[PATCH v2 2/2] Enable arch-specific CFI directives and DWARF instructions only when required by the target (part 1)

[Jan Beulich]

On 25.04.2025 17:53, Matthieu Longo wrote:
> On 2025-04-25 15:45, Jan Beulich wrote:
>> On 25.04.2025 16:02, Matthieu Longo wrote:
>>> On 2025-04-25 14:10, Jan Beulich wrote:
>>>> On 25.04.2025 13:04, Matthieu Longo wrote:
>>>>> --- a/gas/dw2gencfi.c
>>>>> +++ b/gas/dw2gencfi.c
>>>>> @@ -720,9 +720,16 @@ const pseudo_typeS cfi_pseudo_table[] =
>>>>>        { "cfi_same_value", dot_cfi, DW_CFA_same_value },
>>>>>        { "cfi_remember_state", dot_cfi, DW_CFA_remember_state },
>>>>>        { "cfi_restore_state", dot_cfi, DW_CFA_restore_state },
>>>>> -    { "cfi_window_save", dot_cfi, DW_CFA_GNU_window_save },
>>>>> +#if TC_AARCH64
>>>>> +    /* cfi_window_save is an alias of cfi_negate_ra_state which is kept for
>>>>> +       backward-compatibility concerns.  */
>>>>> +    { "cfi_window_save", dot_cfi, DW_CFA_AARCH64_negate_ra_state },
>>>>
>>>> Why's this of concern only for Arm64?
>>>
>>> I am not sure that I understand what your question is here.
>>> cfi_window_save is only used by AArch64 and Sparc.
>>
>> Arm64 uses .cfi_negate_ra_state. .cfi_window_save was wrongly exposed
>> everywhere except for Sparc. Hence as I view it either all other
>> architectures need to retain it for backwards compatibility, or none
>> of them. Yet perhaps I'm missing some peculiarity here.
> 
> None of others architectures has used cfi_window_save, except AArch64. 
> It does not mean that some people out there were not mistakenly adding 
> the directive into their assembly files, but it was not doing anything 
> anyway as it was ignored by the tools.
> 
> You can check this inside GCC, and see that it is only used for 
> unwinding inside Sparc and AArch64 backend.

Hmm, beside this being a gross hack (.cfi_negate_ra_state ought to be
used there), commentary there even alludes to the possibility of using
these elsewhere. Plus you leave aside the possibility of someone having
found a(n) (ab)use for this for another target, perhaps in plain
assembly.

> In the same way in GDB, only the AArch64 and Sparc vendor handler are 
> processing DW_CFA_AARCH64_negate_ra_state and DW_CFA_GNU_window_save.
> aarch64-tdep.c:1410:  if (op == DW_CFA_AARCH64_negate_ra_state)
> sparc-tdep.c:1600:  if (op != DW_CFA_GNU_window_save)
> 
> In my understanding, there is no benefit in keeping the backward 
> compatibility with something that was never used. At worst, the build of 
> those assembly files would fail, and someone would have to remove the 
> directive that was never taken into account anyway.

What do you mean here? gcc isn't the only producer, and gdb isn't the
only consumer. Or at least I know of no way to prove this.

> It seems to me an 
> acceptable risk of making the support of the directive only for Sparc 
> and AArch64.

This may be true. It's largely a policy question, and in such regards
it may well be that I'm overly conservative.

Jan