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

[Lisandro Dalcin]

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/
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