[llvm] [AArch64] Use isKnownNonZero to optimize eligible compares to cmn and ccmn (PR #96349)

[via llvm-commits]

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@@ -3566,13 +3574,21 @@ static SDValue emitConditionalComparison(SDValue LHS, SDValue RHS,
       Opcode = AArch64ISD::CCMN;
       RHS = DAG.getConstant(Imm.abs(), DL, Const->getValueType(0));
     }
-  } else if (RHS.getOpcode() == ISD::SUB) {
-    SDValue SubOp0 = RHS.getOperand(0);
-    if (isNullConstant(SubOp0) && (CC == ISD::SETEQ || CC == ISD::SETNE)) {
-      // See emitComparison() on why we can only do this for SETEQ and SETNE.
-      Opcode = AArch64ISD::CCMN;
-      RHS = RHS.getOperand(1);
-    }
+  } else if (isCMN(RHS, CC, DAG)) {
+    Opcode = AArch64ISD::CCMN;
+    RHS = RHS.getOperand(1);
+  } else if (isCMN(LHS, CC, DAG) &&
+             (isIntEqualitySetCC(CC) ||
+              (isUnsignedIntSetCC(CC) && DAG.isKnownNeverZero(RHS)) ||
+              (isSignedIntSetCC(CC) && cannotBeIntMin(RHS, DAG)))) {
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AtariDreams wrote:

Negation of both sides flips the condition and the swap flips it back. I did this because the CC isn't used in this method except for testing for eligibility, and this is much more feasible than refactoring so that we recompute the condition codes and arm condition codes. 

https://github.com/llvm/llvm-project/pull/96349