Re: [PATCH 0/2] squelch some "gcc -O3 -Wmaybe-uninitialized" warnings

From: Jeff King <peff@peff.net>
Date: Sep 1, 2016, 20:08 UTC
Message-ID: <20160901200837.jbuoay7lf4r7f67x@sigill.intra.peff.net>
In-Reply-To: <xmqqvaygzpbe.fsf@gitster.mtv.corp.google.com>

On Wed, Aug 31, 2016 at 12:55:01PM -0700, Junio C Hamano wrote:

Show 7 quoted lines

> Interesting.  Here is for "gcc -Os" on top to appease gcc 4.8.4 that
> I probably am NOT going to apply.  These are all false positives.
> 
> The ones on config.c is the most curious as these two "ret" needs a
> false initialization, but the one that comes after them
> git_config_ulong() that has the same code structure does not get any
> warning, which made absolutely no sense to me.

Yeah, I'd agree that is really odd. I wondered if perhaps the signedness of the argument mattered (e.g., if we were somehow provoking undefined behavior which caused the compiler to make some assumption), but I just don't see it.

Show 5 quoted lines

>  builtin/update-index.c | 2 +-
>  config.c               | 4 ++--
>  diff.c                 | 2 +-
>  fast-import.c          | 1 +
>  4 files changed, 5 insertions(+), 4 deletions(-)

FWIW, all but the fast-import one have gone away in gcc 6.2.0 (using -Os).

For that one:

Show 12 quoted lines

> diff --git a/fast-import.c b/fast-import.c
> index bf53ac9..abc4519 100644
> --- a/fast-import.c
> +++ b/fast-import.c
> @@ -1377,6 +1377,7 @@ static const char *get_mode(const char *str, uint16_t *modep)
>  	unsigned char c;
>  	uint16_t mode = 0;
>  
> +	*modep = 0;
>  	while ((c = *str++) != ' ') {
>  		if (c < '0' || c > '7')
>  			return NULL;

The complaint actually comes from the caller, who doesn't realize that modep will be set.

It pretty clearly seems to be a false positive, but I don't understand it. If get_mode() is not inlined (or otherwise examined when considering the caller), then it presumably should be treated as a block box that we assume sets "modep". And if it is inlined, then it's pretty obvious that "modep" is initialized in any code path that does not return NULL, and we have:

	p = get_mode(p, &mode);
	if (!p)
		die("Corrupt mode: %s", command_buf.buf);

in the caller (and "die" is marked as NORETURN). So it seems like a pretty easy case to get right, and one that the compiler presumably gets right elsewhere (otherwise we'd have a lot more of these false positives).

Weird.

-Peff