[llvm] [clang] [SPARC] Support reserving arbitrary general purpose registers (PR #74927)

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koachan wrote:

> Doesn’t the ABI require you to emit magic STT_REGISTER or whatever they are symbols to mark this?

Yeah, when targeting 64-bit and we're *not* reserving %g2, %g3, %g6, or %g7 then we should emit STT_SPARC_REGISTER entries for those registers.
However, currently we never emit any of those in the first place. This should be fixed but I think it is better done as a separate PR?

https://github.com/llvm/llvm-project/pull/74927