[Bug target/109456] `-ffixed-reg` cannot prevent using `reg` for ABI-mandated roles (argument register etc) and the behavior should be documented more clearly

[xry111 at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109456

--- Comment #13 from Xi Ruoyao <xry111 at gcc dot gnu.org> ---
(In reply to gccriscvuser from comment #12)
> Updating the documentation is good, but there should also be an error
> diagnostic, right?

It would be a backward-incompatible change.  IMO it's perfectly legal to
disable a register for RA with -ffixed- but still allowing it for fixed roles. 
And I bet there are existing uses of this feature in the wild.