[CMake] [Insight-developers] 64 bit build detection

[Michael Wild]

On Mac OS X one shouldn't do this kind of detection during configure  
step, because as has been mentioned a single file can be compiled  
multiple times for different architectures during one single compiler  
invocation. The size of void* and even endianness can change. It is  
preferable to have a central config.h.in (or similar) containing  
something like this:

#if defined(__APPLE__)
#  if defined(__i386__)
#    undef HAVE_64_BIT
#    undef HAVE_BIG_ENDIAN
#  elif defined(__ppc__)
#    undef HAVE_64_BIT
#    define HAVE_BIG_ENDIAN
#  elif defined(__x86_64__)
#    define HAVE_64_BIT
#    undef HAVE_BIG_ENDIAN
#  elif defined(__ppc64__)
#    define HAVE_64_BIT
#    define HAVE_BIG_ENDIAN
#  else
      // oops
#    error "Unknown architecture!"
#  endif
#else
#  cmakedefine HAVE_64_BIT
#  cmakedefine HAVE_BIG_ENDIAN
#endif


On NOT APPLE platforms you do the normal thing in your CMakeLists.txt:

if( NOT APPLE )
   # check 64 bit
   if( CMAKE_SIZEOF_VOID_P EQUALS 4 )
     set( HAVE_64_BIT 0 )
   else( CMAKE_SIZEOF_VOID_P EQUALS 4 )
     set( HAVE_64_BIT 1 )
   endif( CMAKE_SIZEOF_VOID_P EQUALS 4 )

   # check endianness
   include( TestBigEndian )
   test_big_endian( HAVE_BIG_ENDIAN )
   if( HAVE_BIG_ENDIAN )
     set( HAVE_BIG_ENDIAN 1 )
   else( HAVE_BIG_ENDIAN )
     set( HAVE_BIG_ENDIAN 0 )
   endif( HAVE_BIG_ENDIAN )
endif( NOT APPLE )

# configure config.h
configure_file( ${CMAKE_SOURCE_DIR}/config.h.in ${CMAKE_BINARY_DIR}/ 
config.h )
include_directories( ${CMAKE_BINARY_DIR} )


HTH

Michael

On 12. Jun, 2009, at 15:01, David Cole wrote:

> CMake does not warn.
> And actually, I think I have to back-track on my statement a little  
> bit.
>
> Whether or not a TRY_COMPILE "works" for a multiple arch / config  
> build
> *depends* entirely on what you are trying to compile and what result  
> you are
> expecting to get out of it. If you do a try compile on some code to  
> detect
> the presence of a function, for example, that should work fine on a
> universal binary build (presuming the function you're trying to link  
> against
> is present in universal binary form in the library it lives in...).
>
> The thing that won't work on a multiple arch/config build is trying to
> detect the size of something or the presence/absence of something  
> where the
> thing you're trying to measure is different in the different archs /
> configs. If it's uniform across all archs/configs then you can  
> measure it
> and get an answer that makes sense for all archs/configs. If not,  
> then you
> can't.
>
> Does that make sense?
>
> So... CMake does not warn because it does not know whether what you  
> are
> attempting to measure is valid or not. It's up to you to "do the right
> thing" in these sorts of situations.
>
> You already know that if you are building a universal binary with both
> 32-bit and 64-bit architectures in it that it makes no sense to  
> "measure"
> the size of "void*" at CMake time. You know this because you know  
> there is
> more than one answer depending on which one runs.
>
>
> Hope this clarifies things even further, (rather than muddying  
> anybody's
> waters...)
>
> David
>
>
> On Thu, Jun 11, 2009 at 6:56 PM, Sean McBride <sean at rogue- 
> research.com>wrote:
>
>> On 6/11/09 5:25 PM, David Cole said:
>>
>>> TRY_COMPILE works fine for cross compiles, just not for *mutiple
>>> configs/architectures "simultaneously"* compiles...
>>
>> Thanks for the clarification, that language is clearer.
>>
>> Does CMake detect and warn in such cases?  (it is after all the  
>> common
>> case on OS X.)
>>
>> Cheers,
>>
>> --
>> ____________________________________________________________
>> Sean McBride, B. Eng                 sean at rogue-research.com
>> Rogue Research                        www.rogue-research.com
>> Mac Software Developer              Montréal, Québec, Canada
>>
>>
>>
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