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

[Matthieu Longo]

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.
> 
> Jan

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.

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

Matthieu