[SimplifyCFG] Skip merging return blocks if it would break a CallBr.

[jonpa]

Summary

SimplifyCFG should not merge empty return blocks and leave a CallBr behind with a duplicated destination since the verifier will then trigger an assert. This patch checks for this case and avoids the transformation.

CodeGenPrepare has a similar check which also has a FIXME comment about why this is needed. It seems perhaps better if these two passes would eventually instead update the CallBr instruction instead of just checking and avoiding.

This fixes https://bugs.llvm.org/show_bug.cgi?id=45062.

[craig.topper]

LGTM

llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp

for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
     }
 
     // If this is the first returning block, remember it and keep going.
     if (!RetBlock) {
       RetBlock = &BB;
       continue;
     }
 
+    // Skip merging if this would result in a CallBr instruction with a
+    // duplicate destination. FIXME: See note in CodeGenPrepare.cpp.
+    bool SkipCallBr = false;
+    for (pred_iterator PI = pred_begin(&BB), E = pred_end(&BB);
+         PI != E && !SkipCallBr; ++PI) {
+      if (auto *CBI = dyn_cast<CallBrInst>((*PI)->getTerminator()))
+        for (unsigned i = 0, e = CBI->getNumSuccessors(); i != e; ++i)
+          if (RetBlock == CBI->getSuccessor(i)) {
+            SkipCallBr = true;
+            break;
+          }
+    }
+    if (SkipCallBr)
+      continue;
+
     // Otherwise, we found a duplicate return block.  Merge the two.
     Changed = true;
 
     // Case when there is no input to the return or when the returned values
     // agree is trivial.  Note that they can't agree if there are phis in the
     // blocks.
     if (Ret->getNumOperands() == 0 ||
         Ret->getOperand(0) ==

llvm/test/Transforms/SimplifyCFG/callbr-destinations.ll

+; RUN: opt < %s -simplifycfg -disable-output
+;
+; Test that SimplifyCFG does not cause CallBr instructions to have duplicate
+; destinations, which will cause the verifier to assert.
+
+define void @fun0() {
+entry:
+  callbr void asm sideeffect "", "X"(i8* blockaddress(@fun0, %bb1))
+          to label %bb2 [label %bb1]
+
+bb1:                                              ; preds = %bb
+  ret void
+
+bb2:                                             ; preds = %bb
+  ret void
+}
+
+define void @fun1() {
+entry:
+  callbr void asm sideeffect "", "X"(i8* blockaddress(@fun1, %bb1))
+          to label %bb2 [label %bb1]
+
+bb2:                                             ; preds = %bb
+  ret void
+
+bb1:                                              ; preds = %bb
+  ret void
+}