Re: [Minios-devel] [UNIKRAFT PATCHv4 16/43] uk/arch: Implement ukarch_find_lsbit for Arm64

[Wei Chen <Wei.Chen@xxxxxxx>]

> -----Original Message-----
> From: Sharan Santhanam <sharan.santhanam@xxxxxxxxx>
> Sent: 2018年7月13日 20:45
> To: Julien Grall <Julien.Grall@xxxxxxx>; Wei Chen <Wei.Chen@xxxxxxx>; minios-
> devel@xxxxxxxxxxxxx
> Subject: Re: [Minios-devel] [UNIKRAFT PATCHv4 16/43] uk/arch: Implement
> ukarch_find_lsbit for Arm64
>
> Hello,
>
>
> On 07/13/2018 11:44 AM, Julien Grall wrote:
> > Hi,
> >
> > On 13/07/18 10:20, Wei Chen wrote:
> >>>> +#ifndef __UKARCH_ATOMIC_H__
> >>>> +#error Do not include this header directly
> >>>> +#endif
> >>>> +
> >>>
> >>> For finding the lsbit shouldn't we use the builtin function
> >>> __builtin_ffs, __builtin_ffsl?
> >
> > AFAICT __builtin_ffs is GNU-ism. Do we really want to tie ourself to it?
>
> Yes, this is a good point I had overlooked previously. On further
> looking into it, this builtin function is supported on gcc[1] and
> clang[2]. We may need to consider what other compiler tool chains do we
> need to be compatible with.
>
> > Also, do you know if that is supported correctly on all GCC versions we
> > targets?
> The __builitin_ffs were introduced as a part of the gcc version3.3
> but the earliest documentation where it is been described is in 3.4 [1].
>
>
> >>>
> >>
> >> When I was implementing the ukarch_find_lsbit, I found both Arm32 and
> >> X86_64 were not using the builtin-functions. X86_64 is using "bsfq",
> >> Arm32 is using the same instructions as Arm64.
>
> I agree in terms of consistency, we may use the current implementation.
>
> I wanted to clarify if we made an explicit decision to provide our own
> implementation for the ffs.
>

Later, in other patch series, maybe we can have some method to detect the
__builitin_ffs supported status. If it's supported by current compiler we
can use it, otherwise we will use current implementation.

How do you think about it?

> > To be honest, I would keep the implement as it is.
> >
> > Cheers,
> >
>
> [1]gcc:
> https://gcc.gnu.org/onlinedocs/gcc-3.4.6/gcc/Other-Builtins.html#Other-
> Builtins
>
> [2] Clang:
> https://github.com/llvm-
> mirror/clang/blob/release_26/include/clang/Basic/Builtins.def
>
>
> Thanks & Regards
> Sharan
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