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[Bug libc/12518] memcpy acts randomly (and differently) with overlapping areas
- From: "neleai at seznam dot cz" <sourceware-bugzilla at sourceware dot org>
- To: glibc-bugs at sourceware dot org
- Date: Mon, 19 Aug 2013 23:48:50 +0000
- Subject: [Bug libc/12518] memcpy acts randomly (and differently) with overlapping areas
- Auto-submitted: auto-generated
- References: <bug-12518-131 at http dot sourceware dot org/bugzilla/>
http://sourceware.org/bugzilla/show_bug.cgi?id=12518
--- Comment #29 from Ondrej Bilka <neleai at seznam dot cz> ---
On Mon, Aug 12, 2013 at 05:19:54PM +0000, tudorb at gmail dot com wrote:
> --- Comment #28 from Tudor Bosman <tudorb at gmail dot com> ---
> FYI, this bug has bitten me in a different way: memcpy() copying backwards
> defeats the MADV_SEQUENTIAL flag to madvise(). A trivial file copier
> implementation (mmap source, mmap destination, set MADV_SEQUENTIAL, memcpy from
> source to destination) would perform much worse on machines that support SSSE3
> than on machines that don't because of this bug.
>
Wait, do you have overlapping source and destination areas? If so then a
backward copy is necessary.
Or is backward copy called for not overlapping areas?
I have a patch that could call backward copy only if src-n < dest < src+n
holds.
> (Before anyone tells me that I should copy files using read() and write(), my
> actual usage pattern was more complex, but the details are irrelevant.)
>
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