[0/5] openmp: Support array-of-pointers (multi-segment) noncontiguous array sections and array-shaping casts

  GCC already lets a "target"/"target update" map clause describe a
noncontiguous array section or an array-shaping cast over a single
array or a single pointer indirection, e.g.:

  int a[N][M];
  #pragma omp target update to(a[2:3:4][0:4:2])

  int *p;
  #pragma omp target update to((([N]) p)[2:3:4])

What wasn't supported is a section or shape-cast that crosses more
than one pointer indirection, e.g. an array-of-pointers:

  int *a[N];
  #pragma omp target update to(a[2:3:4][0:4:2])

Each further indirection needs its own, independently-strided
"segment" of dimensions in the noncontiguous-array descriptor that
gets built for the directive and walked on the target side, since the
memory reachable through each pointer in the outer dimension(s) is a
separate, unrelated allocation. This series adds that support, for C
and C++, on top of the existing single-segment (plain array section /
single pointer indirection) machinery [1].

Patch 1 adds the new GOMP_MAP_SHAPE_DIM map kind and the
OMP_CLAUSE_MAP_GRID_DIM_POINTER clause flag used by the rest of the
series to mark a dimension as selecting through a pointer.

Patch 2 teaches the C and C++ front ends to build the right clause
chain for these cases (array-of-pointers sections, array-shaping
casts crossing a pointer, decayed array parameters), with tests
scanning the "original" dump to check the clauses built for each case.

Patches 3 and 4 do the corresponding lowering work: patch 3 is a
pure, non-functional outlining of the (pre-existing, single-segment)
descriptor-construction code in lower_omp_target into its own
function, lower_omp_target_grid_desc; patch 4 generalizes that
function to multiple segments and the new clause kind, and adds the
"lower" dump scans (checking the generated descriptor) to the tests
added in patch 2.

Patch 5 extends the libgomp runtime descriptor and its gather/scatter
loops to walk multiple segments, with C and C++ runtime tests for one,
two, and three segments.

This series targets devel/omp/gcc-16.

[1] The strided patches have not landed in mainline yet. They are on
the devel/omp/gcc-16 branch in the commits:
2a5fe5c4b3f OpenMP: Support strided and shaped-array updates for C++
40617ea1249 OpenMP: Array shaping operator and strided "target update" for C
1b6460d9eb6 OpenMP: Noncontiguous "target update" for Fortran

Paul-Antoine Arras (5):
  openmp: Add GOMP_MAP_SHAPE_DIM and grid-dim pointer marker
    prerequisites
  openmp: Support pointer indirection in array-shaping casts and array
    sections
  openmp: Outline lower_omp_target_grid_desc from lower_omp_target
  openmp: Generalize lower_omp_target_grid_desc to segments and pointer
    sections
  openmp: Support multi-segment noncontiguous descriptors in libgomp

 gcc/c-family/c-omp.cc                         | 151 +++-
 gcc/c/c-parser.cc                             |  59 +-
 gcc/c/c-typeck.cc                             | 144 +++-
 gcc/cp/decl.cc                                |  50 +-
 gcc/cp/parser.cc                              |  69 +-
 gcc/cp/semantics.cc                           | 114 ++-
 gcc/gimplify.cc                               |  18 +-
 gcc/omp-general.cc                            |   5 +-
 gcc/omp-low.cc                                | 798 +++++++++++-------
 .../c-c++-common/gomp/array-section-1.c       |  40 +
 .../c-c++-common/gomp/array-section-2.c       |  22 +
 .../c-c++-common/gomp/array-section-3.c       |  45 +
 .../c-c++-common/gomp/array-section-4.c       |  28 +
 .../c-c++-common/gomp/array-section-5.c       |  32 +
 .../c-c++-common/gomp/array-section-6.c       |  19 +
 .../c-c++-common/gomp/array-section-8.c       |  34 +
 .../c-c++-common/gomp/array-section-9.c       |  28 +
 .../gomp/target-update-iterators-4.c          |  19 +
 gcc/testsuite/g++.dg/gomp/array-shaping-3.C   |  71 ++
 gcc/testsuite/g++.dg/gomp/array-shaping-4.C   |  37 +
 .../g++.dg/gomp/bad-array-shaping-9.C         |  51 ++
 gcc/testsuite/gcc.dg/gomp/array-shaping-9.c   |  25 +
 .../gcc.dg/gomp/bad-array-section-c-9.c       |  60 ++
 .../gcc.dg/gomp/bad-array-shaping-c-8.c       |  45 +
 gcc/tree-pretty-print.cc                      |   9 +
 gcc/tree.h                                    |   7 +
 include/gomp-constants.h                      |   3 +-
 libgomp/libgomp.h                             |  24 +-
 libgomp/target.c                              | 176 +++-
 .../testsuite/libgomp.c++/array-section-1.C   | 204 +++++
 .../testsuite/libgomp.c++/array-section-2.C   | 130 +++
 .../libgomp.c-c++-common/array-section-1.c    | 181 ++++
 .../libgomp.c-c++-common/array-section-10.c   |  69 ++
 .../libgomp.c-c++-common/array-section-11.c   |  73 ++
 .../libgomp.c-c++-common/array-section-2.c    |  75 ++
 .../libgomp.c-c++-common/array-section-3.c    | 135 +++
 .../libgomp.c-c++-common/array-section-4.c    | 134 +++
 .../libgomp.c-c++-common/array-section-5.c    | 136 +++
 .../libgomp.c-c++-common/array-section-6.c    |  75 ++
 .../libgomp.c-c++-common/array-section-7.c    | 104 +++
 .../libgomp.c-c++-common/array-section-8.c    |  87 ++
 .../libgomp.c-c++-common/array-section-9.c    | 120 +++
 42 files changed, 3213 insertions(+), 493 deletions(-)
 create mode 100644 gcc/testsuite/c-c++-common/gomp/array-section-1.c
 create mode 100644 gcc/testsuite/c-c++-common/gomp/array-section-2.c
 create mode 100644 gcc/testsuite/c-c++-common/gomp/array-section-3.c
 create mode 100644 gcc/testsuite/c-c++-common/gomp/array-section-4.c
 create mode 100644 gcc/testsuite/c-c++-common/gomp/array-section-5.c
 create mode 100644 gcc/testsuite/c-c++-common/gomp/array-section-6.c
 create mode 100644 gcc/testsuite/c-c++-common/gomp/array-section-8.c
 create mode 100644 gcc/testsuite/c-c++-common/gomp/array-section-9.c
 create mode 100644 gcc/testsuite/c-c++-common/gomp/target-update-iterators-4.c
 create mode 100644 gcc/testsuite/g++.dg/gomp/array-shaping-3.C
 create mode 100644 gcc/testsuite/g++.dg/gomp/array-shaping-4.C
 create mode 100644 gcc/testsuite/g++.dg/gomp/bad-array-shaping-9.C
 create mode 100644 gcc/testsuite/gcc.dg/gomp/array-shaping-9.c
 create mode 100644 gcc/testsuite/gcc.dg/gomp/bad-array-section-c-9.c
 create mode 100644 gcc/testsuite/gcc.dg/gomp/bad-array-shaping-c-8.c
 create mode 100644 libgomp/testsuite/libgomp.c++/array-section-1.C
 create mode 100644 libgomp/testsuite/libgomp.c++/array-section-2.C
 create mode 100644 libgomp/testsuite/libgomp.c-c++-common/array-section-1.c
 create mode 100644 libgomp/testsuite/libgomp.c-c++-common/array-section-10.c
 create mode 100644 libgomp/testsuite/libgomp.c-c++-common/array-section-11.c
 create mode 100644 libgomp/testsuite/libgomp.c-c++-common/array-section-2.c
 create mode 100644 libgomp/testsuite/libgomp.c-c++-common/array-section-3.c
 create mode 100644 libgomp/testsuite/libgomp.c-c++-common/array-section-4.c
 create mode 100644 libgomp/testsuite/libgomp.c-c++-common/array-section-5.c
 create mode 100644 libgomp/testsuite/libgomp.c-c++-common/array-section-6.c
 create mode 100644 libgomp/testsuite/libgomp.c-c++-common/array-section-7.c
 create mode 100644 libgomp/testsuite/libgomp.c-c++-common/array-section-8.c
 create mode 100644 libgomp/testsuite/libgomp.c-c++-common/array-section-9.c