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Re: aarch64: add HWCAP_ATOMICS to HWCAP_IMPORTANT
- From: Steve Ellcey <sellcey at cavium dot com>
- To: Szabolcs Nagy <szabolcs dot nagy at arm dot com>, GNU C Library <libc-alpha at sourceware dot org>
- Cc: nd at arm dot com
- Date: Thu, 19 Apr 2018 09:08:34 -0700
- Subject: Re: aarch64: add HWCAP_ATOMICS to HWCAP_IMPORTANT
- References: <1d8eb765-e147-534e-ed1e-daa8deb8d5a7@arm.com>
- Reply-to: sellcey at cavium dot com
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On Thu, 2018-04-19 at 12:51 +0100, Szabolcs Nagy wrote:
> This enables searching shared libraries in atomics/ when the hardware
> supports LSE atomics of armv8.1 so one can provide optimized variants
> of libraries in a portable way.
>
> LSE atomics does not affect library abi, the new instructions can
> interoperate with old ones.
>
> I'm not familiar with how this feature of the dynamic linker is used
> in practice by distros or others so comments are welcome.
>
> 2018-04-19 Szabolcs Nagy <szabolcs.nagy@arm.com>
>
> * sysdeps/unix/sysv/linux/aarch64/dl-procinfo.h
> (HWCAP_IMPORTANT): Add
> HWCAP_ATOMICS.
I don't know if this is relavent or not but I checked in changes a few
months ago to use IFUNC in libatomic so that it could use LSE (or not)
depending on the aarch64 hardware it is running on. So if a user is
just using libatomic calls then they don't need a separate library or a
sperate search path. If they want their own libraries with versions
that do or do not use LSE directly (and don't want to use IFUNCs) then
this could still be needed/desired.
https://gcc.gnu.org/ml/gcc-patches/2017-12/msg00187.html
https://gcc.gnu.org/ml/gcc-patches/2017-12/msg00434.html
Steve Ellcey
sellcey@cavium.com