From: Jeff King <peff@peff.net>
To: "René Scharfe" <l.s.r@web.de>
Cc: Duy Nguyen <pclouds@gmail.com>, Git List <git@vger.kernel.org>,
Junio C Hamano <gitster@pobox.com>,
Johannes Schindelin <johannes.schindelin@gmx.de>
Subject: Re: [PATCH 1/5] add SWAP macro
Date: Fri, 28 Apr 2017 17:49:34 -0400 [thread overview]
Message-ID: <20170428214934.tuqihgch6qeen3ni@sigill.intra.peff.net> (raw)
In-Reply-To: <070a4b85-98e0-12a4-6c9c-557f3dfa733c@web.de>
On Fri, Apr 28, 2017 at 07:04:51PM +0200, René Scharfe wrote:
> > What should:
> >
> > SWAP(foo[i], foo[j]);
> >
> > do when i == j? With this code, it ends up calling
> >
> > memcpy(&foo[i], &foo[j], ...);
> >
> > which can cause valgrind to complain about overlapping memory. I suspect
> > in practice that noop copies are better off than partial overlaps, but I
> > think it does still violate the standard.
> >
> > Is it worth comparing the pointers and bailing early?
>
> Hmm, so swapping a value with itself can be a useful thing to do?
> Otherwise an assert would be more appropriate.
No, I doubt that it's useful, and it's probably a sign of a bug
elsewhere. But it's likely a _harmless_ bug, so it may be irritating to
die when we hit it rather than continuing.
I dunno. I could go either way. Or we could leave it as-is, and let
valgrind find the problem. That has zero run-time cost, but of course
nobody bothers to run valgrind outside of the test suite, so the inputs
are not usually very exotic.
> Swapping with *partial* overlap sounds tricky, or even evil. If
> we want to support that for some reason we'd have to use memmove
> in the middle. But that would still corrupt at least one of the
> objects, wouldn't it?
Yes, the overlap case is probably an actual bug. Detecting it is a bit
harder, but definitely possible. I hate to pay the run-time cost for it,
but I wonder if a compiler could optimize it out.
> The line in question is this one:
>
> for (i = 0; i <= (j = (queue->nr - 1) - i); i++)
>
> Assignment in the middle? Hmm. Why not do it like this?
>
> for (i = 0, j = queue->nr - 1; i < j; i++, j--)
>
> Looks less complicated to me.
Yes, see my other reply. :)
-Peff
next prev parent reply other threads:[~2017-04-28 21:49 UTC|newest]
Thread overview: 50+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-01-28 21:13 [PATCH 0/5] introduce SWAP macro René Scharfe
2017-01-28 21:38 ` [PATCH 1/5] add " René Scharfe
2017-01-30 15:39 ` Johannes Schindelin
2017-01-30 16:48 ` René Scharfe
2017-01-30 20:48 ` Johannes Schindelin
2017-01-30 21:46 ` René Scharfe
2017-01-31 12:13 ` Johannes Schindelin
2017-01-31 21:02 ` René Scharfe
2017-02-01 0:44 ` Ramsay Jones
2017-02-01 11:39 ` Johannes Schindelin
2017-01-30 16:01 ` Johannes Schindelin
2017-01-30 16:59 ` René Scharfe
2017-01-30 18:41 ` Johannes Sixt
2017-01-30 21:03 ` Johannes Schindelin
2017-01-30 22:09 ` René Scharfe
2017-01-30 22:21 ` Brandon Williams
2017-01-31 21:03 ` René Scharfe
2017-01-31 21:35 ` Jeff King
2017-01-31 22:29 ` Junio C Hamano
2017-01-31 22:36 ` Jeff King
2017-02-01 11:28 ` Johannes Schindelin
2017-02-01 11:47 ` Jeff King
2017-02-01 18:06 ` René Scharfe
2017-02-01 18:33 ` Junio C Hamano
2017-02-07 22:04 ` René Scharfe
2017-02-07 22:30 ` Junio C Hamano
2017-02-08 15:14 ` Johannes Schindelin
2017-01-31 12:03 ` Johannes Schindelin
2017-04-24 11:29 ` Jeff King
2017-04-24 11:49 ` Jeff King
2017-04-24 13:13 ` Duy Nguyen
2017-04-28 17:04 ` René Scharfe
2017-04-28 21:49 ` Jeff King [this message]
2017-04-29 18:16 ` René Scharfe
2017-04-30 3:11 ` Jeff King
2017-05-02 5:29 ` René Scharfe
2017-01-28 21:40 ` [PATCH 2/5] apply: use " René Scharfe
2017-01-28 21:40 ` [PATCH 3/5] " René Scharfe
2017-01-30 16:03 ` Johannes Schindelin
2017-01-30 17:18 ` René Scharfe
2017-01-30 22:22 ` Junio C Hamano
2017-01-31 21:02 ` René Scharfe
2017-01-28 21:41 ` [PATCH 4/5] diff: " René Scharfe
2017-01-30 16:04 ` Johannes Schindelin
2017-01-30 17:26 ` René Scharfe
2017-01-30 22:22 ` Junio C Hamano
2017-01-28 21:42 ` [PATCH 5/5] graph: " René Scharfe
2017-01-30 16:16 ` Johannes Schindelin
2017-01-30 17:41 ` René Scharfe
2017-01-30 23:20 ` [PATCH 0/5] introduce " Junio C Hamano
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