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Re: [PATCH][AArch64] Enable _STRING_ARCH_unaligned
- From: Andrew Pinski <pinskia at gmail dot com>
- To: Wilco Dijkstra <wdijkstr at arm dot com>
- Cc: GNU C Library <libc-alpha at sourceware dot org>
- Date: Thu, 20 Aug 2015 23:43:52 +0800
- Subject: Re: [PATCH][AArch64] Enable _STRING_ARCH_unaligned
- Authentication-results: sourceware.org; auth=none
- References: <000101d0db53$e96233c0$bc269b40$ at com>
On Thu, Aug 20, 2015 at 10:24 PM, Wilco Dijkstra <email@example.com> wrote:
> Enable _STRING_ARCH_unaligned on AArch64.
> 2015-08-20 Wilco Dijkstra <firstname.lastname@example.org>
> * 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."
> +/* 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.