[llvm] [DAG] canCreateUndefOrPoison - out of range vector insert/extract element indices only generate poison (PR #196720)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon updated https://github.com/llvm/llvm-project/pull/196720

>From 7f748f6d5093a0df6b397c021c02a114135b4931 Mon Sep 17 00:00:00 2001
From: Simon Pilgrim <llvm-dev at redking.me.uk>
Date: Sat, 9 May 2026 13:45:58 +0100
Subject: [PATCH] [DAG] canCreateUndefOrPoison - out of range vector
 insert/extract element indices only generate poison

Matches ValueTracking / GISel implementations - although testing options are limited until DAG has actual uses of UndefPoisonKind::UndefOnly
---
 llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 0b7d8b7946f99..a221df567a10f 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -5993,10 +5993,13 @@ bool SelectionDAG::canCreateUndefOrPoison(SDValue Op, const APInt &DemandedElts,
   case ISD::INSERT_VECTOR_ELT:
   case ISD::EXTRACT_VECTOR_ELT: {
     // Ensure that the element index is in bounds.
-    EVT VecVT = Op.getOperand(0).getValueType();
-    SDValue Idx = Op.getOperand(Opcode == ISD::INSERT_VECTOR_ELT ? 2 : 1);
-    KnownBits KnownIdx = computeKnownBits(Idx, Depth + 1);
-    return KnownIdx.getMaxValue().uge(VecVT.getVectorMinNumElements());
+    if (includesPoison(Kind)) {
+      EVT VecVT = Op.getOperand(0).getValueType();
+      SDValue Idx = Op.getOperand(Opcode == ISD::INSERT_VECTOR_ELT ? 2 : 1);
+      KnownBits KnownIdx = computeKnownBits(Idx, Depth + 1);
+      return KnownIdx.getMaxValue().uge(VecVT.getVectorMinNumElements());
+    }
+    return false;
   }
 
   case ISD::VECTOR_SHUFFLE: {