Gfortran array descriptor update

In some cases it would be nice to improve gfortran's array descriptors (cf. PR37577).

See also discussion at comp.lang.fortran about base_address, base_address + offset, and base_address + virtual_origin. See also a reasoning why virtual origins are a bad idea.

* See discussion about hooks in fortran@

Due to backward compatibility issues, it should be designed carefully so we don't have to do it that often. Libgfortran backward compatibility can be handled with SymbolVersioning , but a more difficult problem will be existing user code (is it feasible at all?).

Features desirable that aren't available in the current descriptor:

* Support rank 15 arrays (as planned for F 2008)

* Save coarray information. int corank might be enough.

* Save type information for (a) polymorphic data types but also (b) for run-time diagnostic.

* Support TR 29113 "Further Interoperability of Fortran with C", preferably as native format. See PR 37577. N1942 (FDTS as sent to SC22, 11 October 2012). F2018 draft contains TR 29113. For comparison, see Intel Fortran array descriptor (mixed language programming chapter), Pathscale array descriptor (Appendix D)

* Note that using the TR 29113 descriptor as the native one has implications for the procedure call interface as well. See ML message

The following is one possible implementation of the descriptor according to the Fortran 2018 2017-12-28 (N2146) draft. Note that the specification does not provide a header file, only textual descriptions of what should be found. See also current version in the fortran-dev branch. 

#ifndef ISO_FORTRAN_BINDING_H
#define ISO_FORTRAN_BINDING_H

#include <stddef.h>

#define CFI_VERSION 0

/* N2137 says that CFI_index_t "is a typedef name for a standard signed integer type capable of
   representing the result of subtracting two pointers". In C89, this is ptrdiff_t.  */
typedef ptrdiff_t CFI_index_t;

/* F2018 (N2146 says):
   CFI_attribute_t is a typedef name for a standard integer type capable
   of representing the values of the attribute codes.

   CFI_type_t is a typedef name for a standard integer type capable of
   representing the values for the supported type specifiers.

   CFI_rank_t is a typedef name for a standard integer type capable of
   representing the largest supported rank.
*/
typedef signed char CFI_type_t;
typedef unsigned char CFI_rank_t;
typedef unsigned char CFI_attribute_t;

typedef struct {
  CFI_index_t  lower_bound,
               extent, /* = number of elements in this dimension.  */
               sm; /* stride multiplier; = pointer difference in bytes between two consecutive elements.  */
} CFI_dim_t;


/* According to N2146, "The first three members of the structure shall be
   base_addr, elem_len, and version in that order. The final member
   shall be dim. All other members shall be between version and dim,
   in any order.". That is, other struct members are allowed, but at
   least the ones below should be present.  */

typedef struct {
 void *           base_addr;          /* base address of object                      */
 size_t           elem_len;           /* length of one element, in bytes             */
 int              version;            /* Must be CFI_VERSION                         */
 CFI_rank_t       rank;               /* object rank, 0 .. CF_MAX_RANK               */
 CFI_type_t       type;               /* identifier for type of object               */
 CFI_attribute_t  attribute;          /* object attribute                            */
 /* Gfortran specific fields here, must be between the version and dim[] fields.  */
 CFI_dim_t        dim[];              /* dimension triples                           */
} CFI_cdesc_t;


/* Sympolic names for attributes of objects   */

#define CFI_attribute_pointer            1
#define CFI_attribute_allocatable        2
#define CFI_attribute_other              3

/* Symbolic names for type identifiers */

#define CFI_type_signed_char                  0
#define CFI_type_short                        1
#define CFI_type_int                          2
#define CFI_type_long                         3
#define CFI_type_long_long                    4
#define CFI_type_size_t                       5
#define CFI_type_int8_                        6
#define CFI_type_int16_t                      7
#define CFI_type_int32_t                      8
#define CFI_type_int64_t                      9
#define CFI_type_int_least8_t                10
#define CFI_type_int_least16_t               11
#define CFI_type_int_least32_t               12
#define CFI_type_int_least64_t               13
#define CFI_type_int_fast8_t                 14
#define CFI_type_int_fast16_t                15
#define CFI_type_int_fast32_t                16
#define CFI_type_int_fast64_t                17
#define CFI_type_intmax_t                    18
#define CFI_type_intptr_                     19
#define CFI_type_ptrdiff_t                   20
#define CFI_type_float                       21
#define CFI_type_double                      22
#define CFI_type_long_double                 23
#define CFI_type_float_Complex               24
#define CFI_type_double_Complex              25
#define CFI_type_long_double_Complex         26
#define CFI_type_Bool                        27
#define CFI_type_char                        28
#define CFI_type_cptr                        29
#define CFI_type_cfunptr                     30
#define CFI_type_struct                      31
#define CFI_type_other                       -2  /* The value for CFI type other shall be negative and distinct from all other type specifiers. */


#endif

The type is e.g. c_int8_t, c_char, etc., "TYPE" (presumably one should distinguish between sequence, bind(c) and extensible etc.). The Fortran descriptor is needed for character string lengths, flags like contiguous (cf. also F2008's contiguous attribute) or whether a pointer target is a pointer (and thus deallocatable). For Fortran 2008 one presumably also needs information about coarrays (e.g. number of co-ranks, number of contained ranks [for OpenMP/threads one can use the descriptor for co-ranks, for MPI not]). And one might want to store data for CLASS and derived-type length parameters. One should also make fdesc extensible.

None: ArrayDescriptorUpdate (last edited 2018-01-24 08:13:57 by JanneBlomqvist)