[llvm] [AArch64][GlobalISel] Take abs scalar codegen closer to SDAG (PR #84886)

[Thorsten Schütt via llvm-commits]

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@@ -3914,8 +3914,12 @@ LegalizerHelper::lower(MachineInstr &MI, unsigned TypeIdx, LLT LowerHintTy) {
   case G_SSHLSAT:
   case G_USHLSAT:
     return lowerShlSat(MI);
-  case G_ABS:
+  case G_ABS: {
+    LLT Ty = MRI.getType(MI.getOperand(0).getReg());
+    if (Ty.isScalar() &&  LI.isLegalOrCustom({G_ICMP, {Ty, Ty}}))
+        return lowerAbsToCNeg(MI);
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tschuett wrote:

My main point is that we should do something akin to the DAG. We check for the legality of icmp and then select a select without checking whether it is legal or not. The icmp check is insufficient.

https://github.com/llvm/llvm-project/pull/84886