From db63949e8dd561e2430bf864b22081b4bc588160 Mon Sep 17 00:00:00 2001
From: Justin Lebar <jlebar@google.com>
Date: Fri, 12 Feb 2016 21:01:36 +0000
Subject: [PATCH] [SimplifyCFG] Don't fold conditional branches that contain
 calls to convergent functions.

Summary:
Performing this optimization duplicates the call to the convergent
function and adds new control-flow dependencies, which is a no-no.

Reviewers: jingyue

Subscribers: broune, hfinkel, tra, resistor, joker.eph, arsenm, llvm-commits, mzolotukhin

Differential Revision: http://reviews.llvm.org/D17128

llvm-svn: 260730
---
 llvm/lib/Transforms/Utils/SimplifyCFG.cpp     | 20 ++++---------
 .../Transforms/SimplifyCFG/attr-convergent.ll | 28 +++++++++++++++++++
 2 files changed, 34 insertions(+), 14 deletions(-)
 create mode 100644 llvm/test/Transforms/SimplifyCFG/attr-convergent.ll

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index f88334a05ea6..3c8317252ee8 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -1690,19 +1690,6 @@ static bool SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *ThenBB,
   return true;
 }
 
-/// \returns True if this block contains a CallInst with the NoDuplicate
-/// attribute.
-static bool HasNoDuplicateCall(const BasicBlock *BB) {
-  for (BasicBlock::const_iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
-    const CallInst *CI = dyn_cast<CallInst>(I);
-    if (!CI)
-      continue;
-    if (CI->cannotDuplicate())
-      return true;
-  }
-  return false;
-}
-
 /// Return true if we can thread a branch across this block.
 static bool BlockIsSimpleEnoughToThreadThrough(BasicBlock *BB) {
   BranchInst *BI = cast<BranchInst>(BB->getTerminator());
@@ -1747,7 +1734,12 @@ static bool FoldCondBranchOnPHI(BranchInst *BI, const DataLayout &DL) {
   // Now we know that this block has multiple preds and two succs.
   if (!BlockIsSimpleEnoughToThreadThrough(BB)) return false;
 
-  if (HasNoDuplicateCall(BB)) return false;
+  // Can't fold blocks that contain noduplicate or convergent calls.
+  if (llvm::any_of(*BB, [](const Instruction &I) {
+        const CallInst *CI = dyn_cast<CallInst>(&I);
+        return CI && (CI->cannotDuplicate() || CI->isConvergent());
+      }))
+    return false;
 
   // Okay, this is a simple enough basic block.  See if any phi values are
   // constants.
diff --git a/llvm/test/Transforms/SimplifyCFG/attr-convergent.ll b/llvm/test/Transforms/SimplifyCFG/attr-convergent.ll
new file mode 100644
index 000000000000..a5f363d055a6
--- /dev/null
+++ b/llvm/test/Transforms/SimplifyCFG/attr-convergent.ll
@@ -0,0 +1,28 @@
+; RUN: opt < %s -simplifycfg -S | FileCheck %s
+
+; Checks that the SimplifyCFG pass won't duplicate a call to a function marked
+; convergent.
+;
+; CHECK: call void @barrier
+; CHECK-NOT: call void @barrier
+define void @check(i1 %cond, i32* %out) {
+entry:
+  br i1 %cond, label %if.then, label %if.end
+
+if.then:
+  store i32 5, i32* %out
+  br label %if.end
+
+if.end:
+  %x = phi i1 [ true, %entry ], [ false, %if.then ]
+  call void @barrier()
+  br i1 %x, label %cond.end, label %cond.false
+
+cond.false:
+  br label %cond.end
+
+cond.end:
+  ret void
+}
+
+declare void @barrier() convergent