[petsc-dev] Cross-compiling/batch systems and getting rid of --know-sizeof-<type> (or at least making it not required at all)

[Smith, Barry F.]

  It would be fantastic if we could avoid the need for the known values and ideally the need for batch completely!!!!
 
  This is a great idea. 

> On May 22, 2019, at 5:02 AM, Lisandro Dalcin via petsc-dev <petsc-dev at mcs.anl.gov> wrote:
> 
> Barry/Satish, you know that making requests without patches is not my style, but this one involves messing with BuildSystem, so please pardon me.
> 
> I have a easy and quick proposal for compile-time determination of sizeof() for the various C types. For example, in our Cray XC40, I'm passing all these flags to configure to avoid the need of running with batch:
> 
> $ grep known-sizeof reconfigure-arch-gnu-opt.py
>     '--known-sizeof-MPI_Comm=4',
>     '--known-sizeof-MPI_Fint=4',
>     '--known-sizeof-char=1',
>     '--known-sizeof-double=8',
>     '--known-sizeof-float=4',
>     '--known-sizeof-int=4',
>     '--known-sizeof-long-long=8',
>     '--known-sizeof-long=8',
>     '--known-sizeof-short=2',
>     '--known-sizeof-size_t=8',
>     '--known-sizeof-void-p=8',
> 
> 
> Look at the following two line C source, TYPE and SIZE have to be passed through the preprocessor in this quick example. Defining main is of course not required if we pass `-c` to the compiler.
> 
> $ cat check-sizeof.c
> typedef char assert_sizeof[(sizeof(TYPE)==SIZE)*2-1];
> int main(int arg, char *argv[]) { return 0;}
> 
> Let's try to determine sizeof(double) by compile-time checks that do not need to run the executable.
> 
> $ cc -DTYPE=double -DSIZE=1 check-sizeof.c
> check-sizeof.c:1:14: error: size of array ‘assert_sizeof’ is negative
>     1 | typedef char assert_sizeof[(sizeof(TYPE)==SIZE)*2-1];
>       |              ^~~~~~~~~~~~~
> 
> $ cc -DTYPE=double -DSIZE=2 check-sizeof.c
> check-sizeof.c:1:14: error: size of array ‘assert_sizeof’ is negative
>     1 | typedef char assert_sizeof[(sizeof(TYPE)==SIZE)*2-1];
>       |              ^~~~~~~~~~~~~
> 
> $ cc -DTYPE=double -DSIZE=4 check-sizeof.c
> check-sizeof.c:1:14: error: size of array ‘assert_sizeof’ is negative
>     1 | typedef char assert_sizeof[(sizeof(TYPE)==SIZE)*2-1];
>       |              ^~~~~~~~~~~~~
> 
> Up to here, sizeof(double) is not 1, nor 2, nor 4.
> 
> Let's try now if sizeof(double) is 8:
> 
> $ cc -DTYPE=double -DSIZE=8 check-sizeof.c
> 
> No compile error. Success! Now we know sizeof(double) is 8, we don't need to run an executable, which is ideal for cross-compiling or to avoid running the configure test with batch.
> 
> 
> -- 
> Lisandro Dalcin
> ============
> Research Scientist
> Extreme Computing Research Center (ECRC)
> King Abdullah University of Science and Technology (KAUST)
> http://ecrc.kaust.edu.sa/