[flang-commits] [flang] [flang] fix possible iterator underflow (PR #115754)

[via flang-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-flang-semantics

Author: None (Originns)

<details>
<summary>Changes</summary>

Replaced do-while with while loop in SemanticsContext::SearchScopeIndex(parser::CharBlock source) to prevent iterator underflow when upper_bound returns begin(). This ensures the iterator check happens before decrementing.

---
Full diff: https://github.com/llvm/llvm-project/pull/115754.diff


1 Files Affected:

- (modified) flang/lib/Semantics/semantics.cpp (+2-2) 


``````````diff
diff --git a/flang/lib/Semantics/semantics.cpp b/flang/lib/Semantics/semantics.cpp
index 58dc1f218b56f4..dac781566f1a8e 100644
--- a/flang/lib/Semantics/semantics.cpp
+++ b/flang/lib/Semantics/semantics.cpp
@@ -415,12 +415,12 @@ auto SemanticsContext::SearchScopeIndex(parser::CharBlock source)
   if (!scopeIndex_.empty()) {
     auto iter{scopeIndex_.upper_bound(source)};
     auto begin{scopeIndex_.begin()};
-    do {
+    while (iter != begin) {
       --iter;
       if (iter->first.Contains(source)) {
         return iter;
       }
-    } while (iter != begin);
+    }
   }
   return scopeIndex_.end();
 }

``````````

</details>


https://github.com/llvm/llvm-project/pull/115754