forked from OSchip/llvm-project
275 lines
9.7 KiB
C++
275 lines
9.7 KiB
C++
//===-- GCSE.cpp - SSA based Global Common Subexpr Elimination ------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file was developed by the LLVM research group and is distributed under
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// the University of Illinois Open Source 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 implementation to identify the common
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// subexpressions, eliminating them when possible.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/iMemory.h"
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#include "llvm/Type.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/Support/InstIterator.h"
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#include "Support/Statistic.h"
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#include <algorithm>
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namespace llvm {
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namespace {
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Statistic<> NumInstRemoved("gcse", "Number of instructions removed");
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Statistic<> NumLoadRemoved("gcse", "Number of loads removed");
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Statistic<> NumNonInsts ("gcse", "Number of instructions removed due "
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"to non-instruction values");
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class GCSE : public FunctionPass {
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std::set<Instruction*> WorkList;
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DominatorSet *DomSetInfo;
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ValueNumbering *VN;
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public:
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virtual bool runOnFunction(Function &F);
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private:
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bool EliminateRedundancies(Instruction *I,std::vector<Value*> &EqualValues);
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Instruction *EliminateCSE(Instruction *I, Instruction *Other);
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void ReplaceInstWithInst(Instruction *First, BasicBlock::iterator SI);
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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<DominatorSet>();
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AU.addRequired<ImmediateDominators>();
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AU.addRequired<ValueNumbering>();
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}
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};
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RegisterOpt<GCSE> X("gcse", "Global Common Subexpression Elimination");
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}
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// createGCSEPass - The public interface to this file...
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FunctionPass *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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DomSetInfo = &getAnalysis<DominatorSet>();
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VN = &getAnalysis<ValueNumbering>();
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// Step #1: Add all instructions in the function to the worklist for
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// processing. All of the instructions are considered to be our
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// subexpressions to eliminate if possible.
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//
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WorkList.insert(inst_begin(F), inst_end(F));
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// Step #2: WorkList processing. Iterate through all of the instructions,
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// checking to see if there are any additionally defined subexpressions in the
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// program. If so, eliminate them!
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//
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while (!WorkList.empty()) {
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Instruction &I = **WorkList.begin(); // Get an instruction from the worklist
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WorkList.erase(WorkList.begin());
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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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if (I.getType() != Type::VoidTy) {
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std::vector<Value*> EqualValues;
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VN->getEqualNumberNodes(&I, EqualValues);
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if (!EqualValues.empty())
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Changed |= EliminateRedundancies(&I, EqualValues);
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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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bool GCSE::EliminateRedundancies(Instruction *I,
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std::vector<Value*> &EqualValues) {
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// If the EqualValues set contains any non-instruction values, then we know
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// that all of the instructions can be replaced with the non-instruction value
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// because it is guaranteed to dominate all of the instructions in the
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// function. We only have to do hard work if all we have are instructions.
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//
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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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// Found a non-instruction. Replace all instructions with the
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// non-instruction.
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//
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Value *Replacement = EqualValues[i];
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// Make sure we get I as well...
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EqualValues[i] = I;
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// Replace all instructions with the Replacement value.
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for (i = 0; i != e; ++i)
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if (Instruction *I = dyn_cast<Instruction>(EqualValues[i])) {
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// Change all users of I to use Replacement.
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I->replaceAllUsesWith(Replacement);
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if (isa<LoadInst>(I))
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++NumLoadRemoved; // Keep track of loads eliminated
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++NumInstRemoved; // Keep track of number of instructions eliminated
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++NumNonInsts; // Keep track of number of insts repl with values
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// Erase the instruction from the program.
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I->getParent()->getInstList().erase(I);
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WorkList.erase(I);
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}
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return true;
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}
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// Remove duplicate entries from EqualValues...
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std::sort(EqualValues.begin(), EqualValues.end());
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EqualValues.erase(std::unique(EqualValues.begin(), EqualValues.end()),
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EqualValues.end());
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// From this point on, EqualValues is logically a vector of instructions.
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//
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bool Changed = false;
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EqualValues.push_back(I); // Make sure I is included...
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while (EqualValues.size() > 1) {
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// FIXME, this could be done better than simple iteration!
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Instruction *Test = cast<Instruction>(EqualValues.back());
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EqualValues.pop_back();
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for (unsigned i = 0, e = EqualValues.size(); i != e; ++i)
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if (Instruction *Ret = EliminateCSE(Test,
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cast<Instruction>(EqualValues[i]))) {
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if (Ret == Test) // Eliminated EqualValues[i]
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EqualValues[i] = Test; // Make sure that we reprocess I at some point
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Changed = true;
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break;
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}
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}
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return Changed;
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}
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// ReplaceInstWithInst - Destroy the instruction pointed to by SI, making all
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// uses of the instruction use First now instead.
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//
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void GCSE::ReplaceInstWithInst(Instruction *First, BasicBlock::iterator SI) {
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Instruction &Second = *SI;
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//cerr << "DEL " << (void*)Second << Second;
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// Add the first instruction back to the worklist
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WorkList.insert(First);
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// Add all uses of the second instruction to the worklist
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for (Value::use_iterator UI = Second.use_begin(), UE = Second.use_end();
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UI != UE; ++UI)
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WorkList.insert(cast<Instruction>(*UI));
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// Make all users of 'Second' now use 'First'
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Second.replaceAllUsesWith(First);
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// Erase the second instruction from the program
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Second.getParent()->getInstList().erase(SI);
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}
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// EliminateCSE - The two instruction I & Other have been found to be common
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// subexpressions. This function is responsible for eliminating one of them,
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// and for fixing the worklist to be correct. The instruction that is preserved
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// is returned from the function if the other is eliminated, otherwise null is
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// returned.
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//
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Instruction *GCSE::EliminateCSE(Instruction *I, Instruction *Other) {
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assert(I != Other);
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WorkList.erase(I);
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WorkList.erase(Other); // Other may not actually be on the worklist anymore...
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// Handle the easy case, where both instructions are in the same basic block
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BasicBlock *BB1 = I->getParent(), *BB2 = Other->getParent();
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Instruction *Ret = 0;
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if (BB1 == BB2) {
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// Eliminate the second occurring instruction. Add all uses of the second
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// instruction to the worklist.
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//
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// Scan the basic block looking for the "first" instruction
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BasicBlock::iterator BI = BB1->begin();
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while (&*BI != I && &*BI != Other) {
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++BI;
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assert(BI != BB1->end() && "Instructions not found in parent BB!");
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}
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// Keep track of which instructions occurred first & second
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Instruction *First = BI;
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Instruction *Second = I != First ? I : Other; // Get iterator to second inst
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BI = Second;
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// Destroy Second, using First instead.
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ReplaceInstWithInst(First, BI);
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Ret = First;
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// Otherwise, the two instructions are in different basic blocks. If one
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// dominates the other instruction, we can simply use it
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//
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} else if (DomSetInfo->dominates(BB1, BB2)) { // I dom Other?
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ReplaceInstWithInst(I, Other);
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Ret = I;
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} else if (DomSetInfo->dominates(BB2, BB1)) { // Other dom I?
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ReplaceInstWithInst(Other, I);
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Ret = Other;
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} else {
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// This code is disabled because it has several problems:
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// One, the actual assumption is wrong, as shown by this code:
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// int "test"(int %X, int %Y) {
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// %Z = add int %X, %Y
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// ret int %Z
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// Unreachable:
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// %Q = add int %X, %Y
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// ret int %Q
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// }
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//
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// Here there are no shared dominators. Additionally, this had the habit of
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// moving computations where they were not always computed. For example, in
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// a case like this:
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// if (c) {
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// if (d) ...
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// else ... X+Y ...
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// } else {
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// ... X+Y ...
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// }
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//
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// In this case, the expression would be hoisted to outside the 'if' stmt,
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// causing the expression to be evaluated, even for the if (d) path, which
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// could cause problems, if, for example, it caused a divide by zero. In
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// general the problem this case is trying to solve is better addressed with
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// PRE than GCSE.
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//
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return 0;
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}
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if (isa<LoadInst>(Ret))
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++NumLoadRemoved; // Keep track of loads eliminated
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++NumInstRemoved; // Keep track of number of instructions eliminated
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// Add all users of Ret to the worklist...
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for (Value::use_iterator I = Ret->use_begin(), E = Ret->use_end(); I != E;++I)
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if (Instruction *Inst = dyn_cast<Instruction>(*I))
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WorkList.insert(Inst);
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return Ret;
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}
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} // End llvm namespace
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