openmp: Conforming device numbers and omp_{initial,invalid}_device

openmp: Conforming device numbers and omp_{initial,invalid}_device

OpenMP 5.2 changed once more what device numbers are allowed.
In 5.1, valid device numbers were [0, omp_get_num_devices()].
5.2 makes also -1 valid (calls it omp_initial_device), which is equivalent
in behavior to omp_get_num_devices() number but has the advantage that it
is a constant.  And it also introduces omp_invalid_device which is
also a constant with implementation defined value < -1.  That value should
act like sNaN, any time any device construct (GOMP_target*) or OpenMP runtime
API routine is asked for such a device, the program is terminated.
And if OMP_TARGET_OFFLOAD=mandatory, all non-conforming device numbers (which
is all but [-1, omp_get_num_devices()] other than omp_invalid_device)
must be treated like omp_invalid_device.

For device constructs, we have a compatibility problem, we've historically
used 2 magic negative values to mean something special.
GOMP_DEVICE_ICV (-1) means device clause wasn't present, pick the
		     omp_get_default_device () number
GOMP_DEVICE_FALLBACK (-2) means the host device (this is used e.g. for
			  #pragma omp target if (cond)
			  where if cond is false, we pass -2
But 5.2 requires that omp_initial_device is -1 (there were discussions
about it, advantage of -1 is that one can say iterate over the
[-1, omp_get_num_devices()-1] range to get all devices starting with
the host/initial one.
And also, if user passes -2, unless it is omp_invalid_device, we need to
treat it like non-conforming with OMP_TARGET_OFFLOAD=mandatory.

So, the patch does on the compiler side some number remapping,
user_device_num >= -2U ? user_device_num - 1 : user_device_num.
This remapping is done at compile time if device clause has constant
argument, otherwise at runtime, and means that for user -1 (omp_initial_device)
we pass -2 to GOMP_* in the runtime library where it treats it like host
fallback, while -2 is remapped to -3 (one of the non-conforming device numbers,
for those it doesn't matter which one is which).
omp_invalid_device is then -4.
For the OpenMP device runtime APIs, no remapping is done.

This patch doesn't deal with the initial default-device-var for
OMP_TARGET_OFFLOAD=mandatory , the spec says that the inital ICV value
for that should in that case depend on whether there are any offloading
devices or not (if not, should be omp_invalid_device), but that means
we can't determine the number of devices lazily (and let libraries have the
possibility to register their offloading data etc.).

2022-06-13  Jakub Jelinek  <jakub@redhat.com>

gcc/
	* omp-expand.cc (expand_omp_target): Remap user provided
	device clause arguments, -1 to -2 and -2 to -3, either
	at compile time if constant, or at runtime.
include/
	* gomp-constants.h (GOMP_DEVICE_INVALID): Define.
libgomp/
	* omp.h.in (omp_initial_device, omp_invalid_device): New enumerators.
	* omp_lib.f90.in (omp_initial_device, omp_invalid_device): New
	parameters.
	* omp_lib.h.in (omp_initial_device, omp_invalid_device): Likewise.
	* target.c (resolve_device): Add remapped argument, handle
	GOMP_DEVICE_ICV only if remapped is true (and clear remapped),
	for negative values, treat GOMP_DEVICE_FALLBACK as fallback only
	if remapped, otherwise treat omp_initial_device that way.  For
	omp_invalid_device, always emit gomp_fatal, even when
	OMP_TARGET_OFFLOAD isn't mandatory.
	(GOMP_target, GOMP_target_ext, GOMP_target_data, GOMP_target_data_ext,
	GOMP_target_update, GOMP_target_update_ext,
	GOMP_target_enter_exit_data): Pass true as remapped argument to
	resolve_device.
	(omp_target_alloc, omp_target_free, omp_target_is_present,
	omp_target_memcpy_check, omp_target_associate_ptr,
	omp_target_disassociate_ptr, omp_get_mapped_ptr,
	omp_target_is_accessible): Pass false as remapped argument to
	resolve_device.  Treat omp_initial_device the same as
	gomp_get_num_devices ().  Don't bypass resolve_device calls if
	device_num is negative.
	(omp_pause_resource): Treat omp_initial_device the same as
	gomp_get_num_devices ().  Call resolve_device.
	* icv-device.c (omp_set_default_device): Always set to device_num
	even when it is negative.
	* libgomp.texi: Document that Conforming device numbers,
	omp_initial_device and omp_invalid_device is implemented.
	* testsuite/libgomp.c/target-41.c (main): Add test with
	omp_initial_device.
	* testsuite/libgomp.c/target-45.c: New test.
	* testsuite/libgomp.c/target-46.c: New test.
	* testsuite/libgomp.c/target-47.c: New test.
	* testsuite/libgomp.c-c++-common/target-is-accessible-1.c (main): Add
	test with omp_initial_device.  Use -5 instead of -1 for negative value
	test.
	* testsuite/libgomp.fortran/target-is-accessible-1.f90 (main):
	Likewise.  Reorder stop numbers.