From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Greg Stark <stark(at)enterprisedb(dot)com> |
Cc: | Postgres <pgsql-hackers(at)postgresql(dot)org>, Oleg Bartunov <oleg(at)sai(dot)msu(dot)su>, Teodor Sigaev <teodor(at)sigaev(dot)ru> |
Subject: | Re: valgrind error in tsvectorin |
Date: | 2009-05-14 23:02:10 |
Message-ID: | 3274.1242342130@sss.pgh.pa.us |
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Lists: | pgsql-hackers |
Greg Stark <stark(at)enterprisedb(dot)com> writes:
>> Yeah, it looks like the memcpy is sometimes unnecessary because res and
>> ptr point to the same place. It might be worth cleaning up just to
>> avoid the valgrind warning, but I doubt it would save any noticeable
>> number of cycles.
> I assume valgrind is warning about it because memcpy's behaviour is
> undefined if the blocks overlap. I'm having trouble imagining an
> implementation that would fail if they're precisely the same pointer
> though.
Me either. A counterexample is that compilers typically implement
structure assignment via memcpy, and the behavior of "*d = *s" is
not undefined merely because d and s point to the same place.
In this particular example it looks like res and ptr might be the same
often enough that adding an "if (res != ptr)" test would save enough
cycles to be worth its cost ... but it's pretty marginal.
regards, tom lane
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