Re: [RFA] MIPS: Introduce struct mips_regnums and accessors

[Kevin Buettner <kevinb at redhat dot com>]

On May 22,  3:40pm, Andrew Cagney wrote:

> +  /* Raw register number initializations.  They are initialized in the
> +     same order that they appear in the struct to make it easier to
> +     verify that they're all initialized.  */
> +  tdep->raw_regnums.zero_regnum = 0;
> +  tdep->raw_regnums.v0_regnum = 2;
> +  tdep->raw_regnums.a0_regnum = 4;
> +  tdep->raw_regnums.t9_regnum = 25;
> +  tdep->raw_regnums.sp_regnum = 29;
> +  tdep->raw_regnums.ra_regnum = 31;
> 
> and, at least for v0_regnum, it doesn't change.  V0 is an offset in the 
> selected block of registers.  It could be either:
> 
> 	enum { V0_OFFSET = 2 };
> 
> 	cookednum->gp0 + V0_OFFSET
> or
> 	rawnum->gp0 + V0_OFFSET
> 
> however, either way, it doesn't change.  The only thing that changes is 
> things like where the general purpose, for floating point, registers start.

I have the following objections to the use of offsets:

    1) They don't work for floating point registers.  (That's assuming
       that I can convince you that it's desirable to have 16 cooked
       registers for o32...)  The reason that it won't work is that,
       e.g, an FPA0_OFFSET will need to have different values for the
       raw and cooked cases.
    2) The conversion from *_REGNUM is much less transparent.
    3) The use of offsets is more error prone.  There's nothing to
       prevent you from adding an offset to the wrong boundary start.
    4) The notation is more cumbersome.

Furthermore, I really don't understand the problem that you had with
my proposed layout.  Could you please explain the underlying reasons
for proposing the offset idea?  E.g, are you concerned about the
initialization and the maintenance thereof?  Or do you just want to
present a minimal view of the raw register numbers to clients outside
of mips-tdep.c?  Or something else altogether?

Kevin