[CMake] FindLAPACK/FindBLAS require Fortran compiler. Why?

[Michael Wild]

On 7. Jul, 2009, at 11:08, Marcel Loose wrote:

[fixed quoting]

> On Tue, 2009-07-07 at 10:32 +0200, Michael Wild wrote:
>> On 7. Jul, 2009, at 10:11, Marcel Loose wrote:
>>
>>> Hi all,
>>>
>>> I noticed that FindLAPACK and FindBLAS require that you enable  
>>> Fortran
>>> as language, which causes CMake to check for a working Fortran
>>> compiler.
>>> IMHO, you only need the fortran runtime libraries, cause you're only
>>> linking against the Lapack/Blas libraries. Most Linux distro's have
>>> separate packages for compiler and runtime libraries.
>>>
>>> Is there a good reason for FindLAPACK and FindBLAS to require a
>>> working
>>> Fortran compiler?
>>>
>>> Best regards,
>>> Marcel Loose.
>>>
>>
>> That's probably because these modules ONLY search for the Fortran
>> interfaces. Usually one needs a different set of libraries for cblas
>> and clapack. Also the function names to check for are different (e.g.
>> cblas_dgemm versus dgemm).
>>
>> Perhaps someone finds the time to write FindCBLAS.cmake and
>> FindCLAPACK.cmake... But honestly, if you look at FindBLAS.cmake,  
>> that
>> is quite tough as the various vendor implementations differ strongly
>> in library names, required compiler flags etc. Linking against Intel
>> MKL is just a night-mare!
>>
>> I hope this explains it a bit (although the answer is probably not
>> very satisfying).
>>
>> Michael
>>
>
> Hi Michael,
>
> I'm not surprised that it's quite difficult to make checks for
> Lapack/Blas work cross-platform and cross-vendor.
>
> But the essence of my question was: why do I need a Fortran compiler!?
>
> As you wrote in your answer: That's probably because these modules  
> ONLY
> search for the Fortran interfaces. Now, that's exactly what they  
> should
> do: search for interfaces! Interfaces: that's libraries in the case of
> Fortran. No compiler is required IMHO, cause I'm only linking against
> (Fortran) libraries.
>
> Best regards,
> Marcel Loose.


Problem is, it is going to be difficult to check a Fortran interface  
with a C-compiler. Sometimes the names are all upper case, sometimes  
all lower case, sometimes there's a trailing underscore at the end of  
all symbols, sometimes even two if the name already contains an  
underscore. All of this makes writing a check_function_exists macro  
quite involved.

An yes, a compiler IS required. How else do you propose to check an  
interface? You could use "nm" on UNIX platforms and "DUMPBIN.exe" on  
Windows, but then you're not sure that the code is going to link  
successfully because there might be an incompatibility of name- 
mangling schemes between compiler, linker and library.

This is exactly one of the reasons why there is CBLAS and CLAPACK.  
Firstly they have interfaces suitable to C-languages, do not require  
the user to provide work-arrays, and all name-mangling issues have  
already been dealt with.

Cheers

Michael