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Factor the PHI and select speculation into a separate rewriter. This

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could probably be factored still further to hoist this logic into
a generic helper, but currently I don't have particularly clean ideas
about how to handle that.

This at least allows us to drop custom load rewriting from the
speculation logic, which in turn allows the existing load rewriting
logic to fire. In theory, this could enable vector promotion or other
tricks after speculation occurs, but I've not dug into such issues. This
is primarily just cleaning up the factoring of the code and the
resulting logic.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@164933 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chandler Carruth 2012-10-01 10:54:05 +00:00
parent 363d1c3c55
commit 1e1b16c485
1 changed files with 294 additions and 263 deletions
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557
lib/Transforms/Scalar/SROA.cpp
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@ -1116,12 +1116,6 @@ Type *AllocaPartitioning::getCommonType(iterator I) const {
UserTy = LI->getType();
} else if (StoreInst *SI = dyn_cast<StoreInst>(UI->U->getUser())) {
UserTy = SI->getValueOperand()->getType();
} else if (SelectInst *SI = dyn_cast<SelectInst>(UI->U->getUser())) {
if (PointerType *PtrTy = dyn_cast<PointerType>(SI->getType()))
UserTy = PtrTy->getElementType();
} else if (PHINode *PN = dyn_cast<PHINode>(UI->U->getUser())) {
if (PointerType *PtrTy = dyn_cast<PointerType>(PN->getType()))
UserTy = PtrTy->getElementType();
}
if (Ty && Ty != UserTy)
@ -1329,6 +1323,7 @@ public:
static char ID;
private:
friend class PHIOrSelectSpeculator;
friend class AllocaPartitionRewriter;
friend class AllocaPartitionVectorRewriter;
@ -1770,6 +1765,278 @@ static bool isIntegerPromotionViable(const TargetData &TD,
}
namespace {
/// \brief Visitor to speculate PHIs and Selects where possible.
class PHIOrSelectSpeculator : public InstVisitor<PHIOrSelectSpeculator> {
// Befriend the base class so it can delegate to private visit methods.
friend class llvm::InstVisitor<PHIOrSelectSpeculator>;
const TargetData &TD;
AllocaPartitioning &P;
SROA &Pass;
public:
PHIOrSelectSpeculator(const TargetData &TD, AllocaPartitioning &P, SROA &Pass)
: TD(TD), P(P), Pass(Pass) {}
/// \brief Visit the users of the alloca partition and rewrite them.
void visitUsers(AllocaPartitioning::const_use_iterator I,
AllocaPartitioning::const_use_iterator E) {
for (; I != E; ++I)
visit(cast<Instruction>(I->U->getUser()));
}
private:
// By default, skip this instruction.
void visitInstruction(Instruction &I) {}
/// PHI instructions that use an alloca and are subsequently loaded can be
/// rewritten to load both input pointers in the pred blocks and then PHI the
/// results, allowing the load of the alloca to be promoted.
/// From this:
/// %P2 = phi [i32* %Alloca, i32* %Other]
/// %V = load i32* %P2
/// to:
/// %V1 = load i32* %Alloca -> will be mem2reg'd
/// ...
/// %V2 = load i32* %Other
/// ...
/// %V = phi [i32 %V1, i32 %V2]
///
/// We can do this to a select if its only uses are loads and if the operand
/// to the select can be loaded unconditionally.
///
/// FIXME: This should be hoisted into a generic utility, likely in
/// Transforms/Util/Local.h
bool isSafePHIToSpeculate(PHINode &PN, SmallVectorImpl<LoadInst *> &Loads) {
// For now, we can only do this promotion if the load is in the same block
// as the PHI, and if there are no stores between the phi and load.
// TODO: Allow recursive phi users.
// TODO: Allow stores.
BasicBlock *BB = PN.getParent();
unsigned MaxAlign = 0;
for (Value::use_iterator UI = PN.use_begin(), UE = PN.use_end();
UI != UE; ++UI) {
LoadInst *LI = dyn_cast<LoadInst>(*UI);
if (LI == 0 || !LI->isSimple()) return false;
// For now we only allow loads in the same block as the PHI. This is
// a common case that happens when instcombine merges two loads through
// a PHI.
if (LI->getParent() != BB) return false;
// Ensure that there are no instructions between the PHI and the load that
// could store.
for (BasicBlock::iterator BBI = &PN; &*BBI != LI; ++BBI)
if (BBI->mayWriteToMemory())
return false;
MaxAlign = std::max(MaxAlign, LI->getAlignment());
Loads.push_back(LI);
}
// We can only transform this if it is safe to push the loads into the
// predecessor blocks. The only thing to watch out for is that we can't put
// a possibly trapping load in the predecessor if it is a critical edge.
for (unsigned Idx = 0, Num = PN.getNumIncomingValues(); Idx != Num;
++Idx) {
TerminatorInst *TI = PN.getIncomingBlock(Idx)->getTerminator();
Value *InVal = PN.getIncomingValue(Idx);
// If the value is produced by the terminator of the predecessor (an
// invoke) or it has side-effects, there is no valid place to put a load
// in the predecessor.
if (TI == InVal || TI->mayHaveSideEffects())
return false;
// If the predecessor has a single successor, then the edge isn't
// critical.
if (TI->getNumSuccessors() == 1)
continue;
// If this pointer is always safe to load, or if we can prove that there
// is already a load in the block, then we can move the load to the pred
// block.
if (InVal->isDereferenceablePointer() ||
isSafeToLoadUnconditionally(InVal, TI, MaxAlign, &TD))
continue;
return false;
}
return true;
}
void visitPHINode(PHINode &PN) {
DEBUG(dbgs() << " original: " << PN << "\n");
SmallVector<LoadInst *, 4> Loads;
if (!isSafePHIToSpeculate(PN, Loads))
return;
assert(!Loads.empty());
Type *LoadTy = cast<PointerType>(PN.getType())->getElementType();
IRBuilder<> PHIBuilder(&PN);
PHINode *NewPN = PHIBuilder.CreatePHI(LoadTy, PN.getNumIncomingValues(),
PN.getName() + ".sroa.speculated");
// Get the TBAA tag and alignment to use from one of the loads. It doesn't
// matter which one we get and if any differ, it doesn't matter.
LoadInst *SomeLoad = cast<LoadInst>(Loads.back());
MDNode *TBAATag = SomeLoad->getMetadata(LLVMContext::MD_tbaa);
unsigned Align = SomeLoad->getAlignment();
// Rewrite all loads of the PN to use the new PHI.
do {
LoadInst *LI = Loads.pop_back_val();
LI->replaceAllUsesWith(NewPN);
Pass.DeadInsts.push_back(LI);
} while (!Loads.empty());
// Inject loads into all of the pred blocks.
for (unsigned Idx = 0, Num = PN.getNumIncomingValues(); Idx != Num; ++Idx) {
BasicBlock *Pred = PN.getIncomingBlock(Idx);
TerminatorInst *TI = Pred->getTerminator();
Use *InUse = &PN.getOperandUse(PN.getOperandNumForIncomingValue(Idx));
Value *InVal = PN.getIncomingValue(Idx);
IRBuilder<> PredBuilder(TI);
LoadInst *Load
= PredBuilder.CreateLoad(InVal, (PN.getName() + ".sroa.speculate.load." +
Pred->getName()));
++NumLoadsSpeculated;
Load->setAlignment(Align);
if (TBAATag)
Load->setMetadata(LLVMContext::MD_tbaa, TBAATag);
NewPN->addIncoming(Load, Pred);
Instruction *Ptr = dyn_cast<Instruction>(InVal);
if (!Ptr)
// No uses to rewrite.
continue;
// Try to lookup and rewrite any partition uses corresponding to this phi
// input.
AllocaPartitioning::iterator PI
= P.findPartitionForPHIOrSelectOperand(InUse);
if (PI == P.end())
continue;
// Replace the Use in the PartitionUse for this operand with the Use
// inside the load.
AllocaPartitioning::use_iterator UI
= P.findPartitionUseForPHIOrSelectOperand(InUse);
assert(isa<PHINode>(*UI->U->getUser()));
UI->U = &Load->getOperandUse(Load->getPointerOperandIndex());
}
DEBUG(dbgs() << " speculated to: " << *NewPN << "\n");
}
/// Select instructions that use an alloca and are subsequently loaded can be
/// rewritten to load both input pointers and then select between the result,
/// allowing the load of the alloca to be promoted.
/// From this:
/// %P2 = select i1 %cond, i32* %Alloca, i32* %Other
/// %V = load i32* %P2
/// to:
/// %V1 = load i32* %Alloca -> will be mem2reg'd
/// %V2 = load i32* %Other
/// %V = select i1 %cond, i32 %V1, i32 %V2
///
/// We can do this to a select if its only uses are loads and if the operand
/// to the select can be loaded unconditionally.
bool isSafeSelectToSpeculate(SelectInst &SI,
SmallVectorImpl<LoadInst *> &Loads) {
Value *TValue = SI.getTrueValue();
Value *FValue = SI.getFalseValue();
bool TDerefable = TValue->isDereferenceablePointer();
bool FDerefable = FValue->isDereferenceablePointer();
for (Value::use_iterator UI = SI.use_begin(), UE = SI.use_end();
UI != UE; ++UI) {
LoadInst *LI = dyn_cast<LoadInst>(*UI);
if (LI == 0 || !LI->isSimple()) return false;
// Both operands to the select need to be dereferencable, either
// absolutely (e.g. allocas) or at this point because we can see other
// accesses to it.
if (!TDerefable && !isSafeToLoadUnconditionally(TValue, LI,
LI->getAlignment(), &TD))
return false;
if (!FDerefable && !isSafeToLoadUnconditionally(FValue, LI,
LI->getAlignment(), &TD))
return false;
Loads.push_back(LI);
}
return true;
}
void visitSelectInst(SelectInst &SI) {
DEBUG(dbgs() << " original: " << SI << "\n");
IRBuilder<> IRB(&SI);
// If the select isn't safe to speculate, just use simple logic to emit it.
SmallVector<LoadInst *, 4> Loads;
if (!isSafeSelectToSpeculate(SI, Loads))
return;
Use *Ops[2] = { &SI.getOperandUse(1), &SI.getOperandUse(2) };
AllocaPartitioning::iterator PIs[2];
AllocaPartitioning::PartitionUse PUs[2];
for (unsigned i = 0, e = 2; i != e; ++i) {
PIs[i] = P.findPartitionForPHIOrSelectOperand(Ops[i]);
if (PIs[i] != P.end()) {
// If the pointer is within the partitioning, remove the select from
// its uses. We'll add in the new loads below.
AllocaPartitioning::use_iterator UI
= P.findPartitionUseForPHIOrSelectOperand(Ops[i]);
PUs[i] = *UI;
P.use_erase(PIs[i], UI);
}
}
Value *TV = SI.getTrueValue();
Value *FV = SI.getFalseValue();
// Replace the loads of the select with a select of two loads.
while (!Loads.empty()) {
LoadInst *LI = Loads.pop_back_val();
IRB.SetInsertPoint(LI);
LoadInst *TL =
IRB.CreateLoad(TV, LI->getName() + ".sroa.speculate.load.true");
LoadInst *FL =
IRB.CreateLoad(FV, LI->getName() + ".sroa.speculate.load.false");
NumLoadsSpeculated += 2;
// Transfer alignment and TBAA info if present.
TL->setAlignment(LI->getAlignment());
FL->setAlignment(LI->getAlignment());
if (MDNode *Tag = LI->getMetadata(LLVMContext::MD_tbaa)) {
TL->setMetadata(LLVMContext::MD_tbaa, Tag);
FL->setMetadata(LLVMContext::MD_tbaa, Tag);
}
Value *V = IRB.CreateSelect(SI.getCondition(), TL, FL,
LI->getName() + ".sroa.speculated");
LoadInst *Loads[2] = { TL, FL };
for (unsigned i = 0, e = 2; i != e; ++i) {
if (PIs[i] != P.end()) {
Use *LoadUse = &Loads[i]->getOperandUse(0);
assert(PUs[i].U->get() == LoadUse->get());
PUs[i].U = LoadUse;
P.use_push_back(PIs[i], PUs[i]);
}
}
DEBUG(dbgs() << " speculated to: " << *V << "\n");
LI->replaceAllUsesWith(V);
Pass.DeadInsts.push_back(LI);
}
}
};
/// \brief Visitor to rewrite instructions using a partition of an alloca to
/// use a new alloca.
///
@ -2337,213 +2604,25 @@ private:
return true;
}
/// PHI instructions that use an alloca and are subsequently loaded can be
/// rewritten to load both input pointers in the pred blocks and then PHI the
/// results, allowing the load of the alloca to be promoted.
/// From this:
/// %P2 = phi [i32* %Alloca, i32* %Other]
/// %V = load i32* %P2
/// to:
/// %V1 = load i32* %Alloca -> will be mem2reg'd
/// ...
/// %V2 = load i32* %Other
/// ...
/// %V = phi [i32 %V1, i32 %V2]
///
/// We can do this to a select if its only uses are loads and if the operand
/// to the select can be loaded unconditionally.
///
/// FIXME: This should be hoisted into a generic utility, likely in
/// Transforms/Util/Local.h
bool isSafePHIToSpeculate(PHINode &PN, SmallVectorImpl<LoadInst *> &Loads) {
// For now, we can only do this promotion if the load is in the same block
// as the PHI, and if there are no stores between the phi and load.
// TODO: Allow recursive phi users.
// TODO: Allow stores.
BasicBlock *BB = PN.getParent();
unsigned MaxAlign = 0;
for (Value::use_iterator UI = PN.use_begin(), UE = PN.use_end();
UI != UE; ++UI) {
LoadInst *LI = dyn_cast<LoadInst>(*UI);
if (LI == 0 || !LI->isSimple()) return false;
// For now we only allow loads in the same block as the PHI. This is
// a common case that happens when instcombine merges two loads through
// a PHI.
if (LI->getParent() != BB) return false;
// Ensure that there are no instructions between the PHI and the load that
// could store.
for (BasicBlock::iterator BBI = &PN; &*BBI != LI; ++BBI)
if (BBI->mayWriteToMemory())
return false;
MaxAlign = std::max(MaxAlign, LI->getAlignment());
Loads.push_back(LI);
}
// We can only transform this if it is safe to push the loads into the
// predecessor blocks. The only thing to watch out for is that we can't put
// a possibly trapping load in the predecessor if it is a critical edge.
for (unsigned Idx = 0, Num = PN.getNumIncomingValues(); Idx != Num;
++Idx) {
TerminatorInst *TI = PN.getIncomingBlock(Idx)->getTerminator();
Value *InVal = PN.getIncomingValue(Idx);
// If the value is produced by the terminator of the predecessor (an
// invoke) or it has side-effects, there is no valid place to put a load
// in the predecessor.
if (TI == InVal || TI->mayHaveSideEffects())
return false;
// If the predecessor has a single successor, then the edge isn't
// critical.
if (TI->getNumSuccessors() == 1)
continue;
// If this pointer is always safe to load, or if we can prove that there
// is already a load in the block, then we can move the load to the pred
// block.
if (InVal->isDereferenceablePointer() ||
isSafeToLoadUnconditionally(InVal, TI, MaxAlign, &TD))
continue;
return false;
}
return true;
}
bool visitPHINode(PHINode &PN) {
DEBUG(dbgs() << " original: " << PN << "\n");
// We would like to compute a new pointer in only one place, but have it be
// as local as possible to the PHI. To do that, we re-use the location of
// the old pointer, which necessarily must be in the right position to
// dominate the PHI.
IRBuilder<> PtrBuilder(cast<Instruction>(OldPtr));
SmallVector<LoadInst *, 4> Loads;
if (!isSafePHIToSpeculate(PN, Loads)) {
Value *NewPtr = getAdjustedAllocaPtr(PtrBuilder, OldPtr->getType());
// Replace the operands which were using the old pointer.
User::op_iterator OI = PN.op_begin(), OE = PN.op_end();
for (; OI != OE; ++OI)
if (*OI == OldPtr)
*OI = NewPtr;
DEBUG(dbgs() << " to: " << PN << "\n");
deleteIfTriviallyDead(OldPtr);
return false;
}
assert(!Loads.empty());
Type *LoadTy = cast<PointerType>(PN.getType())->getElementType();
IRBuilder<> PHIBuilder(&PN);
PHINode *NewPN = PHIBuilder.CreatePHI(LoadTy, PN.getNumIncomingValues());
NewPN->takeName(&PN);
// Get the TBAA tag and alignment to use from one of the loads. It doesn't
// matter which one we get and if any differ, it doesn't matter.
LoadInst *SomeLoad = cast<LoadInst>(Loads.back());
MDNode *TBAATag = SomeLoad->getMetadata(LLVMContext::MD_tbaa);
unsigned Align = SomeLoad->getAlignment();
Value *NewPtr = getAdjustedAllocaPtr(PtrBuilder, OldPtr->getType());
// Replace the operands which were using the old pointer.
User::op_iterator OI = PN.op_begin(), OE = PN.op_end();
for (; OI != OE; ++OI)
if (*OI == OldPtr)
*OI = NewPtr;
// Rewrite all loads of the PN to use the new PHI.
do {
LoadInst *LI = Loads.pop_back_val();
LI->replaceAllUsesWith(NewPN);
Pass.DeadInsts.push_back(LI);
} while (!Loads.empty());
// Inject loads into all of the pred blocks.
for (unsigned Idx = 0, Num = PN.getNumIncomingValues(); Idx != Num; ++Idx) {
BasicBlock *Pred = PN.getIncomingBlock(Idx);
TerminatorInst *TI = Pred->getTerminator();
Value *InVal = PN.getIncomingValue(Idx);
IRBuilder<> PredBuilder(TI);
// Map the value to the new alloca pointer if this was the old alloca
// pointer.
Use *InUse = &PN.getOperandUse(PN.getOperandNumForIncomingValue(Idx));
bool ThisOperand = InUse == OldUse;
if (ThisOperand)
InVal = NewPtr;
LoadInst *Load
= PredBuilder.CreateLoad(InVal, getName(".sroa.speculate." +
Pred->getName()));
++NumLoadsSpeculated;
Load->setAlignment(Align);
if (TBAATag)
Load->setMetadata(LLVMContext::MD_tbaa, TBAATag);
NewPN->addIncoming(Load, Pred);
Instruction *Ptr = dyn_cast<Instruction>(InVal);
if (!Ptr)
// No uses to rewrite.
continue;
// Try to lookup and rewrite any partition uses corresponding to this phi
// input.
AllocaPartitioning::iterator PI
= P.findPartitionForPHIOrSelectOperand(InUse);
if (PI == P.end())
continue;
// Replace the Use in the PartitionUse for this operand with the Use
// inside the load. This will already have been re-written for the
// partition use currently being processed.
// FIXME: This is really gross. We should do PHI and select speculation
// as a pre-processing pass first, and then use the existing
// load-rewriting logic.
AllocaPartitioning::use_iterator UI
= P.findPartitionUseForPHIOrSelectOperand(InUse);
assert(isa<PHINode>(*UI->U->getUser()));
UI->U = &Load->getOperandUse(Load->getPointerOperandIndex());
}
DEBUG(dbgs() << " speculated to: " << *NewPN << "\n");
return NewPtr == &NewAI;
}
/// Select instructions that use an alloca and are subsequently loaded can be
/// rewritten to load both input pointers and then select between the result,
/// allowing the load of the alloca to be promoted.
/// From this:
/// %P2 = select i1 %cond, i32* %Alloca, i32* %Other
/// %V = load i32* %P2
/// to:
/// %V1 = load i32* %Alloca -> will be mem2reg'd
/// %V2 = load i32* %Other
/// %V = select i1 %cond, i32 %V1, i32 %V2
///
/// We can do this to a select if its only uses are loads and if the operand
/// to the select can be loaded unconditionally.
bool isSafeSelectToSpeculate(SelectInst &SI,
SmallVectorImpl<LoadInst *> &Loads) {
Value *TValue = SI.getTrueValue();
Value *FValue = SI.getFalseValue();
bool TDerefable = TValue->isDereferenceablePointer();
bool FDerefable = FValue->isDereferenceablePointer();
for (Value::use_iterator UI = SI.use_begin(), UE = SI.use_end();
UI != UE; ++UI) {
LoadInst *LI = dyn_cast<LoadInst>(*UI);
if (LI == 0 || !LI->isSimple()) return false;
// Both operands to the select need to be dereferencable, either
// absolutely (e.g. allocas) or at this point because we can see other
// accesses to it.
if (!TDerefable && !isSafeToLoadUnconditionally(TValue, LI,
LI->getAlignment(), &TD))
return false;
if (!FDerefable && !isSafeToLoadUnconditionally(FValue, LI,
LI->getAlignment(), &TD))
return false;
Loads.push_back(LI);
}
return true;
DEBUG(dbgs() << " to: " << PN << "\n");
deleteIfTriviallyDead(OldPtr);
return false;
}
bool visitSelectInst(SelectInst &SI) {
@ -2556,68 +2635,12 @@ private:
assert(SI.getFalseValue() != OldPtr && "Pointer is both operands!");
else
assert(SI.getFalseValue() == OldPtr && "Pointer isn't an operand!");
Value *NewPtr = getAdjustedAllocaPtr(IRB, OldPtr->getType());
// If the select isn't safe to speculate, just use simple logic to emit it.
SmallVector<LoadInst *, 4> Loads;
if (!isSafeSelectToSpeculate(SI, Loads)) {
SI.setOperand(IsTrueVal ? 1 : 2, NewPtr);
DEBUG(dbgs() << " to: " << SI << "\n");
deleteIfTriviallyDead(OldPtr);
return false;
}
Use *OtherOp = &SI.getOperandUse(IsTrueVal ? 2 : 1);
AllocaPartitioning::iterator PI
= P.findPartitionForPHIOrSelectOperand(OtherOp);
AllocaPartitioning::PartitionUse OtherUse;
if (PI != P.end()) {
// If the other pointer is within the partitioning, remove the select
// from its uses. We'll add in the new loads below.
AllocaPartitioning::use_iterator UI
= P.findPartitionUseForPHIOrSelectOperand(OtherOp);
OtherUse = *UI;
P.use_erase(PI, UI);
}
Value *TV = IsTrueVal ? NewPtr : SI.getTrueValue();
Value *FV = IsTrueVal ? SI.getFalseValue() : NewPtr;
// Replace the loads of the select with a select of two loads.
while (!Loads.empty()) {
LoadInst *LI = Loads.pop_back_val();
IRB.SetInsertPoint(LI);
LoadInst *TL =
IRB.CreateLoad(TV, getName("." + LI->getName() + ".true"));
LoadInst *FL =
IRB.CreateLoad(FV, getName("." + LI->getName() + ".false"));
NumLoadsSpeculated += 2;
// Transfer alignment and TBAA info if present.
TL->setAlignment(LI->getAlignment());
FL->setAlignment(LI->getAlignment());
if (MDNode *Tag = LI->getMetadata(LLVMContext::MD_tbaa)) {
TL->setMetadata(LLVMContext::MD_tbaa, Tag);
FL->setMetadata(LLVMContext::MD_tbaa, Tag);
}
SelectInst *NewSI
= cast<SelectInst>(IRB.CreateSelect(SI.getCondition(), TL, FL));
NewSI->takeName(LI);
if (PI != P.end()) {
LoadInst *OtherLoad = IsTrueVal ? FL : TL;
Use *OtherLoadUse = &OtherLoad->getOperandUse(0);
assert(OtherUse.U->get() == OtherLoadUse->get());
OtherUse.U = OtherLoadUse;
P.use_push_back(PI, OtherUse);
}
DEBUG(dbgs() << " speculated to: " << *NewSI << "\n");
LI->replaceAllUsesWith(NewSI);
Pass.DeadInsts.push_back(LI);
}
SI.setOperand(IsTrueVal ? 1 : 2, NewPtr);
DEBUG(dbgs() << " to: " << SI << "\n");
deleteIfTriviallyDead(OldPtr);
return NewPtr == &NewAI;
return false;
}
};
@ -2965,6 +2988,14 @@ bool SROA::rewriteAllocaPartition(AllocaInst &AI,
if (P.use_begin(PI) == P.use_end(PI))
return false; // No live uses left of this partition.
DEBUG(dbgs() << "Speculating PHIs and selects in partition "
<< "[" << PI->BeginOffset << "," << PI->EndOffset << ")\n");
PHIOrSelectSpeculator Speculator(*TD, P, *this);
DEBUG(dbgs() << " speculating ");
DEBUG(P.print(dbgs(), PI, ""));
Speculator.visitUsers(P.use_begin(PI), P.use_end(PI));
// Try to compute a friendly type for this partition of the alloca. This
// won't always succeed, in which case we fall back to a legal integer type
// or an i8 array of an appropriate size.