[llvm] [AArch64] Use CMN instruction for negative SUBS immediates (PR #221398)

[Eli Friedman via llvm-commits]

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@@ -4218,6 +4218,18 @@ static bool canEmitConjunction(SelectionDAG &DAG, const SDValue Val,
     // of a SUB operation can be reused.
     PreferFirst = DAG.doesNodeExist(ISD::SUB, DAG.getVTList(VT),
                                     {Val->getOperand(0), Val->getOperand(1)});
+    // The first comparison supports the full 12-bit CMP/CMN immediate range,
+    // while subsequent CCMP/CCMN comparisons only support a 5-bit immediate [0,
+    // 31]. Prefer a wider-immediate comparison first to avoid materializing
+    // constants.
+    if (!PreferFirst && VT.isInteger()) {
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efriedma-quic wrote:

Yes, that's fine.

https://github.com/llvm/llvm-project/pull/221398