[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:

Proof: 

It is known that if a > b, then b < a,

it is known that if b < a, then -b > -a

For the degenerative cases of INT_MIN and 0 for signed and unsigned comparisons, this is where knownbits steps in: https://alive2.llvm.org/ce/z/8YTkie



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