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

[Jed Brown]

Agreed, though need to test that all relevant compilers error
appropriately (and that we accurately detect such errors).  Satish may
remember which are most problematic.

There are a few other --known arguments that we may need to think about.
I think these are tough:

  --known-snrm2-returns-double=0
  --known-sdot-returns-double=0


Byte swapping to/from big endian for integer types can be written in a
portable way that compiles to no-op (at least when optimization is on;
but see [1]), but I don't think that's possible for floating point data.
Of course we could just compile code for both options and select which
one to call at run-time.  Since they operate on arrays instead of
individual values, the dispatch should be negligible.

[1] It's sometimes buggy. https://bugs.llvm.org/show_bug.cgi?id=41545

"Smith, Barry F. via petsc-dev" <petsc-dev at mcs.anl.gov> writes:

>   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/