Hi Paul, thanks for the quick reply ! Would you mind double checking the definition of CFI_is_contiguous() in the current standard ? From https://j3-fortran.org/doc/year/12/12-119r1.pdf (this is a *draft* and not the final standard I do not have)
8.3.5.6 int CFI is contiguous ( const CFI cdesc t * dv ); Description. Test contiguity of an array. Formal Parameter. dv shall be the address of a C descriptor describing an array. The base_addr member of the C descriptor shall not be a null pointer. Result Value. CFI is contiguous returns 1 if the array described is determined to be contiguous, and 0 otherwise.
If the draft has been approved as-is, it means CFI_is_contiguous() is not supposed to return CFI_SUCCESS (and the Intel folks got it right in the 2019 compilers) I can certainly see an interest in copy-in for INTENT(IN) in order to enforce correctness of the Fortran code (assuming this is not prohibited by the standard) *as an option*. Generally speaking, I see that as quite undesirable for performance reasons. For example, in the context of MPI (Message Passing Library) I am primarly interested in, we can double precision :: a(0:1025,0:1025) call mpi_send(a(1:1024,1:1024, 1024*1024, MPI_DOUBLE_PRECISION, ...) in order to send a subarray (and the first parameter of mpi_send() is declared with INTENT(IN)). That currently works with gcc 8, but this involves a temporary (contiguous) array to be allocated/filled/freed at runtime. The primary motivation for using ISO_Fortran_binding.h, is to avoid the need for atemporary array and uses the data "in place" instead (and MPI will internally uses a temporary derived datatype in order to send the non-contiguous subarray, which is much more efficient with respect to performance and memory footprint). As a side note, MPI also provides the mpi_isend() subroutine that returns immediately (and the app must basically call mpi_wait() in order to make sure the message has been sent). In the case of gcc 8, and copy-in in current gcc9, I can see a risk of the temporary array being accessed *after* being freed when mpi_isend returns. No copy-in would avoid this scenario (but to be fully honest, I am not even sure this use-case is correct with respect to the MPI and/or Fortran standards) Feel free to post any patch you are working on and I will be happy to test them. Thanks ! Gilles On 4/4/2019 7:15 PM, Paul Richard Thomas wrote:
Hi Gilles,
Many thanks for your testing of ISO_Fortran_binding with gfortran.
With respect to the base address values, the difference comes about because of the use of INTENT(IN). In order to enforce the "INness", I chose to copy the data. Any other INTENT gives the value that you expect:
module cdesc interface subroutine cdesc_f08(buf, expected) BIND(C, name="cdesc_c") implicit none type(*), dimension(..), INTENT(INOUT) :: buf integer(kind=8),INTENT(IN) :: expected end subroutine cdesc_f08 end interface end module
I will check the standard to see if this copy-in for INTENT(IN) violates any requirement of the standard. I would be happy to get feedback as to whether this is considered to be desirable or not.
The CFI_is_contiguous test exposes one problem with ifort and one with gfortran: (i) According to F2018 18.5.4, "The macro CFI_SUCCESS shall be defined to be the integer constant 0." Thus, ifort is giving the wrong result. Using: #include <stdio.h> #include <stdlib.h> #include <ISO_Fortran_binding.h> void cdesc_c(CFI_cdesc_t* x, long *expected) { printf ("x->rank = %d, CFI_is_contigous(x) = %d, x->base_addr = %p, expected %p\n", x->rank, CFI_is_contiguous(x) == CFI_SUCCESS, x->base_addr, (void *)*expected); } should yield the same result for both, up to and including rank 2.
(ii) The gfortran CFI_is_contigous gives the wrong result for rank
=3. This is a simple, stupid bug that I have already fixed and will commit asap.
Many thanks again.
Paul