[llvm] [TableGen] Fix concatenation of subreg and artificial subregs (PR #114391)

[Jay Foad via llvm-commits]

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+// RUN: llvm-tblgen -gen-register-info -register-info-debug -I %p/../../include %s -o /dev/null 2>&1 | FileCheck %s --check-prefix=CHECK
+include "llvm/Target/Target.td"
+
+// This file tests that when using `isArtificial` for subregisters in
+// combination with `CoveredBySubRegs`, that TableGen infers the
+// correct register classes, subregisters and lane masks, especially
+// when the registers (that consist partially from artificial subregs)
+// are used in tuples.
+//
+// The register hierarchy that this test implements is:
+//
+//                  ssub_hi   ssub
+//                      \    /
+//                       dsub
+//             dsub_hi     /
+//                 \      /
+//                   qsub
+//
+// Where the _hi parts are artificial and where ubregs ssub, dsub and qsub
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jayfoad wrote:

Type "ubregs"

https://github.com/llvm/llvm-project/pull/114391