[llvm] DAG: Preserve poison in some cast folds (PR #225403)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm created https://github.com/llvm/llvm-project/pull/225403

None

>From 2d7dff031edc7e2f58e65856e7169eb9fe2dce46 Mon Sep 17 00:00:00 2001
From: Matt Arsenault <Matthew.Arsenault at amd.com>
Date: Wed, 1 Jul 2026 13:30:22 +0200
Subject: [PATCH] DAG: Preserve poison in some cast folds

---
 llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp | 38 ++++++++++++-------
 1 file changed, 24 insertions(+), 14 deletions(-)

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 17e2ab01bc11f..e86514aed9410 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -16736,14 +16736,24 @@ SDValue DAGCombiner::visitZERO_EXTEND(SDNode *N) {
   return SDValue();
 }
 
+/// For a unary operation, propagate poison or undef from the operand \p N0 to
+/// the result type \p VT. Returns an empty SDValue if \p N0 is neither.
+static SDValue propagateUnaryUndef(SelectionDAG &DAG, SDValue N0, EVT VT) {
+  if (N0.getOpcode() == ISD::POISON)
+    return DAG.getPOISON(VT);
+  if (N0.getOpcode() == ISD::UNDEF)
+    return DAG.getUNDEF(VT);
+  return SDValue();
+}
+
 SDValue DAGCombiner::visitANY_EXTEND(SDNode *N) {
   SDValue N0 = N->getOperand(0);
   EVT VT = N->getValueType(0);
   SDLoc DL(N);
 
-  // aext(undef) = undef
-  if (N0.isUndef())
-    return DAG.getUNDEF(VT);
+  // aext(undef) = undef, aext(poison) = poison
+  if (SDValue R = propagateUnaryUndef(DAG, N0, VT))
+    return R;
 
   if (SDValue Res = tryToFoldExtendOfConstant(N, DL, TLI, DAG, LegalTypes))
     return Res;
@@ -17764,9 +17774,9 @@ SDValue DAGCombiner::visitTRUNCATE(SDNode *N) {
   bool isLE = DAG.getDataLayout().isLittleEndian();
   SDLoc DL(N);
 
-  // trunc(undef) = undef
-  if (N0.isUndef())
-    return DAG.getUNDEF(VT);
+  // trunc(undef) = undef, trunc(poison) = poison
+  if (SDValue R = propagateUnaryUndef(DAG, N0, VT))
+    return R;
 
   // fold (truncate (truncate x)) -> (truncate x)
   if (N0.getOpcode() == ISD::TRUNCATE)
@@ -18305,8 +18315,8 @@ SDValue DAGCombiner::visitBITCAST(SDNode *N) {
   SDValue N0 = N->getOperand(0);
   EVT VT = N->getValueType(0);
 
-  if (N0.isUndef())
-    return DAG.getUNDEF(VT);
+  if (SDValue R = propagateUnaryUndef(DAG, N0, VT))
+    return R;
 
   // If the input is a BUILD_VECTOR with all constant elements, fold this now.
   // Only do this before legalize types, unless both types are integer and the
@@ -20792,8 +20802,8 @@ SDValue DAGCombiner::visitFP_TO_SINT(SDNode *N) {
   SDLoc DL(N);
 
   // fold (fp_to_sint undef) -> undef
-  if (N0.isUndef())
-    return DAG.getUNDEF(VT);
+  if (SDValue R = propagateUnaryUndef(DAG, N0, VT))
+    return R;
 
   // fold (fp_to_sint c1fp) -> c1
   if (SDValue C = DAG.FoldConstantArithmetic(ISD::FP_TO_SINT, DL, VT, {N0}))
@@ -20808,8 +20818,8 @@ SDValue DAGCombiner::visitFP_TO_UINT(SDNode *N) {
   SDLoc DL(N);
 
   // fold (fp_to_uint undef) -> undef
-  if (N0.isUndef())
-    return DAG.getUNDEF(VT);
+  if (SDValue R = propagateUnaryUndef(DAG, N0, VT))
+    return R;
 
   // fold (fp_to_uint c1fp) -> c1
   if (SDValue C = DAG.FoldConstantArithmetic(ISD::FP_TO_UINT, DL, VT, {N0}))
@@ -20824,8 +20834,8 @@ SDValue DAGCombiner::visitXROUND(SDNode *N) {
 
   // fold (lrint|llrint undef) -> undef
   // fold (lround|llround undef) -> undef
-  if (N0.isUndef())
-    return DAG.getUNDEF(VT);
+  if (SDValue R = propagateUnaryUndef(DAG, N0, VT))
+    return R;
 
   // fold (lrint|llrint c1fp) -> c1
   // fold (lround|llround c1fp) -> c1