[llvm] [ARM] Have custom lowering for ucmp and scmp (PR #149315)

[David Green via llvm-commits]

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@@ -10614,6 +10624,181 @@ SDValue ARMTargetLowering::LowerFP_TO_BF16(SDValue Op,
   return DAG.getBitcast(MVT::i32, Res);
 }
 
+SDValue ARMTargetLowering::LowerSCMP(SDValue Op, SelectionDAG &DAG) const {
+  SDLoc dl(Op);
+  SDValue LHS = Op.getOperand(0);
+  SDValue RHS = Op.getOperand(1);
+
+  // For the ARM assembly pattern:
+  // subs r0, r0, r1   ; subtract RHS from LHS and set flags
+  // movgt r0, #1      ; if LHS > RHS, set result to 1
+  // mvnlt r0, #0      ; if LHS < RHS, set result to -1 (mvn #0 = -1)
+  //                   ; if LHS == RHS, result remains 0 from the subs
+
+  // Optimization: if RHS is a subtraction against 0, use ADDC instead of SUBC
+  // Check if RHS is (0 - something), and if so use ADDC with LHS + something
+  SDValue SubResult, Flags;
+  bool CanUseAdd = false;
+  SDValue AddOperand;
+
+  // Check if RHS is a subtraction against 0: (0 - X)
+  if (RHS.getOpcode() == ISD::SUB) {
+    SDValue SubLHS = RHS.getOperand(0);
+    SDValue SubRHS = RHS.getOperand(1);
+
+    // Check if it's 0 - X
+    if (isNullConstant(SubLHS)) {
+      // For SCMP: only if X is known to never be INT_MIN (to avoid overflow)
+      if (RHS->getFlags().hasNoSignedWrap() || !DAG.computeKnownBits(SubRHS)
+                                                    .getSignedMinValue()
+                                                    .isMinSignedValue()) {
+        CanUseAdd = true;
----------------
davemgreen wrote:

Maybe remove CanUseAdd and check for if (AddOperand) below? Or maybe just set `Opcode = ARMISD::SUBC;`, then update it to ARMISD::ADDC in the if, so the code for creating AddWithFlags or SubWithFlags can be shared.

https://github.com/llvm/llvm-project/pull/149315