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

[Sharan Santhanam <sharan.santhanam@xxxxxxxxx>]

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.
> 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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