[AArch64] Signed comparison using CMN is safe when the subtraction is nsw

nsw means no signed wrap, and 0 - INT_MIN is a signed wrap.

Now, this is going to be a point I need to get out of the way:

So is it okay to always transform a > -b  into cmn if it is a signed comparison, even if b is INT_MIN because -INT_MIN is undefined, at least in C.

Taking advantage of UB doesn't introduce bugs. But, this could change behavior in programs with UB.

Author: AZero13

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

@@ -3337,8 +3337,13 @@ static bool isLegalArithImmed(uint64_t C) {
   return IsLegal;
 }
 
-static bool cannotBeIntMin(SDValue CheckedVal, SelectionDAG &DAG) {
-  KnownBits KnownSrc = DAG.computeKnownBits(CheckedVal);
+static bool isSafeSignedCMN(SDValue Op, SelectionDAG &DAG) {
+  // 0 - INT_MIN sign wraps, so no signed wrap means cmn.
+  if (Op->getFlags().hasNoSignedWrap())
+    return true;
+
+  // Maybe nsw was not set here...
+  KnownBits KnownSrc = DAG.computeKnownBits(Op.getOperand(1));
   return !KnownSrc.getSignedMinValue().isMinSignedValue();
 }
 
@@ -3356,7 +3361,7 @@ static bool isCMN(SDValue Op, ISD::CondCode CC, SelectionDAG &DAG) {
   return Op.getOpcode() == ISD::SUB && isNullConstant(Op.getOperand(0)) &&
          (isIntEqualitySetCC(CC) ||
           (isUnsignedIntSetCC(CC) && DAG.isKnownNeverZero(Op.getOperand(1))) ||
-          (isSignedIntSetCC(CC) && cannotBeIntMin(Op.getOperand(1), DAG)));
+          (isSignedIntSetCC(CC) && isSafeSignedCMN(Op, DAG)));
 }
 
 static SDValue emitStrictFPComparison(SDValue LHS, SDValue RHS, const SDLoc &dl,

llvm/test/CodeGen/AArch64/cmp-to-cmn.ll

@@ -434,8 +434,7 @@ entry:
 define i1 @cmn_nsw(i32 %a, i32 %b) {
 ; CHECK-LABEL: cmn_nsw:
 ; CHECK:       // %bb.0:
-; CHECK-NEXT:    neg w8, w1
-; CHECK-NEXT:    cmp w0, w8
+; CHECK-NEXT:    cmn w0, w1
 ; CHECK-NEXT:    cset w0, gt
 ; CHECK-NEXT:    ret
 %sub = sub nsw i32 0, %b
@@ -446,8 +445,7 @@ ret i1 %cmp
 define i1 @cmn_nsw_64(i64 %a, i64 %b) {
 ; CHECK-LABEL: cmn_nsw_64:
 ; CHECK:       // %bb.0:
-; CHECK-NEXT:    neg x8, x1
-; CHECK-NEXT:    cmp x0, x8
+; CHECK-NEXT:    cmn x0, x1
 ; CHECK-NEXT:    cset w0, gt
 ; CHECK-NEXT:    ret
 %sub = sub nsw i64 0, %b