Re: [PATCH] gdb/tdesc: Don't assign custom-group tdesc registers to 'general'

[Luis Machado <luis.machado@arm.com>]

Hi,

On 9/7/23 18:07, Ciaran Woodward wrote:
> The 'Target Description' mechanism in GDB enables the target to
> supply the full set of registers available on the system to gdb
> in an XML format.
> 
> This format enables setting the 'group' of each register, such
> that they can be queried using the 'info registers <group>'
> mechanism.
> 
> However prior to this change, even if a register was explicitly
> assigned to a group, it would still show up in the
> 'info registers general' report. This is unexpected, and also
> disagrees with the comment above the tdesc_register_in_reggroup_p
> function, which says that '-1' should be returned if the register
> group is not-known, not the register group is known, but differs.
> 
> There was a previous change that did address this issue in
> aa66aac47b4dd38f9524ddb5546c08cc09930d37
> but it also caused registers with *no* group in the target
> description to be removed from 'general', so it was reverted in
> 440cf44eb0f70830b8d8ac35289f84129c7a35c1
> as that behaviour was used by some targets.
> 
> The change in this commit enhances the usefulness of the tdesc
> 'group' attribute for adding system configuration registers,
> of which there may be hundreds - very inconvenient to request
> and print on every 'info registers' call.
> ---
> 
> Email archive link to discussion of previously referenced patch:
> https://inbox.sourceware.org/gdb-patches/20200120155315.30333-1-shahab.vahedi@gmail.com/T/#m0ce2983153b2f482b66461ed6b97c4b287b09a89
> 
>  gdb/target-descriptions.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/gdb/target-descriptions.c b/gdb/target-descriptions.c
> index cdedf88c793..c1bb03c3adf 100644
> --- a/gdb/target-descriptions.c
> +++ b/gdb/target-descriptions.c
> @@ -954,14 +954,13 @@ tdesc_register_in_reggroup_p (struct gdbarch *gdbarch, int regno,
>  {
>    struct tdesc_reg *reg = tdesc_find_register (gdbarch, regno);
>  
> -  if (reg != NULL && !reg->group.empty ()
> -      && (reg->group == reggroup->name ()))
> -	return 1;
> -
>    if (reg != NULL
>        && (reggroup == save_reggroup || reggroup == restore_reggroup))
>      return reg->save_restore;
>  
> +  if (reg != NULL && !reg->group.empty ())
> +    return (reg->group == reggroup->name ());
> +
>    return -1;
>  }
>  

Yeah, this is a hard one. Unfortunately the outcome then was that we have a number of debugging stubs out in the open that advertise xml's with less-than-correct group information.

I think even gdb, to this day, is guilty of that.

Though the patch doesn't regress things as much as before, it still makes the 32-bit Arm gdb drop the fpscr register from the general display (info registers).

Depending on what others think, and the benefit of not having a bunch of system registers show up every time, we could cope with this particular regression.