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From: Jeff King <peff@peff.net>
To: Denton Liu <liu.denton@gmail.com>
Cc: Git Mailing List <git@vger.kernel.org>
Subject: Re: [PATCH] Fix -Wmaybe-uninitialized warnings under -O0
Date: Fri, 3 Apr 2020 10:04:47 -0400	[thread overview]
Message-ID: <20200403140447.GA639586@coredump.intra.peff.net> (raw)
In-Reply-To: <20200401140643.GA880372@generichostname>

On Wed, Apr 01, 2020 at 10:06:43AM -0400, Denton Liu wrote:

> > 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.

Ah, OK, I can reproduce easily with -Og (up through gcc-10). Most of
them don't trigger with -O1; just the one in ref-filter.c.

That one's interesting. We have:

  int ret = 0;
  ...
  if (...)
         ...
  else
         ret = for_each_fullref_in_pattern(...);
  ...
  return ret;

So we'd either have 0 or an assigned return. But the bug is actually in
for_each_fullref_in_pattern(), which does this:

  int ret; /* uninitialized! */

  /* a bunch of early return conditionals */
  if (...)
    return ...;

  for_each_string_list_item(...) {
    ret = for_each_fullref_in(...);
  }

  return ret;

but that will return an uninitialized value when there are no patterns.
I doubt we have such a case, but that may explain why -O0 does not
complain (it assumes "in_pattern" will return a useful value) and -O2
does not (it is able to figure out that it always does), but -O1 only
inlines part of it.

Curiously, -Og _does_ find the correct function.

Your patch silences it, but is it doing the right thing? It sets "ret =
0", but we haven't actually iterated anything. Should it be an error
instead?

-Peff

  reply	other threads:[~2020-04-03 14:04 UTC|newest]

Thread overview: 31+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-01  7:30 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 [this message]
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
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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