This is the mail archive of the
libc-alpha@sourceware.org
mailing list for the glibc project.
Re: [PATCH] powerpc: Use aligned stores in memset
- From: Zack Weinberg <zackw at panix dot com>
- To: Florian Weimer <fweimer at redhat dot com>
- Cc: Rajalakshmi Srinivasaraghavan <raji at linux dot vnet dot ibm dot com>, GNU C Library <libc-alpha at sourceware dot org>
- Date: Tue, 12 Sep 2017 08:18:41 -0400
- Subject: Re: [PATCH] powerpc: Use aligned stores in memset
- Authentication-results: sourceware.org; auth=none
- References: <1503033107-20047-1-git-send-email-raji@linux.vnet.ibm.com> <b8fd7e0c-8108-a808-a9a2-0c2df8961275@redhat.com> <e04fa334-d4e1-0660-ec26-024e97024761@linux.vnet.ibm.com> <e7daca03-3e86-8cdf-9d42-4e7effb02c63@redhat.com> <d7115391-1e52-5ecb-dce6-57895aaed268@redhat.com>
On Tue, Sep 12, 2017 at 6:30 AM, Florian Weimer <fweimer@redhat.com> wrote:
>
> I could not find the manual which has the requirement that the mem*
> functions do not use unaligned accesses. Unless they are worded in a
> very peculiar way, right now, the GCC/glibc combination does not comply
> with a requirement that memset & Co. can be used for device memory access.
mem* are required to behave as-if they access memory as an array of
unsigned char. Therefore it is valid to give them arbitrarily
(un)aligned pointers. The C abstract machine doesn't specifically
contemplate the possibility of a CPU that can do unaligned word reads
but maybe not to all memory addresses, but I would argue that if there
is such a CPU, then mem* are obliged to cope with it.
> ...the current glibc
> implementation accesses locations which are outside the specified object
> boundaries.
I think that's technically a defect. Nothing in the C standard
licenses it to do that; we just get away with it because, on the
implementations to date, it's not observable (unless you go past the
end of a page, which you'll note there are a bunch of tests to ensure
we don't do). If an over-read by a single byte is observable, then
mem* is not allowed to do that.
zw