[Mesa-dev] [PATCH] Fix strict-aliasing violations in GLSL shader list implementation

[Matt Turner]

On Tue, Jun 23, 2015 at 7:57 PM, Michel Dänzer <michel at daenzer.net> wrote:
> On 24.06.2015 11:39, Dave Airlie wrote:
>>>
>>> Actually, I'm almost 100% certain that there are lots of other strict
>>> aliasing violations in the Mesa code. That's why we've always disabled it.
>>>
>>> More generally, IMO it's unrealistic to rely on strict aliasing for
>>> optimization, because very few people really understand it (I'm not one
>>> of them).
>>
>> I personally think we should get past the, aliasing is hard, lets go shopping,
>
> I'm not saying that. I'm saying we'll keep getting it wrong, which will
> cause subtle and very hard to debug problems.

As far as I know, we haven't really tried to get it right for a long
time. -fno-strict-aliasing was added to Mesa's build system in 2007.

For what it's worth, gcc has a -Wstrict-aliasing flag [1] (part of
-Wall) that will warn about potential problems. Its default level is
advertised as "Should have very few false positives and few false
negatives."

I recompiled Mesa in my normal configuration (i965 only) with
-fstrict-aliasing -Wstrict-aliasing and it warned about a handful of
things in src/glx, one in the FXT1 compressor, another in swrast and
they all look valid from my understanding of the strict-aliasing
rules.

[1] https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wstrict-aliasing-407