[llvm] [DAGCombiner] Preserve nneg flag from inner zext when we combine (z/s/aext (zext X)) (PR #82199)

[Craig Topper via llvm-commits]

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@@ -13710,8 +13710,12 @@ SDValue DAGCombiner::visitZERO_EXTEND(SDNode *N) {
 
   // fold (zext (zext x)) -> (zext x)
   // fold (zext (aext x)) -> (zext x)
-  if (N0.getOpcode() == ISD::ZERO_EXTEND || N0.getOpcode() == ISD::ANY_EXTEND)
-    return DAG.getNode(ISD::ZERO_EXTEND, DL, VT, N0.getOperand(0));
+  if (N0.getOpcode() == ISD::ZERO_EXTEND || N0.getOpcode() == ISD::ANY_EXTEND) {
+    SDNodeFlags Flags;
+    if (N0.getOpcode() == ISD::ZERO_EXTEND)
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topperc wrote:

nneg is about the sign bit of the input to the zero extend, not the output. It indicates that the zext could be a replaced with a sext.

https://github.com/llvm/llvm-project/pull/82199