[llvm-branch-commits] [llvm] DAG: Preserve poison in more concat_vectors folds (PR #206948)

[Matt Arsenault via llvm-branch-commits]

https://github.com/arsenm created https://github.com/llvm/llvm-project/pull/206948

Avoid degrading poison to undef.

>From 82176c4829e1d2ea56404ee2a8b5d290adc9dc6b Mon Sep 17 00:00:00 2001
From: Matt Arsenault <Matthew.Arsenault at amd.com>
Date: Wed, 1 Jul 2026 13:01:29 +0200
Subject: [PATCH] DAG: Preserve poison in more concat_vectors folds

Avoid degrading poison to undef.
---
 llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 28160dcd8100a..34be92d3c847f 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -7024,7 +7024,9 @@ static SDValue foldCONCAT_VECTORS(const SDLoc &DL, EVT VT,
   SmallVector<SDValue, 16> Elts;
   for (SDValue Op : Ops) {
     EVT OpVT = Op.getValueType();
-    if (Op.isUndef())
+    if (Op.getOpcode() == ISD::POISON)
+      Elts.append(OpVT.getVectorNumElements(), DAG.getPOISON(SVT));
+    else if (Op.getOpcode() == ISD::UNDEF)
       Elts.append(OpVT.getVectorNumElements(), DAG.getUNDEF(SVT));
     else if (Op.getOpcode() == ISD::BUILD_VECTOR)
       Elts.append(Op->op_begin(), Op->op_end());
@@ -7043,7 +7045,9 @@ static SDValue foldCONCAT_VECTORS(const SDLoc &DL, EVT VT,
 
   if (SVT.bitsGT(VT.getScalarType())) {
     for (SDValue &Op : Elts) {
-      if (Op.isUndef())
+      if (Op.getOpcode() == ISD::POISON)
+        Op = DAG.getPOISON(SVT);
+      else if (Op.getOpcode() == ISD::UNDEF)
         Op = DAG.getUNDEF(SVT);
       else
         Op = DAG.getTargetLoweringInfo().isZExtFree(Op.getValueType(), SVT)