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

[jlebar]

Summary

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

[jingyue]

test/Transforms/SimplifyCFG/attr-convergent.ll

21 ↗

(On Diff #47596)

This test doesn't directly reflect your change because %cmp is not a PHINode. In fact, SimplifyCFG optimizes %cmp to PHINode before it worries about whether @barrier is convergent.

It would be more straight-forward to write a test where the containing block of @barrier directly branches at a PHINode directly.

[jlebar]

Simplify test.

This is a lot better, thank you for the suggestion.

[jingyue]

Lgtm

llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp

for (BasicBlock::iterator I = EndBB->begin();
     PN->setIncomingValue(OrigI, V);
     PN->setIncomingValue(ThenI, V);
   }
 
   ++NumSpeculations;
   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());
   unsigned Size = 0;
 
   for (BasicBlock::iterator BBI = BB->begin(); &*BBI != BI; ++BBI) {
     if (isa<DbgInfoIntrinsic>(BBI))
       continue;
static bool FoldCondBranchOnPHI(BranchInst *BI, const DataLayout &DL) {
   if (PN->getNumIncomingValues() == 1) {
     FoldSingleEntryPHINodes(PN->getParent());
     return true;
   }
 
   // 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.
   for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
     ConstantInt *CB = dyn_cast<ConstantInt>(PN->getIncomingValue(i));
     if (!CB || !CB->getType()->isIntegerTy(1)) continue;
 
     // Okay, we now know that all edges from PredBB should be revectored to

llvm/trunk/test/Transforms/SimplifyCFG/attr-convergent.ll

+; 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