[flang] fix possible iterator underflow

[Originns]

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.

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[llvmbot]

@llvm/pr-subscribers-flang-semantics

Author: None (Originns)

Changes

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 --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();
 }

[Originns]

Ping

[Originns]

Ping

[kparzysz]

This will skip the loop when iter == begin, which still points to a valid element.

This is actually ok.

[kparzysz]

Ping

Please add specific reviewers first, otherwise your pings go to no one in particular...

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[Originns]

Ping @sscalpone