[llvm] [MemProf] Fix assert when exists direct recursion (PR #78264)

[Teresa Johnson via llvm-commits]

================
@@ -3470,17 +3470,19 @@ bool MemProfContextDisambiguation::applyImport(Module &M) {
             auto *MIBMD = cast<const MDNode>(MDOp);
             MDNode *StackMDNode = getMIBStackNode(MIBMD);
             assert(StackMDNode);
-            SmallVector<unsigned> StackIdsFromMetadata;
             CallStack<MDNode, MDNode::op_iterator> StackContext(StackMDNode);
-            for (auto ContextIter =
-                     StackContext.beginAfterSharedPrefix(CallsiteContext);
+            auto ContextIterBegin =
+                StackContext.beginAfterSharedPrefix(CallsiteContext);
+            // Skip the checking on the first iteration.
+            uint64_t LastStackContextId = *ContextIterBegin == 0 ? 1 : 0;
----------------
teresajohnson wrote:

I ran into an assert in the operator* method because ContextIterBegin was already at StackContext.end() (after skipping the shared prefix above). I think this should be:
```
uint64_t LastStackContextId = (ContextIter != StackContext.end() && *ContextIterBegin == 0) ? 1 : 0;
```
Can you send a fix?


https://github.com/llvm/llvm-project/pull/78264