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

Hello Wei Chen,

On 07/16/2018 05:06 AM, Wei Chen wrote:

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


On 07/13/2018 11:44 AM, Julien Grall wrote:

On 13/07/18 10:20, Wei Chen wrote:
+#ifndef __UKARCH_ATOMIC_H__
+#error Do not include this header directly

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
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?

I agree.

To be honest, I would keep the implement as it is.



[2] Clang:

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