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From: Jeff King <peff@peff.net>
To: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>
Cc: git@vger.kernel.org, Junio C Hamano <gitster@pobox.com>,
	Denton Liu <liu.denton@gmail.com>,
	Jeff Hostetler <jeffhost@microsoft.com>,
	Johannes Schindelin <Johannes.Schindelin@gmx.de>
Subject: Re: [PATCH v2] trace2: refactor to avoid gcc warning under -O3
Date: Tue, 11 May 2021 13:54:19 -0400	[thread overview]
Message-ID: <YJrEy8IPi+PFWuzT@coredump.intra.peff.net> (raw)
In-Reply-To: <patch-1.1-782555daade-20210511T130231Z-avarab@gmail.com>

On Tue, May 11, 2021 at 03:04:28PM +0200, Ævar Arnfjörð Bjarmason wrote:

> Refactor tr2_dst_try_uds_connect() to avoid a gcc warning[1] that
> appears under -O3 (but not -O2). This makes the build pass under
> DEVELOPER=1 without needing a DEVOPTS=no-error.
> 
> This can be reproduced with GCC Debian 8.3.0-6, but not e.g. with
> clang 7.0.1-8+deb10u2. We've had this warning since
> ee4512ed481 (trace2: create new combined trace facility, 2019-02-22).
> 
> As noted in [2] this warning happens because the compiler doesn't
> assume that errno must be non-zero after a failed syscall. Let's work
> around it as suggested in that analysis. We now return -1 ourselves on
> error, and save away the value of errno in a variable the caller
> passes in.

Thanks, I think this describes the problem nicely.

> On Tue, May 11 2021, Junio C Hamano wrote:
> 
> > What's the concensus if any on this topic?
> 
> Having read Johannes's comments I think it's still most readable to
> just return -1 unconditionally. The resulting code isn't weird, I'd
> argue that it's a better pattern to save away errno like this, but the
> commit messages notes that we're working around a GCC bug.

Agreed. Returning "-1" is the usual style in our code base. And while I
think the original code is correct, I did have to go double-check the C
standard to confirm that it's so.

I slightly disagree with the notion that gcc's behavior is a bug. It
seems more like a lack of feature (it does not have any way to annotate
this special property of errno). But that is neither here nor there for
your patch, and really a matter of opinion. :)

> > In any case, this needs to be signed off before it gets carved into
> > our history.
> 
> Done, and also changed the variable name to minimize the size of the
> diff. A shorter name allowed for less re-flowing of lines.

It's quite short. I'm OK with it for a static-local function with few
callers like this, though.

-Peff

  parent reply	other threads:[~2021-05-11 17:54 UTC|newest]

Thread overview: 31+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-01  7:30 [PATCH] Fix -Wmaybe-uninitialized warnings under -O0 Denton Liu
     [not found] ` <CAPUEspgBkmxszgBee8C9hZnEwqztf-XKEj7LB_jWVFJaJCge0w@mail.gmail.com>
2020-04-01  9:05   ` Denton Liu
2020-04-01  9:52 ` Jeff King
2020-04-01 14:06   ` Denton Liu
2020-04-03 14:04     ` Jeff King
2020-04-03 14:38       ` Jeff King
2020-04-04 12:07       ` Denton Liu
2020-04-04 14:21         ` Jeff King
2021-05-05  8:40           ` [PATCH] trace2: refactor to avoid gcc warning under -O3 Ævar Arnfjörð Bjarmason
2021-05-05  9:47             ` Junio C Hamano
2021-05-05 13:34               ` Jeff King
2021-05-05 14:38             ` Johannes Schindelin
2021-05-06  1:26               ` Junio C Hamano
2021-05-06 20:29                 ` Johannes Schindelin
2021-05-06 21:10                   ` Junio C Hamano
2021-05-11 14:34                     ` Johannes Schindelin
2021-05-11 18:00                       ` Jeff King
2021-05-11 20:58                         ` Junio C Hamano
2021-05-11 21:07                           ` Jeff King
2021-05-11 21:33                             ` Junio C Hamano
2021-05-11  7:03             ` Junio C Hamano
2021-05-11 13:04             ` [PATCH v2] " Ævar Arnfjörð Bjarmason
2021-05-11 16:40               ` Jeff Hostetler
2021-05-11 17:54               ` Jeff King [this message]
2021-05-11 18:08                 ` Jeff King
2021-05-11 21:09                   ` Junio C Hamano
2021-05-20  0:20                   ` Junio C Hamano
2021-05-20 11:05                     ` [PATCH v3] " Ævar Arnfjörð Bjarmason
2021-05-20 13:13                       ` Jeff King
2021-05-20 22:08                         ` Junio C Hamano
2021-05-21  9:34                           ` Jeff King

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