues.size(); i != e; ++i)
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if (!isa<Instruction>(EqualValues[i])) {
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++NumNonInsts; // Keep track of # of insts repl with values
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// Change all users of Inst to use the replacement and remove it
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// from the program.
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ReplaceInstructionWith(Inst, EqualValues[i]);
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Inst = 0;
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EqualValues.clear(); // don't enter the next loop
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break;
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}
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// If there were no non-instruction values that this instruction
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// produces, find a dominating instruction that produces the same
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// value. If we find one, use it's value instead of ours.
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for (unsigned i = 0, e = EqualValues.size(); i != e; ++i) {
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Instruction *OtherI = cast<Instruction>(EqualValues[i]);
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bool Dominates = false;
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if (OtherI->getParent() == BB)
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Dominates = BlockInsts.count(OtherI);
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else
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Dominates = DT.dominates(OtherI->getParent(), BB);
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if (Dominates) {
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// Okay, we found an instruction with the same value as this one
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// and that dominates this one. Replace this instruction with the
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// specified one.
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ReplaceInstructionWith(Inst, OtherI);
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Inst = 0;
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break;
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}
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}
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EqualValues.clear();
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if (Inst) {
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I = Inst; ++I; // Deleted no instructions
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} else if (I == BB->end()) { // Deleted first instruction
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I = BB->begin();
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} else { // Deleted inst in middle of block.
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++I;
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}
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}
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if (Inst)
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BlockInsts.insert(Inst);
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}
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}
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}
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// When the worklist is empty, return whether or not we changed anything...
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return Changed;
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}
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void GCSE::ReplaceInstructionWith(Instruction *I, Value *V) {
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if (isa<LoadInst>(I))
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++NumLoadRemoved; // Keep track of loads eliminated
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if (isa<CallInst>(I))
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++NumCallRemoved; // Keep track of calls eliminated
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++NumInstRemoved; // Keep track of number of insts eliminated
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// Update value numbering
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getAnalysis<ValueNumbering>().deleteValue(I);
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I->replaceAllUsesWith(V);
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if (InvokeInst *II = dyn_cast<InvokeInst>(I)) {
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// Removing an invoke instruction requires adding a branch to the normal
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// destination and removing PHI node entries in the exception destination.
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BranchInst::Create(II->getNormalDest(), II);
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II->getUnwindDest()->removePredecessor(II->getParent());
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}
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// Erase the instruction from the program.
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I->eraseFromParent();
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}
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