[AArch64] Do not try to fold if getSwappedCondition(CC) has no swapped condition

[AZero13]

NFC for now, but if we simplify nodes where we can remove a compare with 0 and we are stuck with condition codes MI or PL, we are in trouble, as you cannot swap that.

[llvmbot]

@llvm/pr-subscribers-backend-aarch64

Author: AZero13 (AZero13)

Changes

NFC for now, but if we simplify nodes where we can remove a compare with 0 and we are stuck with condition codes MI or PL, we are in trouble, as you cannot swap that.

---

Full diff: https://github.com/llvm/llvm-project/pull/160753.diff

1 Files Affected:

• (modified) llvm/lib/Target/AArch64/AArch64ISelLowering.cpp (+4-1)

diff --git a/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp b/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
index a4c1e265f0e63..0e68d5b570c28 100644
--- a/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
+++ b/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
@@ -25764,8 +25764,11 @@ static SDValue reassociateCSELOperandsForCSE(SDNode *N, SelectionDAG &DAG) {
     // Try again with the operands of the SUBS instruction and the condition
     // swapped. Due to canonicalization, this only helps for non-constant
     // operands of the SUBS instruction.
+    auto NewCC = getSwappedCondition(CC);
+    if (NewCC == AArch64CC::AL)
+      return SDValue();
     std::swap(CmpOpToMatch, CmpOpOther);
-    if (SDValue R = Fold(getSwappedCondition(CC), CmpOpToMatch, CmpOpToMatch))
+    if (SDValue R = Fold(NewCC, CmpOpToMatch, CmpOpToMatch))
       return R;
     return SDValue();
   }

[AZero13]

@davemgreen

[AZero13]

@topperc