/// 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.
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))
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
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;
}
// 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;
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