[Python-Dev] what environment variable should contain compiler warning suppression flags?

[Jeffrey Yasskin]

On Sat, Jun 26, 2010 at 4:37 PM, M.-A. Lemburg <mal at egenix.com> wrote:
> Brett Cannon wrote:
>> On Wed, Jun 23, 2010 at 14:53, Brett Cannon <brett at python.org> wrote:
>>> I finally realized why clang has not been silencing its warnings about
>>> unused return values: I have -Wno-unused-value set in CFLAGS which
>>> comes before OPT (which defines -Wall) as set in PY_CFLAGS in
>>> Makefile.pre.in.
>>>
>>> I could obviously set OPT in my environment, but that would override
>>> the default OPT settings Python uses. I could put it in EXTRA_CFLAGS,
>>> but the README says that's for stuff that tweak binary compatibility.
>>>
>>> So basically what I am asking is what environment variable should I
>>> use? If CFLAGS is correct then does anyone have any issues if I change
>>> the order of things for PY_CFLAGS in the Makefile so that CFLAGS comes
>>> after OPT?
>>>
>>
>> Since no one objected I swapped the order in r82259. In case anyone
>> else uses clang to compile Python, this means that -Wno-unused-value
>> will now work to silence the warning about unused return values that
>> is caused by some macros. Probably using -Wno-empty-body is also good
>> to avoid all the warnings triggered by the UCS4 macros in cjkcodecs.
>
> I think you need to come up with a different solution and revert
> the change...
>
> OPT has historically been the only variable to use for
> adjusting the Python C compiler settings.
>
> As the name implies this was usually used to adjust the
> optimizer settings, including raising the optimization level
> from the default or disabling it.
>
> With your change CFLAGS will always override OPT and thus
> any optimization definitions made in OPT will no longer
> have an effect.
>
> Note that CFLAGS defines -O2 on many platforms.
>
> In your particular case, you should try setting OPT to
> "... -Wno-unused-value ..." (ie. replace -Wall with your
> setting).

The python configure environment variables are really confused. If OPT
is intended to be user-overridden for optimization settings, it
shouldn't be used to set -Wall and -Wstrict-prototypes. If it's
intended to set warning options, it shouldn't also set optimization
options. Setting the user-visible customization option on the
configure command line shouldn't stomp unrelated defaults.

In configure-based systems, CFLAGS is traditionally
(http://sources.redhat.com/automake/automake.html#Flag-Variables-Ordering)
the way to tack options onto the end of the command line. Python
breaks this by threading flags through CFLAGS in the makefile, which
means they all get stomped if the user sets CFLAGS on the make command
line. We should instead use another spelling ("CFlags"?) for the
internal variable, and append $(CFLAGS) to it.

AC_PROG_CC is the macro that sets CFLAGS to -g -O2 on gcc-based
systems (http://www.gnu.org/software/hello/manual/autoconf/C-Compiler.html#index-AC_005fPROG_005fCC-842).
If Python's configure.in sets an otherwise-empty CFLAGS to -g before
calling AC_PROG_CC, AC_PROG_CC won't change it. Or we could just
preserve the users CFLAGS setting across AC_PROG_CC regardless of
whether it's set, to let the user set CFLAGS on the configure line
without stomping any defaults.