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Re: adding specific C flags for a SINGLE source file

From: Sander Niemeijer
Subject: Re: adding specific C flags for a SINGLE source file
Date: Fri, 10 Dec 2004 11:12:50 +0100

this comes *really* close to working. Problem is that in the Makefile one
ends up with:

$(libboincbenchmark_a_CPPFLAGS) $(CPPFLAGS) $(libboincbenchmark_a_CXXFLAGS) $(CXXFLAGS)

Notice (grrrr!!) that user's $(CXXFLAGS) is *last*. So I can't over-ride
it with libboincbenchmark_a_CXXFLAGS.

In other words:
and I end up with
  -O3 -g -O2.

Any idea how to get around this?

I had exactly the same problem. We had a single (very large) source file in a libtool library that we wanted to compile without compiler optimization turned on (i.e. we wanted to use -O0) in order to bring down the compilation time (from 20 minutes to within a minute for this specific file).

This was the cleanest solution I could come up with:

FOO_O = foo.o
$(FOO_O): foo.c
        $(MAKE) foo.o CFLAGS="$(CFLAGS) $(DISOPT_FLAG)" FOO_O=dummy-foo.o

FOO_LO = foo.lo
$(FOO_LO): foo.c
        $(MAKE) foo.lo CFLAGS="$(CFLAGS) $(DISOPT_FLAG)" FOO_LO=dummy-foo.lo

What this does is create a special rule for foo.o and foo.lo (yes, even though it goes against the advice that was given, unfortunately we _need_ a special rule for "foo.o: foo.c" to get this to work). Fortunately the rules that are executed are very clean. What happens is that once the foo.o target gets build the rule disables itself again by redefining FOO_O to something else and then it calls make again with a modified CFLAGS setting. In this way you still use the default .c.o build rules from automake (including all the nice dependency information generation), but now with a modified CFLAGS setting.

There are some pitfalls however:
- we didn't use a libname_la_CFLAGS setting, but if you do you would have to setup rules for libname-foo.o and libname-foo.lo - If the source file foo.c is not in the same directory as your Makefile(.in), make won't be able to find foo.c. To solve this problem we added the following (ugly) 'copy' rule:
foo.c: $(foo_dir)/foo.c
cp `test -f '$(foo_dir)/foo.c' || echo '$(srcdir)/'`$(foo_dir)/foo.c foo.c

I still don't like this approach very much, but it was the best approach that I could come up with.

As a general remark, automake is not that clear in the precedence that compiler flags take (will FOOFLAGS come before or after AM_FOOFLAGS/target_FOOFLAGS) and if/how one would be able to overrule these flags. Important is also to notice that CXXFLAGS/CFLAGS or overruled by adding a new option at the end (e.g. '-O2 -O3' would end up meaning '-O3'), but with include/library search paths in CPPFLAGS/LDFLAGS it is exactly the other way around (e.g. '-Ifoo -Ibar' ends up meaning '-Ifoo' when you have a foo/foo.h and bar/foo.h). I think some notes about this in the documentation or FAQ would also be very helpful.

Best regards,
Sander Niemeijer

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