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

From: Junio C Hamano <gitster@pobox.com>
To: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>
Cc: git@vger.kernel.org, Jeff King <peff@peff.net>,
	Denton Liu <liu.denton@gmail.com>,
	Jeff Hostetler <jeffhost@microsoft.com>
Subject: Re: [PATCH] trace2: refactor to avoid gcc warning under -O3
Date: Tue, 11 May 2021 16:03:35 +0900	[thread overview]
Message-ID: <xmqqv97pn454.fsf@gitster.g> (raw)
In-Reply-To: <patch-1.1-87d9bcf1095-20210505T083951Z-avarab@gmail.com> (=?utf-8?B?IsOGdmFyCUFybmZqw7Zyw7A=?= Bjarmason"'s message of "Wed, 5 May 2021 10:40:32 +0200")

Ævar Arnfjörð Bjarmason  <avarab@gmail.com> writes:

> 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.
>
> 1.
>
>     trace2/tr2_dst.c: In function ‘tr2_dst_get_trace_fd.part.5’:
>     trace2/tr2_dst.c:296:10: warning: ‘fd’ may be used uninitialized in this function [-Wmaybe-uninitialized]
>       dst->fd = fd;
>       ~~~~~~~~^~~~
>     trace2/tr2_dst.c:229:6: note: ‘fd’ was declared here
>       int fd;
>           ^~
> 2. https://lore.kernel.org/git/20200404142131.GA679473@coredump.intra.peff.net/
> ---
>  trace2/tr2_dst.c | 27 +++++++++++++++------------
>  1 file changed, 15 insertions(+), 12 deletions(-)

What's the concensus if any on this topic?

In any case, this needs to be signed off before it gets carved into
our history.

Thanks.