Gfortran array descriptor update
See also LibgfortranAbiCleanup - when the ABI is changed, the clean up should be done.
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.
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:
Stride in bytes rather than sizeof(array_element_type). This is needed in order to efficiently support derived type components? PR ...? PR 40737, PR 38471
Flags for allocated and associated status. This avoids ugly workarounds like allocating a 1 byte array for zero sized allocations, see e.g. PR 35719, and is probably necessary for solving PR 35718.
Store each dimension as a (lbound, stride, extent) triplet rather than the current (lbound, stride, ubound). With the former ubound can be calculated as ubound = lbound + stride * extent which is essentially free whereas with the current setup extent must be calculated as extent = (ubound - lbound) / stride where the division is quite expensive. See e.g. tables with instruction timings and throughput.
* 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.
For comparison, see Intel Fortran array descriptor (mixed language programming chapter), Pathscale array descriptor (Appendix D)
The TR 29113 draft as of 2008-11-06 had the following as a test case "to ensure what the TR says is compilable with gcc" - before implementing, check whether this has been approved meanwhile or whether a newer draft is available and check the exact meaning of the elements, the wording below is at least partially not the official one.
typedef struct {
intptr_t lower_bound,
extent, /* = number of elements in this dimension. */
sm; /* stride multiplier; = pointer difference betwixt two consecutive elements. */
} CFI_dim_t;
typedef struct {
void * base_addr; /* base address of object */
size_t elem_len; /* length of one element, in bytes */
int rank; /* object rank, 0 .. CF_MAX_RANK */
int type; /* identifier for type of object */
int attribute; /* object attribute: 0..2, or -1 */
int state; /* allocation/association state: 0 or 1 */
void * fdesc; /* pointer to corresponding Fortran descriptor */
CFI_dim_t dim[CFI_MAX_RANK]; /* dimension triples */
} CFI_cdesc_t;
/* Sympolic names for attributes of objects */
#define CFI_attribute_assumed 0
#define CFI_attribute_allocatable 1
#define CFI_attribute_pointer 2
/* Symbolic names for type identifiers */
#define CFI_type_unknown 0
#define CFI_type_struct 100
#define CFI_type_signed_char 1
#define CFI_type_short 3
#define CFI_type_int 5
#define CFI_type_long 7
#define CFI_type_long_long 9
#define CFI_type_size_t 11
#define CFI_type_int8_t 12
#define CFI_type_int16_t 14
#define CFI_type_int32_t 16
#define CFI_type_int64_t 18
#define CFI_type_int_least8_t 20
#define CFI_type_int_least16_t 22
#define CFI_type_int_least32_t 24
#define CFI_type_int_least64_t 26
#define CFI_type_int_fast8_t 28
#define CFI_type_int_fast16_t 30
#define CFI_type_int_fast32_t 32
#define CFI_type_int_fast64_t 34
#define CFI_type_intmax_t 36
#define CFI_type_intptr_t 37
#define CFI_type_float 38
#define CFI_type_double 39
#define CFI_type_long_double 40
#define CFI_type_float_Complex 41
#define CFI_type_double_Complex 42
#define CFI_type_long_double_Complex 43
#define CFI_type_Bool 44
#define CFI_type_char 45The 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 needs to 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.