[llvm] DAG: Use poison for some vector element folds (PR #225406)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm updated https://github.com/llvm/llvm-project/pull/225406

>From 410c754db84fedc65265310ed24f6e0eaa8d2641 Mon Sep 17 00:00:00 2001
From: Matt Arsenault <Matthew.Arsenault at amd.com>
Date: Wed, 1 Jul 2026 13:42:07 +0200
Subject: [PATCH 1/2] DAG: Use poison for some vector element folds

---
 llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 4f20198244883..eea52a2dcf4fb 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -25914,7 +25914,10 @@ SDValue DAGCombiner::visitEXTRACT_VECTOR_ELT(SDNode *N) {
   SDValue Index = N->getOperand(1);
   EVT ScalarVT = N->getValueType(0);
   EVT VecVT = VecOp.getValueType();
-  if (VecOp.isUndef())
+  if (VecOp.getOpcode() == ISD::POISON)
+    return DAG.getPOISON(ScalarVT);
+
+  if (VecOp.getOpcode() == ISD::UNDEF)
     return DAG.getUNDEF(ScalarVT);
 
   // extract_vector_elt (insert_vector_elt vec, val, idx), idx) -> val
@@ -26063,9 +26066,9 @@ SDValue DAGCombiner::visitEXTRACT_VECTOR_ELT(SDNode *N) {
     // Find the new index to extract from.
     int OrigElt = Shuf->getMaskElt(IndexC->getZExtValue());
 
-    // Extracting an undef index is undef.
+    // Extracting an undef index is poison.
     if (OrigElt == -1)
-      return DAG.getUNDEF(ScalarVT);
+      return DAG.getPOISON(ScalarVT);
 
     // Select the right vector half to extract from.
     SDValue SVInVec;
@@ -26262,7 +26265,7 @@ SDValue DAGCombiner::visitEXTRACT_VECTOR_ELT(SDNode *N) {
 
   // If Idx was -1 above, Elt is going to be -1, so just return undef.
   if (Elt == -1)
-    return DAG.getUNDEF(LVT);
+    return DAG.getPOISON(LVT);
 
   if (SDValue Scalarized =
           TLI.scalarizeExtractedVectorLoad(LVT, DL, VecVT, Index, LN0, DAG)) {

>From 36797affb52a1df4e0197c43b4b97de1177ddc60 Mon Sep 17 00:00:00 2001
From: Matt Arsenault <Matthew.Arsenault at amd.com>
Date: Mon, 28 Sep 2026 16:45:51 +0200
Subject: [PATCH 2/2] helper

---
 llvm/include/llvm/CodeGen/SelectionDAG.h      | 5 +++++
 llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp | 7 ++-----
 2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/llvm/include/llvm/CodeGen/SelectionDAG.h b/llvm/include/llvm/CodeGen/SelectionDAG.h
index 764858702b732..0f4a035424bd1 100644
--- a/llvm/include/llvm/CodeGen/SelectionDAG.h
+++ b/llvm/include/llvm/CodeGen/SelectionDAG.h
@@ -1174,6 +1174,11 @@ class SelectionDAG {
   /// Return a POISON node. POISON does not have a useful SDLoc.
   SDValue getPOISON(EVT VT) { return getNode(ISD::POISON, SDLoc(), VT); }
 
+  SDValue getUNDEFOrPOISON(SDValue V) {
+    assert(V.isUndef() && "UNDEF/POISON node expected");
+    return getNode(V.getOpcode(), SDLoc(), V.getValueType());
+  }
+
   /// Return a node that represents the runtime scaling 'MulImm * RuntimeVL'.
   LLVM_ABI SDValue getVScale(const SDLoc &DL, EVT VT, APInt MulImm);
 
diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index eea52a2dcf4fb..1a0cdf3976699 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -25914,11 +25914,8 @@ SDValue DAGCombiner::visitEXTRACT_VECTOR_ELT(SDNode *N) {
   SDValue Index = N->getOperand(1);
   EVT ScalarVT = N->getValueType(0);
   EVT VecVT = VecOp.getValueType();
-  if (VecOp.getOpcode() == ISD::POISON)
-    return DAG.getPOISON(ScalarVT);
-
-  if (VecOp.getOpcode() == ISD::UNDEF)
-    return DAG.getUNDEF(ScalarVT);
+  if (VecOp.isUndef())
+    return DAG.getUNDEFOrPOISON(VecOp);
 
   // extract_vector_elt (insert_vector_elt vec, val, idx), idx) -> val
   //