Re: [PATCH] MIPS bit field failures in gdb.base/store.exp

Luis Machado
2014-09-29T12:36:24Z

Hi,

On 09/26/2014 12:16 PM, Pedro Alves wrote:
> On 09/25/2014 08:31 PM, Luis Machado wrote:
>
>> ping! Any ideas on different approaches suitable for this problem or is
>> the proposed fix ok (with either passing a value struct or a bit size)?
>
> Sorry, it's not easy to have a quick opinion without thinking this
> through...
>
> So, in value_assign, the case in question, we see:
>
> gdbarch = get_frame_arch (frame);
> if (gdbarch_convert_register_p (gdbarch, VALUE_REGNUM (toval), type))
> {
> /* If TOVAL is a special machine register requiring
> conversion of program values to a special raw
> format. */
> gdbarch_value_to_register (gdbarch, frame,
> VALUE_REGNUM (toval), type,
> value_contents (fromval));
> }
>
> Notice how gdbarch_value_to_register takes the fromval's contents
> as a buffer, only, and isn't passed down anything that would make it
> possible to find out whether it's writing to a bitfield, so that
> the implementation could do a read-modify-write itself and
> write to the proper bitfield offset.
>
> So, it seems to me that until we find an arch that needs to handle
> bitfields especially (I'm having trouble imagining why that
> would be necessary), we should just change value_assign's
> lval_register handling from:
>
> if (gdbarch_convert_register_p (gdbarch, VALUE_REGNUM (toval), type))
> {
> gdbarch_value_to_register ();
> }
> else
> {
> if (value_bitsize (toval))
> {
> // read-modify-write
> }
> else
> {
> put_frame_register_bytes ();
> }
> }
>
> to:
>
> if (value_bitsize (toval))
> {
> // read-modify-write
> }
> else
> {
> if (gdbarch_convert_register_p (gdbarch, VALUE_REGNUM (toval), type))
> {
> gdbarch_value_to_register ();
> }
> else
> {
> put_frame_register_bytes ();
> }
> }

Though a bit less generic, that also seems to be a reasonable solution
for now, and it fixes the failures i saw for MIPS. Out of the top of my
head i also don't recall a target that handles bit fields in a special
way. Should i go with this patch for the next submission or do you want
to author it?

Thanks,
Luis