Re: configure sets CFLAGS or how to disable default CFLAGS='-g -O2' for gcc?

[Chris Pickett]

Jacob Meuser wrote:
> On Tue, Apr 04, 2006 at 02:20:39PM -0400, Chris Pickett wrote:
> presumably, if you are adding stuff, you already know exactly why
> you are adding it, exactly what it does, and exactly what the
> consequences are.  no different than anything else in computing.

I guess this is really it: I don't and can't know; there's just not 
enough time to learn everything that my computer does.  I fix things 
when I find out they are problems.  Of course I try to be proactive 
about finding problems, primarily by testing early and often, but I'm 
not perfect.  For example, I didn't know AC_PROG_CC set "-g -O2", even 
though I had been using it for a while.

> you want simple?  nothing is simpler than "default".
>
> sure, if you are releasing _binaries_, you can build with whatever
> CFLAGS, since presumably you are also testing the binaries.  but for
> source distributions, it's best to be conservative.

So, how about providing special flags for known platforms, and a safe 
default for unknown platforms?  Add to this protection by configure 
options so the user really has to want my flags to get them.  Is this an 
acceptable compromise?

> you need stuff like:
>
> # only add -O3 on i386/gcc versions >= 3.0
> if USING_GCC
> if GCC_VERSION_3
> if ARCH_I386
> AM_CFLAGS="-O3"
> endif
> endif
> endif
>
> where USING_GCC, GCC_VERSION_3 and ARCH_I386 are being set in tests
> in configure.  not the greatest example, but hopefully you get the
> idea.

Thanks for this example; I can imagine it gets much more complicated 
when you start testing out program fragments to see that they don't break.

> > Shifting the problem onto 
> > the user's shoulders doesn't really do anything for me because they are 
> > the same shoulders!
>
>
> so just do it in your script that runs configure.  trust me, adding
> optimization flags is going to annoy people who are trying to build
> your software on a platform you don't know about ... and you are
> going to have a really hard time trying to help them, since you do
> not know that platform!
>
> see, adding these flags makes your job harder!

Yes, I see.

> of course, if you are not planning on distributin the software, and
> really only _you_ will be the user ... but then why would you be
> using autotools in that case?

For now, it's the case that I am the only user; hopefully in the future 
that won't be so.  Regardless, the Autotools are still useful right now 
because they help me develop and build on multiple systems.

> > There was a confusion here.  I meant to say "command-line CFLAGS"
> > instead of "environment CFLAGS".  The Automake manual specifically says
> > not to clobber whatever the command-line CFLAGS are, to let the user
> > have the last say, and I was trying to do that by using AM_CFLAGS and 
> > friends.
>
>
> _but_, AM_CFLAGS will _always_ be added to the compile command.
> if you have AM_CFLAGS="-O3 -fomit-frame-pointer", the user _cannot_
> remove those without editing the build system.  you must check
> that AM_CFLAGS is not going to cause problems on any platform your
> software will be used on.

How about ./configure CFLAGS="-O2 -fno-omit-frame-pointer"?

But anyway, I think I see your point: don't piss off the user (and 
ultimately make my job harder) by forcing them to use my compiler flags. 
 So, unless they specifically pass --use-my-brain-dead-opt-flags to 
configure, they won't see any of them.

The whole irony of this that I actually wanted -O0 but AC_PROG_CC was 
killing it.  :)

Cheers,
Chris