Re: [PATCH] vcs-svn: Fix some compiler warnings

From: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Date: Feb 2, 2012, 18:24 UTC
Message-ID: <4F2AD4CF.7020303@ramsay1.demon.co.uk>
In-Reply-To: <20120131192053.GC12443@burratino>

Jonathan Nieder wrote:

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> Ramsay Jones wrote:
> 
>> In particular, some versions of gcc complains as follows:
>>
>>         CC vcs-svn/sliding_window.o
>>     vcs-svn/sliding_window.c: In function `check_overflow':
>>     vcs-svn/sliding_window.c:36: warning: comparison is always false \
>>         due to limited range of data type
> 
> Yuck.  Suppressing this warning would presumably also suppress the
> optimization that notices the comparison is always false.

I didn't check, but I would assume so. I would prefer not to loose any chance at optimizing the code, but in this case, given that we are talking about a fatal error path, I don't think it is much of a loss.

> The -Wtype-limits warning also triggers in some other perfectly
> reasonable situations: see <http://gcc.gnu.org/PR51712>.

[...]

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>> Note that the "some versions of gcc" which complain includes 3.4.4 and
>> 4.1.2, whereas gcc version 4.4.0 compiles the code without complaint.
> 
> Thanks for tracking this down.  Interesting.  -Wtype-limits was split
> out from the default set of warnings (!) in gcc 4.3 to address
> <http://gcc.gnu.org/PR12963>, among other bugs (r124875, 2007-05-20).

Thanks for the above references, and for taking the time to track them down.

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> Is there some less ugly way to write the condition "if this value is
> not representable in this type"?
> 
> I guess I could live with something like the following (please don't
> take the names too seriously):
> 
> 	static inline off_t off_t_or_die(uintmax_t val, const char *msg_if_bad)
> 	{
> 		if (val > maximum_signed_value_of_type(off_t))
> 			die("%s", msg_if_bad);
> 		return (off_t) val;
> 	}
> 
> 	...
> 
> 		off_t delta_len = off_t_or_die(len, "enormous delta");
> 		postimage_len = apply_delta(delta_len, input, ...);
> 
> What do you think?

An static inline function was actually my first thought (although I had something more like Junio's suggestion [elsewhere in this thread] in mind), but I didn't want to place it in git-compat-util.h and could not find a suitable place in the vcs-svn directory.

Hmm, I will send a v2 patch along these lines ...

ATB, Ramsay Jones