[Developers] how to use CCTK_RESTRICT

Thomas Radke tradke at aei.mpg.de
Fri Oct 7 10:46:35 CDT 2005


Erik Schnetter wrote:
> On Oct 7, 2005, at 08:20:52, Thomas Radke wrote:
> 
>> Hi,
>>
>> I'm currently doing some investigation on Cactus performance  
>> profiling and optimisation. One of the things I've tried was using  
>> restrict pointers in MoL (whether this is a reasonable thing to do  
>> I'll ask in a separate thread on this mailing list) in the way that  I 
>> define them as in
>>
>>   const CCTK_REAL* CCTK_RESTRICT RHSVar;
>>   CCTK_REAL* CCTK_RESTRICT UpdateVar;
>>
>> To my surprise I found that the CCTK_RESTRICT macro expands to an  
>> empty string so that my changes had no effect at all. I made sure  
>> that the C/C++ compiler (in my case it's Intel) does support the  
>> qualifier, and the Cactus configure script does recognise it.
>>
>> What I don't understand is the logic in lib/make/cctk_Config.h.in  
>> which defines/undefines CCTK_RESTRICT depending on the value of  
>> CCTK_C_RESTRICT (and CCTK_CXX_RESTRICT respectively for C++). I  
>> believe it is a bug that the macro expands to nothing iff  
>> CCTK_C_RESTRICT is undefined. Or am I missing something here ?
> 
> 
> MoL is written in C, and "restrict" is a keyword there, defined in  ANSI 
> C.  You should be able to use "restrict" instead of  "CCTK_RESTRICT".  
> Cactus autodetects it properly.  Only C++ code has  problems, because 
> Cactus does not #define "restrict" to anything in C+ +, since "restrict" 
> is not a keyword in the C++ standard.

I just learned from Jonathan that the "restrict" keyword is there in the 
C99 standard, not (necessarily?) in earlier versions.

Are you suggesting to generally use "restrict" when writing Cactus C/C++ 
code, and then rely on cctk_Config.h to do the right thing ? Then we 
should rename "CCTK_RESTRICT" into some Cactus internal macro name such 
as "CCTKi_RESTRICT", shouldn't we ?
Or should we rather use "CCTK_RESTRICT" in which case the logic in 
cctk_Config.h (and in your patch) could be simplified ?


-- 
Cheers, Thomas.



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