Re: [PATCH][AArch64] Enable _STRING_ARCH_unaligned

[Andrew Pinski <pinskia at gmail dot com>]

On Thu, Aug 20, 2015 at 11:43 PM, Andrew Pinski <pinskia@gmail.com> wrote:
> On Thu, Aug 20, 2015 at 10:24 PM, Wilco Dijkstra <wdijkstr@arm.com> wrote:
>> Enable _STRING_ARCH_unaligned on AArch64.
>>
>> 2015-08-20  Wilco Dijkstra  <wdijkstr@arm.com>
>>
>>         * sysdeps/aarch64/bits/string.h: New file.
>>         (_STRING_ARCH_unaligned): Define.
>>
>> ---
>>  sysdeps/aarch64/bits/string.h | 24 ++++++++++++++++++++++++
>>  1 file changed, 24 insertions(+)
>>  create mode 100644 sysdeps/aarch64/bits/string.h
>>
>> diff --git a/sysdeps/aarch64/bits/string.h b/sysdeps/aarch64/bits/string.h
>> new file mode 100644
>> index 0000000..5221e69
>> --- /dev/null
>> +++ b/sysdeps/aarch64/bits/string.h
>> @@ -0,0 +1,24 @@
>> +/* Optimized, inlined string functions.  AArch64 version.
>> +   Copyright (C) 2015 Free Software Foundation, Inc.
>> +   This file is part of the GNU C Library.
>> +
>> +   The GNU C Library is free software; you can redistribute it and/or
>> +   modify it under the terms of the GNU Lesser General Public
>> +   License as published by the Free Software Foundation; either
>> +   version 2.1 of the License, or (at your option) any later version.
>> +
>> +   The GNU C Library is distributed in the hope that it will be useful,
>> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
>> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
>> +   Lesser General Public License for more details.
>> +
>> +   You should have received a copy of the GNU Lesser General Public
>> +   License along with the GNU C Library; if not, see
>> +   <http://www.gnu.org/licenses/>.  */
>> +
>> +#ifndef _STRING_H
>> +# error "Never use <bits/string.h> directly; include <string.h> instead."
>> +#endif
>> +
>> +/* AArch64 implementations support efficient unaligned access.  */
>> +#define _STRING_ARCH_unaligned 1
>
> I don't think this is 100% true.  On ThunderX, an unaligned store or
> load takes an extra 8 cycles (a full pipeline flush) as all unaligned
> load/stores have to be replayed.
> I think we should also benchmark  there to find out if this is a win
> because I doubt it is a win but I could be proved wrong.

Are there benchmarks for each of the uses of _STRING_ARCH_unaligned
so I can do the benchmarking on ThunderX?
Also I don't see any benchmark results even for any of the other
AARCH64 processors.

Thanks,
Andrew

>
> Thanks,
> Andrew Pinski
>
>> --
>> 1.9.1
>>
>>