Re: Best means to override CXXFLAGS locally

[Dan McMahill]

Akim Demaille wrote:
> > What would be the cleanest means to handle this exception?
>
>
> Here is what I did:
> - Force automake to generate an object specific rule by
>   using some foo_CXXFLAGS
> - Open Makefile.in, get the specific rules.
> - Use LTCOMPILECXX to factor the common part
> - Append $(MY_NOPTION_CXXFLAGS) to it
> - rename the (object) output file name Automake used
> - include all this in the Makefile.am
> - remove the foo_CXXFLAGS so that Automake do not
>   produce a rule for this object
> - and therefore my own rule is obeyed with conflict
> - and finally pray for no significant changes in the
>   future versions of Automake ;)
>
> # This is an edited copy of Automake's compilation rule so that we can
> # insert $(PARSER_CXXFLAGS) *after* $(CXXFLAGS) to override -O2.
> # Using foo_CXXFLAGS does not suffice, since CXXFLAGS, a user
> # variable, appears last.
> ugrammar.lo: ugrammar.cc
> @am__fastdepCXX_TRUE@    if $(LTCXXCOMPILE) $(PARSER_CXXFLAGS) -MT  
> ugrammar.lo -MD -MP -MF "$(DEPDIR)/ugrammar.Tpo" -c -o ugrammar.lo  
> `test -f 'ugrammar.cc' || echo '$(srcdir)/'`ugrammar.cc; \
> @am__fastdepCXX_TRUE@    then mv -f "$(DEPDIR)/ugrammar.Tpo" "$(DEPDIR)/ 
> ugrammar.Plo"; else rm -f "$(DEPDIR)/ugrammar.Tpo"; exit 1; fi
> @AMDEP_TRUE@@am__fastdepCXX_FALSE@    source='ugrammar.cc'  
> object='ugrammar.lo' libtool=yes @AMDEPBACKSLASH@
> @AMDEP_TRUE@@am__fastdepCXX_FALSE@    DEPDIR=$(DEPDIR) $(CXXDEPMODE) $ 
> (depcomp) @AMDEPBACKSLASH@
> @am__fastdepCXX_FALSE@    $(LTCXXCOMPILE) $(PARSER_CXXFLAGS) -c -o  
> ugrammar.lo `test -f 'ugrammar.cc' || echo '$(srcdir)/'`ugrammar.cc
>
>
> Is there anything better to do?  Is there a trap I just fell in?

Does it work with non-gcc compilers?

I've used something like this:

# work around buggy compilers when compiling the parser with optimization
# make sure no one sneaks a -O* in on us via one of these variables
# set in the environment
CXX_NOOPT=$(CXX:-O%=)
CPPFLAGS_NOOPT=$(CPPFLAGS:-O%=)
CXXFLAGS_NOOPT=$(CXXFLAGS:-O%=)

And if you're using libtool, you can define a LTCXXCOMPILE_NOOPT in a 
similar way.

It is a common enough problem that I wonder if it is worth requesting a 
automake feature that lists a set of files which may have problems with 
optimizing compilers.  As much as I hate to suggest it, you could then 
use autoconf to define a conditional where you tabulate known bad 
compilers.  You could also just across the board prevent optimization on 
a particular set of files.

-Dan