Re: [PATCH] Fix -Wmaybe-uninitialized warnings under -O0

From: Denton Liu <liu.denton@gmail.com>
To: Jeff King <peff@peff.net>
Cc: Git Mailing List <git@vger.kernel.org>
Subject: Re: [PATCH] Fix -Wmaybe-uninitialized warnings under -O0
Date: Wed, 1 Apr 2020 10:06:43 -0400	[thread overview]
Message-ID: <20200401140643.GA880372@generichostname> (raw)
In-Reply-To: <20200401095255.GA60227@coredump.intra.peff.net>

Hi Peff,

On Wed, Apr 01, 2020 at 05:52:55AM -0400, Jeff King wrote:
> On Wed, Apr 01, 2020 at 03:30:16AM -0400, Denton Liu wrote:
> 
> > When compiling Git under -O0, gcc (Arch Linux 9.3.0-1) 9.3.0 produces
> > many -Wmaybe-uninitialized warnings. These are false positives since
> > when Git is compiled under -O2, gcc is smart enough to see that the
> > code paths that use these variables all initialise them beforehand.
> > Nonetheless, these warnings block the compilation process when
> > DEVELOPER=1 is enabled (which enables -Werror).
> > 
> > Fix these warnings by initializing these variables with dummy values (0,
> > -1 or NULL as appropriate).
> 
> Hmph. I almost always compile with -O0 and have been using gcc 9.3.0
> since it was packaged for Debian a few weeks ago, but I don't see any of
> these warnings.
> 
> The current version in Debian unstable is 9.3.0-8, which picks up some
> extra patches from the upstream gcc-9 branch. But even if I download a
> snapshot of the original 9.3.0 release, it builds fine.
> 
> So why does your version behave differently? And if this is a temporary
> state for a buggy version of gcc (that may be fixed in the next point
> release), is it worth changing our source code to appease it?

A correction to the earlier message... It seems like I wasn't reporting
the correct settings. I was actually compiling with -Og, not -O0
(whoops!).

I tested it with gcc-8 and it seems like it also reports the same
problem. Also, -O1 reports warnings as well.

I guess the question is do we only support -O0 and -O2 or should we
support the levels in between as well?

Sorry for the confusion,

Denton

> 
> -Peff