[llvm] [SelectionDAG][X86] Fold `sub(x, mul(divrem(x,y)[0], y))` to `divrem(x, y)[1]` (PR #136565)

[Matt Arsenault via llvm-commits]

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@@ -3867,6 +3867,60 @@ static SDValue foldSubCtlzNot(SDNode *N, SelectionDAG &DAG) {
   return Matcher.getNode(ISD::CTLZ_ZERO_UNDEF, DL, VT, Not);
 }
 
+// Fold sub(x, mul(divrem(x,y)[0], y)) to divrem(x, y)[1]
+static SDValue foldSubOfQuotientToRem(SDNode *N, SelectionDAG &DAG) {
+  assert(N->getOpcode() == ISD::SUB && "Node must be a SUB");
+  SDValue Sub0 = N->getOperand(0);
+  SDValue Sub1 = N->getOperand(1);
+  SDLoc DL(N);
+
+  auto CheckAndFoldMulCase = [&](SDValue DivRem, SDValue MaybeY) -> SDValue {
+    if ((DivRem.getOpcode() == ISD::SDIVREM ||
+         DivRem.getOpcode() == ISD::UDIVREM) &&
+        DivRem.getResNo() == 0 && DivRem.getOperand(0) == Sub0 &&
+        DivRem.getOperand(1) == MaybeY) {
+      return SDValue(DivRem.getNode(), 1);
+    }
+    return SDValue();
+  };
+
+  if (Sub1.getOpcode() == ISD::MUL) {
+    // (sub x, (mul divrem(x,y)[0], y))
+    SDValue Mul0 = Sub1.getOperand(0);
+    SDValue Mul1 = Sub1.getOperand(1);
+
+    SDValue Res = CheckAndFoldMulCase(Mul0, Mul1);
+    if (Res.getNode())
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arsenm wrote:

Don't need .getNode() 

https://github.com/llvm/llvm-project/pull/136565