Re: Best means to override CXXFLAGS locally

[Sander Niemeijer]

Hi Ralf,

I have been struggling with the 'single file disable optimization  
issue' for a while and used to have a somewhat ugly solution up to  
this point.
The recent fixes in automake that ensure that target_??FLAGS always  
overrides AM_??FLAGS make your suggestion a valid solution now.

In our project we have a single file (part of a libtool library  
target) that is too big to be compiled with optimization turned on.
Based on your suggestion I now use the following approach:

In configure.ac I have the following (at the bottom of the file!,  
just before AC_OUTPUT):
----
# here are some rules that set the value for DISOPT_FLAG for certain  
compilers. For gcc this is -O0
# ...
AC_SUBST(DISOPT_FLAG)

# store CFLAGS in CFG_CFLAGS and clear CFLAGS so we can override the  
flags (e.g. DISOPT_FLAG) in our Makefile(s)
CFG_CFLAGS=$CFLAGS
CFLAGS=
AC_SUBST(CFG_CFLAGS)
----

Then in each Makefile.am I have at the top:
---
CFG_CFLAGS = @CFG_CFLAGS@
AM_CFLAGS = $(CFG_CFLAGS)
---
and for each target_CFLAGS setting in a Makefile.am I add '$ 
(CFG_CFLAGS)'

The single file that has to be build with optimization turned off is  
now build as a libtool 'convenience library':
---
noinst_LTLIBRARIES = bigfile.la

bigfile_la_sources = bigfile.c
bigfile_la_CFLAGS = $(CFG_CFLAGS) $(DISOPT_FLAG)
---
and this convenience library is added to the original library by  
adding 'libbigfile.la' to the _la_LIBADD setting of the original  
library.

The catch with this approach is that running a 'make CFLAGS=<some  
other flags>' (if you ever do such a thing) will no longer replace  
the initial CFLAGS settings that were provided to the configure  
script (or set by AC_PROG_CC). It will just add them at the end (and  
also override the DISOPT_FLAG!).
With the way I have set up the CFG_CFLAGS setting in Makefile.am you  
can now still replace the original CFLAGS by using 'make  
CFG_CFLAGS=<some other flags>'.

Best regards,
Sander

On 22-okt-2006, at 13:46, Ralf Wildenhues wrote:

> Hello Akim,
>
> * Akim Demaille wrote on Sun, Oct 22, 2006 at 01:10:02PM CEST:
> > > What would be the cleanest means to handle this exception?
> >
> > Here is what I did:
>
> I think it's much better to do the munging in configure.ac.  Store the
> CFLAGS set by the user and/or AC_PROG_CC in AM_CFLAGS, and override
> CFLAGS.  Depending upon whether the changed flags settings apply to  
> only
> a few, non-portability-problematic files in your project, the rest of
> the configure tests should run with the flags set by the user, and you
> should only override at the end, or the converse.
>
> Much cleaner, involves much less changes.  If you have something that
> works nicely and is reasonably generally useful, post a summary, or
> even better, a FAQ addition to the manual.  ;-)
>
> Cheers,
> Ralf