Re: [PATCH v2] trace2: refactor to avoid gcc warning under -O3

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