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Re: [PATCH][AArch64] Enable _STRING_ARCH_unaligned


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.

Thanks,
Andrew Pinski

> --
> 1.9.1
>
>


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