Ernest Turro ernest.turro at ic.ac.uk
Tue Mar 20 23:13:10 CET 2007

On 20 Mar 2007, at 21:32, Kasper Daniel Hansen wrote:

> On Mar 20, 2007, at 7:58 PM, Ernest Turro wrote:
>> Why is it that R places CFLAGS after PKG_CFLAGS and not before when
>> compiling a package (e.g. through R CMD build pkg)? This can be
>> problematic if, for instance, you want to use -O3, but -O2 is in
>> R_HOME/etc/Makeconf. If -O2 (in CFLAGS) appears after -O3 (in
>> PKG_CFLAGS), you are left with what you didn't want: -O2.
>> In R-exts, it says that "Flags which are set in file etc/Makeconf can
>> be overridden by the environment variable MAKEFLAGS (at least for
>> systems using GNU make), as in (Bourne shell syntax)" but this
>> doesn't work if I set either MAKEFLAGS or CFLAGS/CXXFLAGS in my
>> configure.ac script or package Makevars.
> In you example above you want to force the user to use a higher  
> optimization flag. But (s)he may have very valid reasons for not  
> doing so - and are you really sure that you are comfortable setting  
> -O3 on _all_ platforms? Also -O. is GCC specific so it does not  
> work for all compilers.

My configure script checks for GCC before setting -O3 (and -ffast-math).

> If a user really wants a super fast R (s)he will (should) compile  
> it with -O3.

I'm compiling a MCMC simulation package. It is very intensive and so - 
O3  should definitely be the default level on systems with GCC.

> Having said that, I think it is problematic that one cannot   
> _downgrade_ the optimization. I am maintaining a package including  
> an external library (outside of my control) which does not work  
> with -O2 on some platforms, due to alignment problems.
>> Does anyone have any ideas on how to reliably override the default
>> CFLAGS/CXXFLAGS given in Makeconf?
> I was given the following code some while ago by Simon Urbanek:
> all: $(SHLIB)
> %.o: %.cpp
>         $(CXX) $(ALL_CPPFLAGS) $(ALL_CXXFLAGS) $(MYCXXFLAGS) -c $< - 
> o $@
> (this is for C++, I imagine the syntax is straightforward for C).  
> Put it in src/Makevars.

Thanks very much. That does indeed allow me to place my flags _after_  
the flags from R_HOME/etc/Makeconf. It would be nice, though, if the  
PKG_CXXFLAGS/PKG_CFLAGS were automatically placed _after_ CXXFLAGS/ 
CFLAGS by R... I vaguely recall the Windows version placing them in  
that order.



> But as I said above, I think it is a bad idea to raise the  
> optimization level for all users.
> Kasper
>> Many thanks,
>> Ernest
>> ______________________________________________
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