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Rewrite the GCSE pass to be *substantially* simpler, a bit more efficient,

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and a bit more powerful


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@12817 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2004-04-10 21:11:11 +00:00
parent 326c0f3013
commit 38e66bd9d1
1 changed files with 122 additions and 215 deletions
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337
lib/Transforms/Scalar/GCSE.cpp
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@ -15,14 +15,15 @@
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar.h"
#include "llvm/iMemory.h"
#include "llvm/iOther.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/Support/InstIterator.h"
#include "llvm/Transforms/Utils/Local.h"
#include "Support/DepthFirstIterator.h"
#include "Support/Statistic.h"
#include "Support/Debug.h"
#include <algorithm>
using namespace llvm;
@ -33,23 +34,17 @@ namespace {
Statistic<> NumNonInsts ("gcse", "Number of instructions removed due "
"to non-instruction values");
class GCSE : public FunctionPass {
std::set<Instruction*> WorkList;
DominatorSet *DomSetInfo;
ValueNumbering *VN;
public:
struct GCSE : public FunctionPass {
virtual bool runOnFunction(Function &F);
private:
bool EliminateRedundancies(Instruction *I,std::vector<Value*> &EqualValues);
Instruction *EliminateCSE(Instruction *I, Instruction *Other);
void ReplaceInstWithInst(Instruction *First, BasicBlock::iterator SI);
void ReplaceInstructionWith(Instruction *I, Value *V);
// This transformation requires dominator and immediate dominator info
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesCFG();
AU.addRequired<DominatorSet>();
AU.addRequired<ImmediateDominators>();
AU.addRequired<DominatorTree>();
AU.addRequired<ValueNumbering>();
}
};
@ -67,32 +62,89 @@ bool GCSE::runOnFunction(Function &F) {
bool Changed = false;
// Get pointers to the analysis results that we will be using...
DomSetInfo = &getAnalysis<DominatorSet>();
VN = &getAnalysis<ValueNumbering>();
DominatorSet &DS = getAnalysis<DominatorSet>();
ValueNumbering &VN = getAnalysis<ValueNumbering>();
DominatorTree &DT = getAnalysis<DominatorTree>();
// Step #1: Add all instructions in the function to the worklist for
// processing. All of the instructions are considered to be our
// subexpressions to eliminate if possible.
//
WorkList.insert(inst_begin(F), inst_end(F));
std::vector<Value*> EqualValues;
// Step #2: WorkList processing. Iterate through all of the instructions,
// checking to see if there are any additionally defined subexpressions in the
// program. If so, eliminate them!
//
while (!WorkList.empty()) {
Instruction &I = **WorkList.begin(); // Get an instruction from the worklist
WorkList.erase(WorkList.begin());
// 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();
// If this instruction computes a value, try to fold together common
// instructions that compute it.
//
if (I.getType() != Type::VoidTy) {
std::vector<Value*> EqualValues;
VN->getEqualNumberNodes(&I, EqualValues);
// Remember which instructions we've seen in this basic block as we scan.
std::set<Instruction*> BlockInsts;
if (!EqualValues.empty())
Changed |= EliminateRedundancies(&I, EqualValues);
for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) {
Instruction *Inst = I++;
// If this instruction computes a value, try to fold together common
// instructions that compute it.
//
if (Inst->getType() != Type::VoidTy) {
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 = DS.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);
}
}
}
@ -100,193 +152,48 @@ bool GCSE::runOnFunction(Function &F) {
return Changed;
}
bool GCSE::EliminateRedundancies(Instruction *I,
std::vector<Value*> &EqualValues) {
// If the EqualValues set contains any non-instruction values, then we know
// that all of the instructions can be replaced with the non-instruction value
// because it is guaranteed to dominate all of the instructions in the
// function. We only have to do hard work if all we have are instructions.
//
for (unsigned i = 0, e = EqualValues.size(); i != e; ++i)
if (!isa<Instruction>(EqualValues[i])) {
// Found a non-instruction. Replace all instructions with the
// non-instruction.
//
Value *Replacement = EqualValues[i];
// Make sure we get I as well...
EqualValues[i] = I;
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
// Replace all instructions with the Replacement value.
for (i = 0; i != e; ++i)
if (Instruction *I = dyn_cast<Instruction>(EqualValues[i])) {
// Change all users of I to use Replacement.
I->replaceAllUsesWith(Replacement);
// If we are not replacing the instruction with a constant, we cannot do
// anything special.
if (!isa<Constant>(V)) {
I->replaceAllUsesWith(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 instructions eliminated
++NumNonInsts; // Keep track of number of insts repl with values
// Erase the instruction from the program.
I->getParent()->getInstList().erase(I);
return;
}
// Erase the instruction from the program.
I->getParent()->getInstList().erase(I);
WorkList.erase(I);
}
return true;
}
Constant *C = cast<Constant>(V);
std::vector<User*> Users(I->use_begin(), I->use_end());
// Perform the replacement.
I->replaceAllUsesWith(C);
// Erase the instruction from the program.
I->getParent()->getInstList().erase(I);
// Remove duplicate entries from EqualValues...
std::sort(EqualValues.begin(), EqualValues.end());
EqualValues.erase(std::unique(EqualValues.begin(), EqualValues.end()),
EqualValues.end());
// Check each user to see if we can constant fold it.
while (!Users.empty()) {
Instruction *U = cast<Instruction>(Users.back());
Users.pop_back();
// From this point on, EqualValues is logically a vector of instructions.
//
bool Changed = false;
EqualValues.push_back(I); // Make sure I is included...
while (EqualValues.size() > 1) {
// FIXME, this could be done better than simple iteration!
Instruction *Test = cast<Instruction>(EqualValues.back());
EqualValues.pop_back();
for (unsigned i = 0, e = EqualValues.size(); i != e; ++i)
if (Instruction *Ret = EliminateCSE(Test,
cast<Instruction>(EqualValues[i]))) {
if (Ret == Test) // Eliminated EqualValues[i]
EqualValues[i] = Test; // Make sure that we reprocess I at some point
Changed = true;
break;
if (Constant *C = ConstantFoldInstruction(U)) {
ReplaceInstructionWith(U, C);
// If the instruction used I more than once, it could be on the user list
// multiple times. Make sure we don't reprocess it.
std::vector<User*>::iterator It = std::find(Users.begin(), Users.end(),U);
while (It != Users.end()) {
Users.erase(It);
It = std::find(Users.begin(), Users.end(), U);
}
}
return Changed;
}
// ReplaceInstWithInst - Destroy the instruction pointed to by SI, making all
// uses of the instruction use First now instead.
//
void GCSE::ReplaceInstWithInst(Instruction *First, BasicBlock::iterator SI) {
Instruction &Second = *SI;
DEBUG(std::cerr << "GCSE: Substituting %" << First->getName() << " for: "
<< Second);
//cerr << "DEL " << (void*)Second << Second;
// Add the first instruction back to the worklist
WorkList.insert(First);
// Add all uses of the second instruction to the worklist
for (Value::use_iterator UI = Second.use_begin(), UE = Second.use_end();
UI != UE; ++UI)
WorkList.insert(cast<Instruction>(*UI));
// Make all users of 'Second' now use 'First'
Second.replaceAllUsesWith(First);
// Erase the second instruction from the program
Second.getParent()->getInstList().erase(SI);
}
// EliminateCSE - The two instruction I & Other have been found to be common
// subexpressions. This function is responsible for eliminating one of them,
// and for fixing the worklist to be correct. The instruction that is preserved
// is returned from the function if the other is eliminated, otherwise null is
// returned.
//
Instruction *GCSE::EliminateCSE(Instruction *I, Instruction *Other) {
assert(I != Other);
WorkList.erase(I);
WorkList.erase(Other); // Other may not actually be on the worklist anymore...
// Handle the easy case, where both instructions are in the same basic block
BasicBlock *BB1 = I->getParent(), *BB2 = Other->getParent();
Instruction *Ret = 0;
if (BB1 == BB2) {
// Eliminate the second occurring instruction. Add all uses of the second
// instruction to the worklist.
//
// Scan the basic block looking for the "first" instruction
BasicBlock::iterator BI = BB1->begin();
while (&*BI != I && &*BI != Other) {
++BI;
assert(BI != BB1->end() && "Instructions not found in parent BB!");
}
// Keep track of which instructions occurred first & second
Instruction *First = BI;
Instruction *Second = I != First ? I : Other; // Get iterator to second inst
BI = Second;
if (isa<LoadInst>(Second))
++NumLoadRemoved; // Keep track of loads eliminated
if (isa<CallInst>(Second))
++NumCallRemoved; // Keep track of calls eliminated
// Destroy Second, using First instead.
ReplaceInstWithInst(First, BI);
Ret = First;
// Otherwise, the two instructions are in different basic blocks. If one
// dominates the other instruction, we can simply use it
//
} else if (DomSetInfo->dominates(BB1, BB2)) { // I dom Other?
if (isa<LoadInst>(Other))
++NumLoadRemoved; // Keep track of loads eliminated
if (isa<CallInst>(Other))
++NumCallRemoved; // Keep track of calls eliminated
ReplaceInstWithInst(I, Other);
Ret = I;
} else if (DomSetInfo->dominates(BB2, BB1)) { // Other dom I?
if (isa<LoadInst>(I))
++NumLoadRemoved; // Keep track of loads eliminated
if (isa<CallInst>(I))
++NumCallRemoved; // Keep track of calls eliminated
ReplaceInstWithInst(Other, I);
Ret = Other;
} else {
// This code is disabled because it has several problems:
// One, the actual assumption is wrong, as shown by this code:
// int "test"(int %X, int %Y) {
// %Z = add int %X, %Y
// ret int %Z
// Unreachable:
// %Q = add int %X, %Y
// ret int %Q
// }
//
// Here there are no shared dominators. Additionally, this had the habit of
// moving computations where they were not always computed. For example, in
// a case like this:
// if (c) {
// if (d) ...
// else ... X+Y ...
// } else {
// ... X+Y ...
// }
//
// In this case, the expression would be hoisted to outside the 'if' stmt,
// causing the expression to be evaluated, even for the if (d) path, which
// could cause problems, if, for example, it caused a divide by zero. In
// general the problem this case is trying to solve is better addressed with
// PRE than GCSE.
//
return 0;
}
++NumInstRemoved; // Keep track of number of instructions eliminated
// Add all users of Ret to the worklist...
for (Value::use_iterator I = Ret->use_begin(), E = Ret->use_end(); I != E;++I)
if (Instruction *Inst = dyn_cast<Instruction>(*I))
WorkList.insert(Inst);
return Ret;
}