[llvm] [OpenMPOpt] avoid OOB array write (PR #178686)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Jameson Nash (vtjnash)

<details>
<summary>Changes</summary>

When analysis reaches code with UB at runtime, this write needs to be ignored to avoid corrupting memory with UB at compile time.

Drive-by finding during review of
https://github.com/llvm/llvm-project/pull/178356.

---
Full diff: https://github.com/llvm/llvm-project/pull/178686.diff


1 Files Affected:

- (modified) llvm/lib/Transforms/IPO/OpenMPOpt.cpp (+5-2) 


``````````diff
diff --git a/llvm/lib/Transforms/IPO/OpenMPOpt.cpp b/llvm/lib/Transforms/IPO/OpenMPOpt.cpp
index 1f52f9a78b907..f415fa3cfcf97 100644
--- a/llvm/lib/Transforms/IPO/OpenMPOpt.cpp
+++ b/llvm/lib/Transforms/IPO/OpenMPOpt.cpp
@@ -909,8 +909,11 @@ struct OffloadArray {
           GetPointerBaseWithConstantOffset(S->getPointerOperand(), Offset, DL);
       if (Dst == &Array) {
         int64_t Idx = Offset / PointerSize;
-        StoredValues[Idx] = getUnderlyingObject(S->getValueOperand());
-        LastAccesses[Idx] = S;
+        // Ignore updates that must be UB (probably in dead code at runtime)
+        if ((uint64_t)Idx < NumValues) {
+          StoredValues[Idx] = getUnderlyingObject(S->getValueOperand());
+          LastAccesses[Idx] = S;
+        }
       }
     }
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/178686