-fno-strict-aliasing in CWARNFLAGS?

Jeremy Huddleston jeremyhu at apple.com
Tue Feb 2 17:34:53 PST 2010

On Feb 2, 2010, at 17:11, Gaetan Nadon wrote:

> On Tue, 2010-02-02 at 14:00 -0800, Jeremy Huddleston wrote:
>> On Feb 2, 2010, at 13:18, Gaetan Nadon wrote:
>>> I have not seen any compelling reasons to turn off this optimization.
>>> Maybe 10 years ago when it was first introduced. I have seen reports of
>>> large number of warnings, but from older gcc versions. As it is today,
>>> we are losing some optimization that could be beneficial.
>>> This option has been there for so long (most likely copied along), I
>>> doubt you will will get a clear answer for each of the 240 xorg modules.
>>> It would take a few modules to try it out first.
>> I see it in libX11 has historically used -fno-strict-aliasing:
>> http://cgit.freedesktop.org/xorg/lib/libX11/commit/?id=db7c6fdeeaef9475458498e4cf09d6b1329e9aa3
>> but adding XORG_CWARNFLAGS to XORG_DEFAULT_OPTIONS has caused this to change for other modules.
>> Looking at historic versions of modules, I see it present in:
>> libICE
>> libSM
>> libX11
>> libXau
>> libXfont
>> libXft
>> libXpm
>> libXres
>> xorg-server
>> of course most of these seem to have just copied the entire GCC_WARNINGS block and probably didn't actually need -fno-strict-aliasing
> Of course. Whatever the reasons were, if anyone remembers, may not apply
> anymore. Devising a plan for it's removal will not be an easy task. I
> see 3 options:
> 1) take it out of macros, 1 patch

I sent that patch already.

> 2) transfer it to all makefiles and then removing it gradually, that's
> 2*240 +1 patches

I don't think that's wise.  If anything, you should just put it in the ones where it was prior to XORG_CWARNFLAGS (the 9 mentioned above).

That being said, I also enabled the warning which should mention when it discovers code sensitive to the -fstrict-aliasing optimization, and none of the libs in the list above spewed such a warning.

> 3) override in 'n' makefiles until proven safe. Then take it out macros.
> That's 2*n +1 patches.

We could do that for the 9 modules above.  I'm fairly confident that the libs don't need it, but I haven't rebuilt all of the server to be confident it doesn't spew any warnings about strict-aliasing.

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