[llvm] [DAG] Fold (umin (sub a b) a) -> (usubo a b); (select usubo.1 a usubo.0) (PR #161651)

[Simon Pilgrim via llvm-commits]

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@@ -6199,6 +6199,28 @@ SDValue DAGCombiner::visitIMINMAX(SDNode *N) {
                                         SDLoc(N), VT, N0, N1))
     return SD;
 
+  // (umin (sub a, b) a) -> (usubo a, b); (select usubo.1, a, usubo.0)
+  //
+  // IR:
+  //   %sub  = sub %a, %b
+  //   %cond = umin %sub, %a
+  //   ->
+  //   %usubo    = usubo %a, %b
+  //   %overflow = extractvalue %usubo, 1
+  //   %sub      = extractvalue %usubo, 0
+  //   %cond     = select %overflow, %a, %sub
+  if (N0.getOpcode() == ISD::SUB) {
+    SDValue A, B, C;
+    if (sd_match(N, m_UMin(m_Sub(m_Value(A), m_Value(B)), m_Value(C)))) {
+      EVT AVT = A.getValueType();
+      if (A == C && TLI.isOperationLegalOrCustom(ISD::USUBO, AVT)) {
+        SDVTList VTs = DAG.getVTList(AVT, MVT::i1);
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RKSimon wrote:

MVT::i1 ?

https://github.com/llvm/llvm-project/pull/161651