From ee994236c60e5ed7b78e46e90b4e938c8e51cc19 Mon Sep 17 00:00:00 2001
From: Davide Italiano <davide@freebsd.org>
Date: Tue, 31 Jan 2017 18:09:05 +0000
Subject: [PATCH] [Instcombine] Combine consecutive identical fences

Differential Revision:  https://reviews.llvm.org/D29314

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@293661 91177308-0d34-0410-b5e6-96231b3b80d8
---
 .../InstCombine/InstCombineCalls.cpp          |  9 ++++
 .../InstCombine/InstCombineInternal.h         |  1 +
 .../InstCombine/consecutive-fences.ll         | 47 +++++++++++++++++++
 3 files changed, 57 insertions(+)
 create mode 100644 test/Transforms/InstCombine/consecutive-fences.ll

diff --git a/lib/Transforms/InstCombine/InstCombineCalls.cpp b/lib/Transforms/InstCombine/InstCombineCalls.cpp
index 19746651d99..56983dc6036 100644
--- a/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -3268,6 +3268,15 @@ Instruction *InstCombiner::visitCallInst(CallInst &CI) {
   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) {
diff --git a/lib/Transforms/InstCombine/InstCombineInternal.h b/lib/Transforms/InstCombine/InstCombineInternal.h
index 9c684f11087..9f16a6f6e1e 100644
--- a/lib/Transforms/InstCombine/InstCombineInternal.h
+++ b/lib/Transforms/InstCombine/InstCombineInternal.h
@@ -289,6 +289,7 @@ public:
   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);
diff --git a/test/Transforms/InstCombine/consecutive-fences.ll b/test/Transforms/InstCombine/consecutive-fences.ll
new file mode 100644
index 00000000000..6f1c4127738
--- /dev/null
+++ b/test/Transforms/InstCombine/consecutive-fences.ll
@@ -0,0 +1,47 @@
+; 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
+}
-- 
2.50.1