On Thu, Sep 8, 2011 at 1:56 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Daniel Farina <daniel@heroku.com> writes:
>> On Tue, Sep 6, 2011 at 12:00 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>> I'm still of the opinion that there's no real need to avoid memcpy with
>>> identical source and destination, so I didn't apply this first patch.
>
>> I am leery of memcpy with overlapping regions. I know that it can
>> cause an infinite loop on ssse3 architectures, having to do with some
>> backwards-iteration it does:
>> https://bugs.launchpad.net/ubuntu/+source/apache2/+bug/609290
>
> The linked page offers no support for your claim of an infinite loop,
> and in any case it's discussing a case involving *overlapping* regions,
> not *identical* regions.
What do you mean? As per my original bug report, or in this case
(possibly both; I should have dumped the registers, which I'll do if I
see it again...)? I'm able to believe that things are fine with all
known memcpys today in this case.
> The reason why this concern is irrelevant for identical regions is that
> no matter what order the memcpy routine copies the bytes in, it's
> necessarily storing the exact same data into each byte that was there
> before. The only way that strange results could be produced is if the
> routine transiently set some byte to a value other than its final value,
> which would mean that it must be intending to store to that location
> more than once, which would be silly and inefficient.
This is a good point, I do understand there is a distinction between
the degenerate-case memcpy-to-identical region and the
overlapping-case.
In my naivety, I'd ask what the costs are of pedantic adherence to
this common guideline and, in event someone somewhere does something
that is not expected (or, has a slow-but-not-technically-buggy memcpy)
are broken, how likely the failure will be easy to pick out. But I
don't seriously expect an answer, because I don't think this code path
has been a problem for so many years and measuring those things are
pretty hard.
--
fdr
