[llvm-branch-commits] [llvm] DAG: Preserve poison in all-undef build_vector/concat_vectors folds (PR #206947)

[Matt Arsenault via llvm-branch-commits]

https://github.com/arsenm created https://github.com/llvm/llvm-project/pull/206947

None

>From 08ed6143c0ab992e53ee57fdac5d1f1524262022 Mon Sep 17 00:00:00 2001
From: Matt Arsenault <Matthew.Arsenault at amd.com>
Date: Wed, 1 Jul 2026 12:48:04 +0200
Subject: [PATCH] DAG: Preserve poison in all-undef build_vector/concat_vectors
 folds

---
 llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp | 16 ++++++++++++----
 llvm/test/CodeGen/AMDGPU/vector-reduce-or.ll   |  1 -
 2 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index fa0b564b25006..28160dcd8100a 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -6934,8 +6934,12 @@ static SDValue FoldBUILD_VECTOR(const SDLoc &DL, EVT VT,
          "Incorrect element count in BUILD_VECTOR!");
 
   // BUILD_VECTOR of UNDEFs is UNDEF.
-  if (llvm::all_of(Ops, [](SDValue Op) { return Op.isUndef(); }))
-    return DAG.getUNDEF(VT);
+  bool AllPoison = true;
+  if (llvm::all_of(Ops, [&AllPoison](SDValue Op) {
+        AllPoison &= Op.getOpcode() == ISD::POISON;
+        return Op.isUndef();
+      }))
+    return AllPoison ? DAG.getPOISON(VT) : DAG.getUNDEF(VT);
 
   // BUILD_VECTOR of seq extract/insert from the same vector + type is Identity.
   SDValue IdentitySrc;
@@ -6976,8 +6980,12 @@ static SDValue foldCONCAT_VECTORS(const SDLoc &DL, EVT VT,
     return Ops[0];
 
   // Concat of UNDEFs is UNDEF.
-  if (llvm::all_of(Ops, [](SDValue Op) { return Op.isUndef(); }))
-    return DAG.getUNDEF(VT);
+  bool AllPoison = true;
+  if (llvm::all_of(Ops, [&AllPoison](SDValue Op) {
+        AllPoison &= Op.getOpcode() == ISD::POISON;
+        return Op.isUndef();
+      }))
+    return AllPoison ? DAG.getPOISON(VT) : DAG.getUNDEF(VT);
 
   // Scan the operands and look for extract operations from a single source
   // that correspond to insertion at the same location via this concatenation:
diff --git a/llvm/test/CodeGen/AMDGPU/vector-reduce-or.ll b/llvm/test/CodeGen/AMDGPU/vector-reduce-or.ll
index bf6babede48b5..3f11691def1b5 100644
--- a/llvm/test/CodeGen/AMDGPU/vector-reduce-or.ll
+++ b/llvm/test/CodeGen/AMDGPU/vector-reduce-or.ll
@@ -29,7 +29,6 @@ define i8 @test_vector_reduce_or_v2i8(<2 x i8> %v) {
 ; GFX7-SDAG-NEXT:    s_waitcnt vmcnt(0) expcnt(0) lgkmcnt(0)
 ; GFX7-SDAG-NEXT:    v_or_b32_e32 v0, v0, v1
 ; GFX7-SDAG-NEXT:    v_and_b32_e32 v0, 0xff, v0
-; GFX7-SDAG-NEXT:    v_or_b32_e32 v0, 0xff00, v0
 ; GFX7-SDAG-NEXT:    s_setpc_b64 s[30:31]
 ;
 ; GFX7-GISEL-LABEL: test_vector_reduce_or_v2i8: