Re: [PATCH 4/8] regcache: Zero-extend small registers described by a register map.

[Andrew Burgess <andrew.burgess@embecosm.com>]

* John Baldwin <jhb@FreeBSD.org> [2021-07-14 07:07:37 -0700]:

> When registers are supplied via regcache_supply_register from a register
> block described by a register map, registers may be stored in slots smaller
> than GDB's native register size (e.g. x86 segment registers are 16 bits,
> but the GDB registers for those are 32-bits).  regcache_collect_regset
> is careful to zero-extend slots larger than a register size, but
> regcache_supply_regset just used regcache::raw_supply_part and did not
> initialize the upper bytes of a register value.
> 
> trad_frame_set_reg_regmap assumes these semantics (zero-extending
> short registers) as I had misread the implementation of
> regcache::transfer_regset and assumed it zero-extended short
> registers.  In my specific use case (x86 segment registers stored as
> 16-bit values), I need the semantics of zero-extending a register
> value in a smaller slot.

I don't claim to know if anyone relies on the old behaviour of
transfer_regset_register, but the change you propose seems reasonable.

However, the second paragraph of your commit message really confuses
me.

It seems to say that a mistake was made in trad_frame_set_reg_regmap,
and so transfer_regset_register should change, then you just jump to
saying you need the zero extend.  I don't really understand the
connection between all these ideas.

Also, when you say "In my specific use case ....  I need the semantics
of zero-extending ...", but you don't say what your use case is, so if
I wanted to look at which piece of code is calling this for your
specific use case, I can't...

Thanks,
Andrew

> ---
>  gdb/regcache.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/gdb/regcache.c b/gdb/regcache.c
> index 672da0556f..396923c8a5 100644
> --- a/gdb/regcache.c
> +++ b/gdb/regcache.c
> @@ -1169,7 +1169,12 @@ regcache::transfer_regset_register (struct regcache *out_regcache, int regnum,
>  	memset (out_buf + offs + reg_size, 0, slot_size - reg_size);
>      }
>    else if (in_buf != nullptr)
> -    out_regcache->raw_supply_part (regnum, 0, reg_size, in_buf + offs);
> +    {
> +      /* Zero-extend the register value if the slot is smaller than the register.  */
> +      if (slot_size < register_size (gdbarch, regnum))
> +	out_regcache->raw_supply_zeroed (regnum);
> +      out_regcache->raw_supply_part (regnum, 0, reg_size, in_buf + offs);
> +    }
>    else
>      {
>        /* Invalidate the register.  */
> -- 
> 2.31.1
>