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" <[email protected]> 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 >> <[email protected]> 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/
