[PATCH] D17128: [SimplifyCFG] Don't fold conditional branches that contain calls to convergent functions.

[Justin Lebar via llvm-commits]

jlebar created this revision.
jlebar added a reviewer: jingyue.
jlebar added subscribers: mzolotukhin, llvm-commits, arsenm, joker.eph, resistor, tra, hfinkel, broune.

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

http://reviews.llvm.org/D17128

Files:
  lib/Transforms/Utils/SimplifyCFG.cpp
  test/Transforms/SimplifyCFG/attr-convergent.ll

Index: test/Transforms/SimplifyCFG/attr-convergent.ll
===================================================================
--- /dev/null
+++ test/Transforms/SimplifyCFG/attr-convergent.ll
@@ -0,0 +1,33 @@
+; 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(i32 %cond, i32* %out) {
+entry:
+  %out1 = getelementptr i32, i32* %out, i32 1
+  %out2 = getelementptr i32, i32* %out, i32 2
+  %cmp = icmp eq i32 %cond, 0
+  br i1 %cmp, label %if.then, label %if.end
+
+if.then:
+  store i32 5, i32* %out
+  br label %if.end
+
+if.end:
+  call void @barrier()
+  br i1 %cmp, label %cond.end, label %cond.false
+
+cond.false:
+  store i32 5, i32* %out1
+  br label %cond.end
+
+cond.end:
+  %value = phi i32 [ 1, %cond.false ], [ 0, %if.end ]
+  store i32 %value, i32* %out2
+  ret void
+}
+
+declare void @barrier() convergent
Index: lib/Transforms/Utils/SimplifyCFG.cpp
===================================================================
--- lib/Transforms/Utils/SimplifyCFG.cpp
+++ lib/Transforms/Utils/SimplifyCFG.cpp
@@ -1690,19 +1690,6 @@
   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 @@
   // 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.


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D17128.47596.patch
Type: text/x-patch
Size: 2378 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20160211/3abba87b/attachment.bin>