[llvm] [SelectionDAG] Return poison instead of undef for out-of-bounds EXTRACT_VECTOR_ELT (PR #192844)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-selectiondag

Author: Xinlong Chen (Xinlong-Chen)

<details>
<summary>Changes</summary>

Out-of-bounds EXTRACT_VECTOR_ELT on fixed-length vectors is undefined behavior. 

Return poison instead of undef to be consistent with LangRef semantics.

Prep work to help with https://github.com/llvm/llvm-project/pull/190307

---
Full diff: https://github.com/llvm/llvm-project/pull/192844.diff


1 Files Affected:

- (modified) llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp (+2-2) 


``````````diff
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 86245e4044925..6c8d7742a80e4 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -8379,12 +8379,12 @@ SDValue SelectionDAG::getNode(unsigned Opcode, const SDLoc &DL, EVT VT,
     if (N1.isUndef() || N2.isUndef())
       return getUNDEF(VT);
 
-    // EXTRACT_VECTOR_ELT of out-of-bounds element is an UNDEF for fixed length
+    // EXTRACT_VECTOR_ELT of out-of-bounds element is an POISON for fixed length
     // vectors. For scalable vectors we will provide appropriate support for
     // dealing with arbitrary indices.
     if (N2C && N1.getValueType().isFixedLengthVector() &&
         N2C->getAPIntValue().uge(N1.getValueType().getVectorNumElements()))
-      return getUNDEF(VT);
+      return getPOISON(VT);
 
     // EXTRACT_VECTOR_ELT of CONCAT_VECTORS is often formed while lowering is
     // expanding copies of large vectors from registers. This only works for

``````````

</details>


https://github.com/llvm/llvm-project/pull/192844