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This patch implements two things (sorry).

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First, it allows SRA of globals that have embedded arrays, implementing
GlobalOpt/globalsra-partial.llx.  This comes up infrequently, but does allow,
for example, deleting several stores to dead parts of globals in dhrystone.

Second, this implements GlobalOpt/malloc-promote-*.llx, which is the
following nifty transformation:

Basically if a global pointer is initialized with malloc, and we can tell
that the program won't notice, we transform this:

struct foo *FooPtr;
...
  FooPtr = malloc(sizeof(struct foo));
...
  FooPtr->A   FooPtr->B

Into:

struct foo FooPtrBody;
...
   FooPtrBody.A  FooPtrBody.B

This comes up occasionally, for example, the 'disp' global in 183.equake (where
the xform speeds the CBE version of the program up from 56.16s to 52.40s (7%)
on apoc), and the 'desired_accept', 'fixLRBT', 'macroArray', & 'key_queue'
globals in 300.twolf (speeding it up from 22.29s to 21.55s (3.4%)).

The nice thing about this xform is that it exposes the resulting global to
global variable optimization and makes alias analysis easier in addition to
eliminating a few loads.

llvm-svn: 16916
This commit is contained in:
Chris Lattner 2004-10-11 05:54:41 +00:00
parent a306f07a3b
commit 9d662982e0
1 changed files with 182 additions and 25 deletions
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207
lib/Transforms/IPO/GlobalOpt.cpp
View File

@ -21,6 +21,8 @@
#include "llvm/Module.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/StringExtras.h"
#include <set>
@ -36,7 +38,14 @@ namespace {
Statistic<> NumGlobUses ("globalopt", "Number of global uses devirtualized");
struct GlobalOpt : public ModulePass {
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequired<TargetData>();
}
bool runOnModule(Module &M);
private:
bool ProcessInternalGlobal(GlobalVariable *GV, Module::giterator &GVI);
};
RegisterOpt<GlobalOpt> X("globalopt", "Global Variable Optimizer");
@ -161,10 +170,22 @@ static bool AnalyzeGlobal(Value *V, GlobalStatus &GS,
}
} else if (I->getOpcode() == Instruction::GetElementPtr) {
if (AnalyzeGlobal(I, GS, PHIUsers)) return true;
// Theoretically we could SRA globals with GEP insts if all indexes are
// constants. In practice, these GEPs would already be constant exprs
// if that was the case though.
GS.isNotSuitableForSRA = true;
// If the first two indices are constants, this can be SRA'd.
if (isa<GlobalVariable>(I->getOperand(0))) {
if (I->getNumOperands() < 3 || !isa<Constant>(I->getOperand(1)) ||
!cast<Constant>(I->getOperand(1))->isNullValue() ||
!isa<ConstantInt>(I->getOperand(2)))
GS.isNotSuitableForSRA = true;
} else if (ConstantExpr *CE = dyn_cast<ConstantExpr>(I->getOperand(0))){
if (CE->getOpcode() != Instruction::GetElementPtr ||
CE->getNumOperands() < 3 || I->getNumOperands() < 2 ||
!isa<Constant>(I->getOperand(0)) ||
!cast<Constant>(I->getOperand(0))->isNullValue())
GS.isNotSuitableForSRA = true;
} else {
GS.isNotSuitableForSRA = true;
}
} else if (I->getOpcode() == Instruction::Select) {
if (AnalyzeGlobal(I, GS, PHIUsers)) return true;
GS.isNotSuitableForSRA = true;
@ -323,7 +344,7 @@ static GlobalVariable *SRAGlobal(GlobalVariable *GV) {
else
assert(0 && "Unknown aggregate sequential type!");
if (NumElements > 16) return 0; // It's not worth it.
if (NumElements > 16 && GV->use_size() > 16) return 0; // It's not worth it.
NewGlobals.reserve(NumElements);
for (unsigned i = 0, e = NumElements; i != e; ++i) {
Constant *In = getAggregateConstantElement(Init,
@ -341,38 +362,66 @@ static GlobalVariable *SRAGlobal(GlobalVariable *GV) {
if (NewGlobals.empty())
return 0;
DEBUG(std::cerr << "PERFORMING GLOBAL SRA ON: " << *GV);
Constant *NullInt = Constant::getNullValue(Type::IntTy);
// Loop over all of the uses of the global, replacing the constantexpr geps,
// with smaller constantexpr geps or direct references.
while (!GV->use_empty()) {
ConstantExpr *CE = cast<ConstantExpr>(GV->use_back());
assert(CE->getOpcode() == Instruction::GetElementPtr &&
"NonGEP CE's are not SRAable!");
User *GEP = GV->use_back();
assert(((isa<ConstantExpr>(GEP) &&
cast<ConstantExpr>(GEP)->getOpcode()==Instruction::GetElementPtr)||
isa<GetElementPtrInst>(GEP)) && "NonGEP CE's are not SRAable!");
// Ignore the 1th operand, which has to be zero or else the program is quite
// broken (undefined). Get the 2nd operand, which is the structure or array
// index.
unsigned Val = cast<ConstantInt>(CE->getOperand(2))->getRawValue();
unsigned Val = cast<ConstantInt>(GEP->getOperand(2))->getRawValue();
if (Val >= NewGlobals.size()) Val = 0; // Out of bound array access.
Constant *NewPtr = NewGlobals[Val];
Value *NewPtr = NewGlobals[Val];
// Form a shorter GEP if needed.
if (CE->getNumOperands() > 3) {
std::vector<Constant*> Idxs;
Idxs.push_back(NullInt);
for (unsigned i = 3, e = CE->getNumOperands(); i != e; ++i)
Idxs.push_back(CE->getOperand(i));
NewPtr = ConstantExpr::getGetElementPtr(NewPtr, Idxs);
}
CE->replaceAllUsesWith(NewPtr);
CE->destroyConstant();
if (GEP->getNumOperands() > 3)
if (ConstantExpr *CE = dyn_cast<ConstantExpr>(GEP)) {
std::vector<Constant*> Idxs;
Idxs.push_back(NullInt);
for (unsigned i = 3, e = CE->getNumOperands(); i != e; ++i)
Idxs.push_back(CE->getOperand(i));
NewPtr = ConstantExpr::getGetElementPtr(cast<Constant>(NewPtr), Idxs);
} else {
GetElementPtrInst *GEPI = cast<GetElementPtrInst>(GEP);
std::vector<Value*> Idxs;
Idxs.push_back(NullInt);
for (unsigned i = 3, e = GEPI->getNumOperands(); i != e; ++i)
Idxs.push_back(GEPI->getOperand(i));
NewPtr = new GetElementPtrInst(NewPtr, Idxs,
GEPI->getName()+"."+utostr(Val), GEPI);
}
GEP->replaceAllUsesWith(NewPtr);
if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(GEP))
GEPI->getParent()->getInstList().erase(GEPI);
else
cast<ConstantExpr>(GEP)->destroyConstant();
}
// Delete the old global, now that it is dead.
Globals.erase(GV);
++NumSRA;
return NewGlobals[0];
// Loop over the new globals array deleting any globals that are obviously
// dead. This can arise due to scalarization of a structure or an array that
// has elements that are dead.
unsigned FirstGlobal = 0;
for (unsigned i = 0, e = NewGlobals.size(); i != e; ++i)
if (NewGlobals[i]->use_empty()) {
Globals.erase(NewGlobals[i]);
if (FirstGlobal == i) ++FirstGlobal;
}
return FirstGlobal != NewGlobals.size() ? NewGlobals[FirstGlobal] : 0;
}
/// AllUsesOfValueWillTrapIfNull - Return true if all users of the specified
@ -535,10 +584,98 @@ static bool OptimizeAwayTrappingUsesOfLoads(GlobalVariable *GV, Constant *LV) {
return Changed;
}
/// ConstantPropUsersOf - Walk the use list of V, constant folding all of the
/// instructions that are foldable.
static void ConstantPropUsersOf(Value *V) {
for (Value::use_iterator UI = V->use_begin(), E = V->use_end(); UI != E; )
if (Instruction *I = dyn_cast<Instruction>(*UI++))
if (Constant *NewC = ConstantFoldInstruction(I)) {
I->replaceAllUsesWith(NewC);
// Back up UI to avoid invalidating it!
bool AtBegin = false;
if (UI == V->use_begin())
AtBegin = true;
else
--UI;
I->getParent()->getInstList().erase(I);
if (AtBegin)
UI = V->use_begin();
else
++UI;
}
}
/// OptimizeGlobalAddressOfMalloc - This function takes the specified global
/// variable, and transforms the program as if it always contained the result of
/// the specified malloc. Because it is always the result of the specified
/// malloc, there is no reason to actually DO the malloc. Instead, turn the
/// malloc into a global, and any laods of GV as uses of the new global.
static GlobalVariable *OptimizeGlobalAddressOfMalloc(GlobalVariable *GV,
MallocInst *MI) {
DEBUG(std::cerr << "PROMOTING MALLOC GLOBAL: " << *GV << " MALLOC = " <<*MI);
ConstantInt *NElements = cast<ConstantInt>(MI->getArraySize());
if (NElements->getRawValue() != 1) {
// If we have an array allocation, transform it to a single element
// allocation to make the code below simpler.
Type *NewTy = ArrayType::get(MI->getAllocatedType(),
NElements->getRawValue());
MallocInst *NewMI =
new MallocInst(NewTy, Constant::getNullValue(Type::UIntTy),
MI->getName(), MI);
std::vector<Value*> Indices;
Indices.push_back(Constant::getNullValue(Type::IntTy));
Indices.push_back(Indices[0]);
Value *NewGEP = new GetElementPtrInst(NewMI, Indices,
NewMI->getName()+".el0", MI);
MI->replaceAllUsesWith(NewGEP);
MI->getParent()->getInstList().erase(MI);
MI = NewMI;
}
// Create the new global variable.
Constant *Init = Constant::getNullValue(MI->getAllocatedType());
GlobalVariable *NewGV = new GlobalVariable(MI->getAllocatedType(), false,
GlobalValue::InternalLinkage, Init,
GV->getName()+".body");
GV->getParent()->getGlobalList().insert(GV, NewGV);
// Anything that used the malloc now uses the global directly.
MI->replaceAllUsesWith(NewGV);
MI->getParent()->getInstList().erase(MI);
Constant *RepValue = NewGV;
if (NewGV->getType() != GV->getType()->getElementType())
RepValue = ConstantExpr::getCast(RepValue, GV->getType()->getElementType());
// Loop over all uses of GV, processing them in turn.
while (!GV->use_empty())
if (LoadInst *LI = dyn_cast<LoadInst>(GV->use_back())) {
LI->replaceAllUsesWith(RepValue);
LI->getParent()->getInstList().erase(LI);
} else {
StoreInst *SI = cast<StoreInst>(GV->use_back());
SI->getParent()->getInstList().erase(SI);
}
// Now the GV is dead, nuke it.
GV->getParent()->getGlobalList().erase(GV);
// To further other optimizations, loop over all users of NewGV and try to
// constant prop them. This will promote GEP instructions with constant
// indices into GEP constant-exprs, which will allow global-opt to hack on it.
ConstantPropUsersOf(NewGV);
if (RepValue != NewGV)
ConstantPropUsersOf(RepValue);
return NewGV;
}
// OptimizeOnceStoredGlobal - Try to optimize globals based on the knowledge
// that only one value (besides its initializer) is ever stored to the global.
static bool OptimizeOnceStoredGlobal(GlobalVariable *GV, Value *StoredOnceVal) {
static bool OptimizeOnceStoredGlobal(GlobalVariable *GV, Value *StoredOnceVal,
Module::giterator &GVI, TargetData &TD) {
if (CastInst *CI = dyn_cast<CastInst>(StoredOnceVal))
StoredOnceVal = CI->getOperand(0);
else if (GetElementPtrInst *GEPI =dyn_cast<GetElementPtrInst>(StoredOnceVal)){
@ -567,15 +704,35 @@ static bool OptimizeOnceStoredGlobal(GlobalVariable *GV, Value *StoredOnceVal) {
// Optimize away any trapping uses of the loaded value.
if (OptimizeAwayTrappingUsesOfLoads(GV, SOVC))
return true;
} else if (MallocInst *MI = dyn_cast<MallocInst>(StoredOnceVal)) {
// If we have a global that is only initialized with a fixed size malloc,
// and if all users of the malloc trap, and if the malloc'd address is not
// put anywhere else, transform the program to use global memory instead
// of malloc'd memory. This eliminates dynamic allocation (good) and
// exposes the resultant global to further GlobalOpt (even better). Note
// that we restrict this transformation to only working on small
// allocations (2048 bytes currently), as we don't want to introduce a 16M
// global or something.
if (ConstantInt *NElements = dyn_cast<ConstantInt>(MI->getArraySize()))
if (MI->getAllocatedType()->isSized() &&
NElements->getRawValue()*
TD.getTypeSize(MI->getAllocatedType()) < 2048 &&
AllUsesOfLoadedValueWillTrapIfNull(GV)) {
// FIXME: do more correctness checking to make sure the result of the
// malloc isn't squirrelled away somewhere.
GVI = OptimizeGlobalAddressOfMalloc(GV, MI);
return true;
}
}
//if (isa<MallocInst>(StoredOnceValue))
}
return false;
}
/// ProcessInternalGlobal - Analyze the specified global variable and optimize
/// it if possible. If we make a change, return true.
static bool ProcessInternalGlobal(GlobalVariable *GV, Module::giterator &GVI) {
bool GlobalOpt::ProcessInternalGlobal(GlobalVariable *GV,
Module::giterator &GVI) {
std::set<PHINode*> PHIUsers;
GlobalStatus GS;
PHIUsers.clear();
@ -625,7 +782,6 @@ static bool ProcessInternalGlobal(GlobalVariable *GV, Module::giterator &GVI) {
return true;
} else if (!GS.isNotSuitableForSRA &&
!GV->getInitializer()->getType()->isFirstClassType()) {
DEBUG(std::cerr << "PERFORMING GLOBAL SRA ON: " << *GV);
if (GlobalVariable *FirstNewGV = SRAGlobal(GV)) {
GVI = FirstNewGV; // Don't skip the newly produced globals!
return true;
@ -633,7 +789,8 @@ static bool ProcessInternalGlobal(GlobalVariable *GV, Module::giterator &GVI) {
} else if (GS.StoredType == GlobalStatus::isStoredOnce) {
// Try to optimize globals based on the knowledge that only one value
// (besides its initializer) is ever stored to the global.
if (OptimizeOnceStoredGlobal(GV, GS.StoredOnceValue))
if (OptimizeOnceStoredGlobal(GV, GS.StoredOnceValue, GVI,
getAnalysis<TargetData>()))
return true;
}
}