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

[John Baldwin]

On 10/19/21 9:31 AM, John Baldwin wrote:
> On 10/19/21 1:36 AM, Andrew Burgess wrote:
>> * 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.

Here's an updated log message that hopefully is clearer:

     regcache: Zero-extend small registers described by a register map.
     
     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).  Upcoming patches also require these semantics for
     handling x86 segment register values stored in 16-bit slots on
     FreeBSD.  Note that architecturally x86 segment registers are 16 bits,
     but the x86 gdb architectures treat these registers as 32 bits.

-- 
John Baldwin