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). 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 2012-02-16 (N1904) 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

/* N1904 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;

/* If C99 were always available, we could use int_fast8_t, but signed char is probably good enough.  */
typedef signed char CFI_type_t;
typedef signed char CFI_rank_t;
typedef signed char CFI_attribute_t;

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


/* According to N1904, "It shall contain at least the following
   members. The first three members of the structure shall be
   base_addr, elem_len, and version in that order. The final member
   shall be dim, with the other members after version and before 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_t                            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_t                        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 2013-04-24 13:31:36 by TobiasBurnus)