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[Bug libc/17801] memcpy is slower on amd64 than on i686 with a Sandy Bridge CPU
- From: "neleai at seznam dot cz" <sourceware-bugzilla at sourceware dot org>
- To: glibc-bugs at sourceware dot org
- Date: Tue, 06 Jan 2015 13:12:19 +0000
- Subject: [Bug libc/17801] memcpy is slower on amd64 than on i686 with a Sandy Bridge CPU
- Auto-submitted: auto-generated
- References: <bug-17801-131 at http dot sourceware dot org/bugzilla/>
https://sourceware.org/bugzilla/show_bug.cgi?id=17801
--- Comment #1 from Ondrej Bilka <neleai at seznam dot cz> ---
On Mon, Jan 05, 2015 at 10:23:18PM +0000, bugs at linkmauve dot fr wrote:
> On amd64 memcpy is actually calling __memcpy_avx_unaligned, and on i686 itâs
> calling __memcpy_ssse3_rep, and with a Sandy Bridge CPU, AVX is slower than
> SSSE3, despite being newer.
>
> I tested by disabling the AVX implementation and got nearly the same speed on
> amd64 than with the i686 version of the same program.
>
> Other functions (like memmove or strncmp) may also be affected, but I havenât
> checked them.
>
> Other CPUs as well, Iâve heard that Ivy Bridge also has a slower AVX
> implementation, maybe some AMD ones too.
>
No, that is typo, that implementation was aimed for avx2 only, specially
haswell where its fast.
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