[llvm] efec798 - DAG: Preserve poison in some cast folds (#225403)

[via llvm-commits]

Author: Matt Arsenault
Date: 2026-09-22T17:55:08+02:00
New Revision: efec7984808669b2dc75fbfab7664658ff45f74a

URL: https://github.com/llvm/llvm-project/commit/efec7984808669b2dc75fbfab7664658ff45f74a
DIFF: https://github.com/llvm/llvm-project/commit/efec7984808669b2dc75fbfab7664658ff45f74a.diff

LOG: DAG: Preserve poison in some cast folds (#225403)

Added: 
    

Modified: 
    llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

Removed: 
    


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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