Re: debug builds with NO optimizations

[John Calcote]

When you override CFLAGS on the configure command line, you give
configure the opportunity to add more flags to the CFLAGS variable that
are then passed on to make. On the other hand, when you modify CFLAGS on
the make command line, you are overriding all settings provided to the
Makefile by the configure script when the Makefile was built. For
example, I have a (generated) Makefile that contains the following
definition for CFLAGS:
 
CFLAGS = -g -O2 -DLINUX -Wall -Werror -O3
The -g -O2 comes from the AC_PROG_CC macro in my configure.ac file. The
-DLINUX comes from some hand-written script in that same configure.ac
that determines my platform (based on host_os), while thet -Wall -Werror
comes from some more script that determines which compiler I'm using.
Finally, the -O3 is determined by yet another piece of script that
determines based on --enable features specified how optimal I want the
code to be.
 
If you say "make CFLAGS=-g", you've just lost the definition for LINUX
and the warning options defined in the configure script. Before you all
flame me on this, let me say that I'm already aware that I should be
passing the -D option in the CPPFLAGS, but I would argue that CFLAGS is
the only viable place to pass the -W options, and that these options
should probably be set by a configure script that can figure out which
compiler you're using. I would also like to point out that this is a
simple example - I know of projects that also much more carefully craft
optimization flags - they pass a dozen or more -f flags based on
compiler and platform characteristics.
 
This means that, in general (IMHO), it's better to configure your build
tree with the flags you want when running the configure script, and then
just run make with no arguments (other than specific sub-targets when
needed). This allows programmed configuration options (--enable, --with,
etc) to affect CFLAGS as well - perhaps with esoteric -f options that
you will never remember properly without lots of research right before
you type make. :) 
 
I reserve setting CFLAGS on the make command line for when I absolutely
need to specify the entire set of CFLAGS in a build - for instance, when
I'm experimenting with something new.
 
John

>>> Brian Dessent <address@hidden> 2/6/2006 9:09:19 pm >>>
Harlan Stenn wrote:

> That didn't used to be the case, as I remember from the docs.
> 
> I do remember (perhaps incorrectly) that the difference was apparent
> when running "config.status --recheck".

I am sure there is more to it that I am not aware of.  Otherwise,
there
would be no controversy.  Maybe someone who actually knows something
about autoconf can say for sure.  But from what I can tell for setting
CFLAGS, CXXFLAGS, LDFLAGS, and so on, either way of doing it should
record the choice equally in generated files.

There is also the the issue of overriding during make, such as "make
CFLAGS=-O2" which from what I can tell sometimes works and sometimes
is
a bad idea.  It also depends on your platform's make since I think
there
are differences in how these kind of overrides are handled.  But that
would be another route to try if you wanted to change the build
without
reconfiguring and without making separate build directories.

Brian


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