[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 #14 from Xi Ruoyao <xry111 at gcc dot gnu.org> ---
(In reply to Andreas Schwab from comment #10)
> Or "other ABI-mandated fixed roles".  This also includes return value
> registers.

Hmm, even "ABI-mandated fixed roles" is not enough.  For example, or RISC-V we
have

extern int foo(char *);

int test()
{
    char x[12345678];
    return foo(x);
}

Then t0 is used by the function prologue, and -ffixed-t0 cannot stop it
(because the use of t0 is hard coded in the target hook).