[llvm] [DAGCombiner] Fold subtraction if above threshold to `umin` (PR #134235)

[Craig Topper via llvm-commits]

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@@ -4247,6 +4247,19 @@ SDValue DAGCombiner::visitSUB(SDNode *N) {
       sd_match(N1, m_UMaxLike(m_Specific(A), m_Specific(B))))
     return DAG.getNegative(DAG.getNode(ISD::ABDU, DL, VT, A, B), DL, VT);
 
+  // (sub x, (select (ult x, y), 0, y)) -> (umin x, (sub x, y))
+  auto LK = TLI.getTypeConversion(*DAG.getContext(), VT);
+  if ((LK.first == TargetLoweringBase::TypeLegal ||
+       LK.first == TargetLoweringBase::TypePromoteInteger) &&
+      TLI.isOperationLegal(ISD::UMIN, LK.second)) {
+    SDValue Y;
+    if (sd_match(N1, m_OneUse(m_Select(m_SetCC(m_Specific(N0), m_Value(Y),
+                                               m_SpecificCondCode(ISD::SETULT)),
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topperc wrote:

Here is a test where InstCombine doesn't change the uge to ult https://godbolt.org/z/oTafcozaG

https://github.com/llvm/llvm-project/pull/134235