Re: [PATCH 0/2] -Wuninitialized

From: Ramsay Jones <ramsay@ramsayjones.plus.com>
Date: Mar 20, 2018, 22:41 UTC
Message-ID: <ce46b318-2c37-4562-7e57-526f511cd6a8@ramsayjones.plus.com>
In-Reply-To: <20180320043225.GB13302@sigill.intra.peff.net>

On 20/03/18 04:32, Jeff King wrote:

> On Mon, Mar 19, 2018 at 05:53:09PM +0000, Ramsay Jones wrote:

Show 21 quoted lines

>> If we now add a patch to remove all self-initialization, which would be the
>> first patch plus the obvious change to 'saved_namelen' in read-cache.c, then
>> note the warnings issued by various compilers at various optimization levels
>> on several different platforms [5]:
>>
>>                     O0      O1      O2      O3      Os       Og
>>  1) gcc 4.8.3   |   -      1,20     1    1,18-19  1-4,21-23  1,5-17
>>  2) gcc 4.8.4   |   -      1,20     1       1     1-4,21-23  1,5-8,10-13,15-16 
>>  3) clang 3.4   |   -       -       -       -        -       n/a 
>>  4) gcc 5.4.0   |   -       1       1       1     1,3-4,21   1,5-8,10-13,16-16
>>  5) clang 3.8.0 |   -       -       -       -        -       n/a 
>>  6) gcc 5.4.0   |   -       1       1       1       1-4     1,5-17 
>>  7) clang 3.8.0 |   -       -       -       -        -       n/a 
>>  8) gcc 6.4.0   |   -       1       1    1,18-19    1,4     1,5-17
>>  9) clang 5.0.1 |   -       -       -       -        -        -
>> 10) gcc 7.2.1   |   -       1       1       1       1,4     1,5-17
> 
> So I guess this could create headaches for people using DEVELOPER=1 on
> as ancient a compiler as 4.8.4, but most other people should be OK. I
> think I can live with that as a cutoff, and the Travis builds should
> work there.

Yeah, I have an even older laptop (Windows 95 era), but I'm not sure if it will even boot these days. (It does have gcc on it IIRC, but who knows which version). ;-)

Show 6 quoted lines

> (And if we do the detect-compiler stuff from the other nearby thread,
> somebody who cares can even loosen the warnings for those old gcc
> versions).
> 
> I'm neglecting anybody doing -O3 or -Os here, but IMHO those are
> sufficiently rare that the builder can tweak their own settings.

I have occasionally used -O3, never used -Os (except for this exercise) and have been meaning to try -Og (in anger) for a while. ;-)

> I wonder if people use -Og, though? I don't (I usually do -O0 for my
> edit-compile-debug cycle).

In the gcc documentation, the -Og description says:

  "... It should be the optimization level of choice for the standard
   edit-compile-debug cycle, ..."

Like you, I use -O0 (very old habits). As I said above, I have been meaning to try -Og, but, well round tuits ... ;-)

[BTW, gcc also supports -Ofast, but I don't think we want to go there ...]

ATB, Ramsay Jones