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enhance SRoA to promote allocas that are used by PHI nodes. This often

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occurs because instcombine sinks loads and inserts phis.  This kicks in 
on such apps as 175.vpr, eon, 403.gcc, xalancbmk and a bunch of times in
spec2006 in some app that uses std::deque.

This resolves the last of rdar://7339113.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@124090 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2011-01-24 01:07:11 +00:00
parent c87c50a39c
commit e3357863aa
2 changed files with 181 additions and 29 deletions
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183
lib/Transforms/Scalar/ScalarReplAggregates.cpp
View File

@ -874,7 +874,7 @@ public:
/// to:
/// %V1 = load i32* %Alloca -> will be mem2reg'd
/// %V2 = load i32* %Other
/// %V = select i1 %cond, i32 %V1, i32* %V2
/// %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.
@ -887,7 +887,7 @@ static bool isSafeSelectToSpeculate(SelectInst *SI, const TargetData *TD) {
LoadInst *LI = dyn_cast<LoadInst>(*UI);
if (LI == 0 || LI->isVolatile()) return false;
// Both operands to the select need to be deferencable, either absolutely
// 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(SI->getTrueValue(), LI,
LI->getAlignment(), TD))
@ -900,6 +900,77 @@ static bool isSafeSelectToSpeculate(SelectInst *SI, const TargetData *TD) {
return true;
}
/// isSafePHIToSpeculate - 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.
static bool isSafePHIToSpeculate(PHINode *PN, const TargetData *TD) {
// 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->isVolatile()) 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());
}
// Okay, we know that we have one or more loads in the same block as the PHI.
// We can 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 i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
BasicBlock *Pred = PN->getIncomingBlock(i);
// If the predecessor has a single successor, then the edge isn't critical.
if (Pred->getTerminator()->getNumSuccessors() == 1)
continue;
Value *InVal = PN->getIncomingValue(i);
// If the InVal is an invoke in the pred, we can't put a load on the edge.
if (InvokeInst *II = dyn_cast<InvokeInst>(InVal))
if (II->getParent() == Pred)
return false;
// 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, Pred->getTerminator(), MaxAlign, TD))
continue;
return false;
}
return true;
}
/// tryToMakeAllocaBePromotable - This returns true if the alloca only has
/// direct (non-volatile) loads and stores to it. If the alloca is close but
@ -946,6 +1017,21 @@ static bool tryToMakeAllocaBePromotable(AllocaInst *AI, const TargetData *TD) {
continue;
}
if (PHINode *PN = dyn_cast<PHINode>(U)) {
if (PN->use_empty()) { // Dead PHIs can be stripped.
InstsToRewrite.insert(PN);
continue;
}
// If it is safe to turn "load (phi [AI, ptr, ...])" into a PHI of loads
// in the pred blocks, then we can transform this by rewriting the PHI.
if (!isSafePHIToSpeculate(PN, TD))
return false;
InstsToRewrite.insert(PN);
continue;
}
return false;
}
@ -957,35 +1043,80 @@ static bool tryToMakeAllocaBePromotable(AllocaInst *AI, const TargetData *TD) {
// If we have instructions that need to be rewritten for this to be promotable
// take care of it now.
for (unsigned i = 0, e = InstsToRewrite.size(); i != e; ++i) {
// Selects in InstsToRewrite only have load uses. Rewrite each as two
// loads with a new select.
SelectInst *SI = cast<SelectInst>(InstsToRewrite[i]);
for (Value::use_iterator UI = SI->use_begin(), E = SI->use_end(); UI != E;){
LoadInst *LI = cast<LoadInst>(*UI++);
if (SelectInst *SI = dyn_cast<SelectInst>(InstsToRewrite[i])) {
// Selects in InstsToRewrite only have load uses. Rewrite each as two
// loads with a new select.
while (!SI->use_empty()) {
LoadInst *LI = cast<LoadInst>(SI->use_back());
IRBuilder<> Builder(LI);
LoadInst *TrueLoad =
Builder.CreateLoad(SI->getTrueValue(), LI->getName()+".t");
LoadInst *FalseLoad =
Builder.CreateLoad(SI->getFalseValue(), LI->getName()+".t");
// Transfer alignment and TBAA info if present.
TrueLoad->setAlignment(LI->getAlignment());
FalseLoad->setAlignment(LI->getAlignment());
if (MDNode *Tag = LI->getMetadata(LLVMContext::MD_tbaa)) {
TrueLoad->setMetadata(LLVMContext::MD_tbaa, Tag);
FalseLoad->setMetadata(LLVMContext::MD_tbaa, Tag);
IRBuilder<> Builder(LI);
LoadInst *TrueLoad =
Builder.CreateLoad(SI->getTrueValue(), LI->getName()+".t");
LoadInst *FalseLoad =
Builder.CreateLoad(SI->getFalseValue(), LI->getName()+".t");
// Transfer alignment and TBAA info if present.
TrueLoad->setAlignment(LI->getAlignment());
FalseLoad->setAlignment(LI->getAlignment());
if (MDNode *Tag = LI->getMetadata(LLVMContext::MD_tbaa)) {
TrueLoad->setMetadata(LLVMContext::MD_tbaa, Tag);
FalseLoad->setMetadata(LLVMContext::MD_tbaa, Tag);
}
Value *V = Builder.CreateSelect(SI->getCondition(), TrueLoad, FalseLoad);
V->takeName(LI);
LI->replaceAllUsesWith(V);
LI->eraseFromParent();
}
Value *V = Builder.CreateSelect(SI->getCondition(), TrueLoad, FalseLoad);
V->takeName(LI);
LI->replaceAllUsesWith(V);
// Now that all the loads are gone, the select is gone too.
SI->eraseFromParent();
continue;
}
// Otherwise, we have a PHI node which allows us to push the loads into the
// predecessors.
PHINode *PN = cast<PHINode>(InstsToRewrite[i]);
if (PN->use_empty()) {
PN->eraseFromParent();
continue;
}
const Type *LoadTy = cast<PointerType>(PN->getType())->getElementType();
PHINode *NewPN = PHINode::Create(LoadTy, PN->getName()+".ld", 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>(PN->use_back());
MDNode *TBAATag = SomeLoad->getMetadata(LLVMContext::MD_tbaa);
unsigned Align = SomeLoad->getAlignment();
// Rewrite all loads of the PN to use the new PHI.
while (!PN->use_empty()) {
LoadInst *LI = cast<LoadInst>(PN->use_back());
LI->replaceAllUsesWith(NewPN);
LI->eraseFromParent();
}
// Now that all the loads are gone, the select is gone too.
SI->eraseFromParent();
// Inject loads into all of the pred blocks. Keep track of which blocks we
// insert them into in case we have multiple edges from the same block.
DenseMap<BasicBlock*, LoadInst*> InsertedLoads;
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
BasicBlock *Pred = PN->getIncomingBlock(i);
LoadInst *&Load = InsertedLoads[Pred];
if (Load == 0) {
Load = new LoadInst(PN->getIncomingValue(i),
PN->getName() + "." + Pred->getName(),
Pred->getTerminator());
Load->setAlignment(Align);
if (TBAATag) Load->setMetadata(LLVMContext::MD_tbaa, TBAATag);
}
NewPN->addIncoming(Load, Pred);
}
PN->eraseFromParent();
}
++NumAdjusted;

27
test/Transforms/ScalarRepl/phi-select.ll
View File

@ -25,8 +25,8 @@ entry:
}
; CHECK: @test2
; CHECK: %A.0 = alloca i32
; CHECK: %A.1 = alloca i32
; CHECK: %X.ld = phi i32 [ 1, %entry ], [ 2, %T ]
; CHECK-NEXT: ret i32 %X.ld
define i32 @test2(i1 %c) {
entry:
%A = alloca {i32, i32}
@ -35,7 +35,7 @@ entry:
br i1 %c, label %T, label %F
T:
%C = getelementptr {i32, i32}* %A, i32 0, i32 1
store i32 2, i32* %B
store i32 2, i32* %C
br label %F
F:
%X = phi i32* [%B, %entry], [%C, %T]
@ -130,3 +130,24 @@ define i32 @test7(i32 %x, i1 %c) nounwind readnone ssp {
; CHECK: ret i32 %r
}
;; Promote allocs that are PHI'd together by moving the loads.
define i32 @test8(i32 %x) nounwind readnone ssp {
; CHECK: @test8
; CHECK-NOT: load i32
; CHECK-NOT: store i32
; CHECK: %p.0.ld = phi i32 [ 2, %entry ], [ 1, %T ]
; CHECK-NEXT: ret i32 %p.0.ld
entry:
%a = alloca i32, align 8
%b = alloca i32, align 8
store i32 1, i32* %a, align 8
store i32 2, i32* %b, align 8
%c = icmp eq i32 %x, 0
br i1 %c, label %T, label %Cont
T:
br label %Cont
Cont:
%p.0 = phi i32* [%b, %entry],[%a, %T]
%r = load i32* %p.0, align 8
ret i32 %r
}