llvm-6502/lib/Transforms/Scalar/GCSE.cpp
2006-08-27 22:42:52 +00:00

200 lines
7.3 KiB
C++

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