[Instcombine] Combine consecutive identical fences

[Bigcheese]

Looks fine to me as long as we've resolved the semantic issues here. I'd wait for what Chandler &&/|| David have to say.

[davide]

In D29314#661269, @Bigcheese wrote:

Looks fine to me as long as we've resolved the semantic issues here. I'd wait for what Chandler &&/|| David have to say.

Thanks Michael. I'll wait a bit and see if there are objections.

[majnemer]

LGTM

lib/Transforms/InstCombine/InstCombineCalls.cpp

3275–3276 ↗

(On Diff #86380)

I think you could just do if (FI.isIdenticalTo(NFI))

[davide]

lib/Transforms/InstCombine/InstCombineCalls.cpp

3275–3276 ↗

(On Diff #86380)

I was actually looking for that function (but didn't find it), thanks for the review.

llvm/trunk/lib/Transforms/InstCombine/InstCombineCalls.cpp

if (match(II->getNextNode(),
       return eraseInstFromFunction(*II);
     break;
   }
   }
 
   return visitCallSite(II);
 }
 
+// Fence instruction simplification
+Instruction *InstCombiner::visitFenceInst(FenceInst &FI) {
+  // Remove identical consecutive fences.
+  if (auto *NFI = dyn_cast<FenceInst>(FI.getNextNode()))
+    if (FI.isIdenticalTo(NFI))
+      return eraseInstFromFunction(FI);
+  return nullptr;
+}
+
 // InvokeInst simplification
 //
 Instruction *InstCombiner::visitInvokeInst(InvokeInst &II) {
   return visitCallSite(&II);
 }
 
 /// If this cast does not affect the value passed through the varargs area, we
 /// can eliminate the use of the cast.

llvm/trunk/lib/Transforms/InstCombine/InstCombineInternal.h

public:
   Instruction *visitPHINode(PHINode &PN);
   Instruction *visitGetElementPtrInst(GetElementPtrInst &GEP);
   Instruction *visitAllocaInst(AllocaInst &AI);
   Instruction *visitAllocSite(Instruction &FI);
   Instruction *visitFree(CallInst &FI);
   Instruction *visitLoadInst(LoadInst &LI);
   Instruction *visitStoreInst(StoreInst &SI);
   Instruction *visitBranchInst(BranchInst &BI);
+  Instruction *visitFenceInst(FenceInst &FI);
   Instruction *visitSwitchInst(SwitchInst &SI);
   Instruction *visitReturnInst(ReturnInst &RI);
   Instruction *visitInsertValueInst(InsertValueInst &IV);
   Instruction *visitInsertElementInst(InsertElementInst &IE);
   Instruction *visitExtractElementInst(ExtractElementInst &EI);
   Instruction *visitShuffleVectorInst(ShuffleVectorInst &SVI);
   Instruction *visitExtractValueInst(ExtractValueInst &EV);
   Instruction *visitLandingPadInst(LandingPadInst &LI);

llvm/trunk/test/Transforms/InstCombine/consecutive-fences.ll

+; RUN: opt -instcombine -S %s | FileCheck %s
+
+; Make sure we collapse the fences in this case
+
+; CHECK-LABEL: define void @tinkywinky
+; CHECK-NEXT:   fence seq_cst
+; CHECK-NEXT:   fence singlethread acquire
+; CHECK-NEXT:   ret void
+; CHECK-NEXT: }
+
+define void @tinkywinky() {
+  fence seq_cst
+  fence seq_cst
+  fence seq_cst
+  fence singlethread acquire
+  fence singlethread acquire
+  fence singlethread acquire
+  ret void
+}
+
+; CHECK-LABEL: define void @dipsy
+; CHECK-NEXT:   fence seq_cst
+; CHECK-NEXT:   fence singlethread seq_cst
+; CHECK-NEXT:   ret void
+; CHECK-NEXT: }
+
+define void @dipsy() {
+  fence seq_cst
+  fence singlethread seq_cst
+  ret void
+}
+
+; CHECK-LABEL: define void @patatino
+; CHECK-NEXT:   fence acquire
+; CHECK-NEXT:   fence seq_cst
+; CHECK-NEXT:   fence acquire
+; CHECK-NEXT:   fence seq_cst
+; CHECK-NEXT:   ret void
+; CHECK-NEXT: }
+
+define void @patatino() {
+  fence acquire
+  fence seq_cst
+  fence acquire
+  fence seq_cst
+  ret void
+}