openacc: Add support for gang local storage allocation in shared memory [PR90115]

openacc: Add support for gang local storage allocation in shared memory [PR90115]

This patch implements a method to track the "private-ness" of
OpenACC variables declared in offload regions in gang-partitioned,
worker-partitioned or vector-partitioned modes. Variables declared
implicitly in scoped blocks and those declared "private" on enclosing
directives (e.g. "acc parallel") are both handled. Variables that are
e.g. gang-private can then be adjusted so they reside in GPU shared
memory.

The reason for doing this is twofold: correct implementation of OpenACC
semantics, and optimisation, since shared memory might be faster than
the main memory on a GPU. Handling of private variables is intimately
tied to the execution model for gangs/workers/vectors implemented by
a particular target: for current targets, we use (or on mainline, will
soon use) a broadcasting/neutering scheme.

That is sufficient for code that e.g. sets a variable in worker-single
mode and expects to use the value in worker-partitioned mode. The
difficulty (semantics-wise) comes when the user wants to do something like
an atomic operation in worker-partitioned mode and expects a worker-single
(gang private) variable to be shared across each partitioned worker.
Forcing use of shared memory for such variables makes that work properly.

In terms of implementation, the parallelism level of a given loop is
not fixed until the oaccdevlow pass in the offload compiler, so the
patch delays fixing the parallelism level of variables declared on or
within such loops until the same point. This is done by adding a new
internal UNIQUE function (OACC_PRIVATE) that lists (the address of) each
private variable as an argument, and other arguments set so as to be able
to determine the correct parallelism level to use for the listed
variables. This new internal function fits into the existing scheme for
demarcating OpenACC loops, as described in comments in the patch.

Two new target hooks are introduced: TARGET_GOACC_ADJUST_PRIVATE_DECL and
TARGET_GOACC_EXPAND_VAR_DECL.  The first can tweak a variable declaration
at oaccdevlow time, and the second at expand time.  The first or both
of these target hooks can be used by a given offload target, depending
on its strategy for implementing private variables.

This patch updates the TARGET_GOACC_ADJUST_PRIVATE_DECL target hook in
the AMD GCN backend to the current name and prototype. (An earlier
version of the hook was already present, but dormant.)

	gcc/
	PR middle-end/90115
	* doc/tm.texi.in (TARGET_GOACC_EXPAND_VAR_DECL)
	(TARGET_GOACC_ADJUST_PRIVATE_DECL): Add documentation hooks.
	* doc/tm.texi: Regenerate.
	* expr.c (expand_expr_real_1): Expand decls using the
	expand_var_decl OpenACC hook if defined.
	* internal-fn.c (expand_UNIQUE): Handle IFN_UNIQUE_OACC_PRIVATE.
	* internal-fn.h (IFN_UNIQUE_CODES): Add OACC_PRIVATE.
	* omp-low.c (omp_context): Add oacc_privatization_candidates
	field.
	(lower_oacc_reductions): Add PRIVATE_MARKER parameter.  Insert
	before fork.
	(lower_oacc_head_tail): Add PRIVATE_MARKER parameter.  Modify
	private marker's gimple call arguments, and pass it to
	lower_oacc_reductions.
	(oacc_privatization_scan_clause_chain)
	(oacc_privatization_scan_decl_chain, lower_oacc_private_marker):
	New functions.
	(lower_omp_for, lower_omp_target, lower_omp_1): Use these.
	* omp-offload.c (convert.h): Include.
	(oacc_loop_xform_head_tail): Treat private-variable markers like
	fork/join when transforming head/tail sequences.
	(struct var_decl_rewrite_info): Add struct.
	(oacc_rewrite_var_decl, is_sync_builtin_call): New functions.
	(execute_oacc_device_lower): Support rewriting gang-private
	variables using target hook, and fix up addr_expr and var_decl
	nodes afterwards.
	* target.def (adjust_private_decl, expand_var_decl): New hooks.
	* config/gcn/gcn-protos.h (gcn_goacc_adjust_gangprivate_decl):
	Rename to...
	(gcn_goacc_adjust_private_decl): ...this.
	* config/gcn/gcn-tree.c (gcn_goacc_adjust_gangprivate_decl):
	Rename to...
	(gcn_goacc_adjust_private_decl): ...this. Add LEVEL parameter.
	* config/gcn/gcn.c (TARGET_GOACC_ADJUST_GANGPRIVATE_DECL): Rename
	definition using gcn_goacc_adjust_gangprivate_decl...
	(TARGET_GOACC_ADJUST_PRIVATE_DECL): ...to this, using
	gcn_goacc_adjust_private_decl.
	* config/nvptx/nvptx.c (tree-pretty-print.h): Include.
	(gang_private_shared_size): New global variable.
	(gang_private_shared_align): Likewise.
	(gang_private_shared_sym): Likewise.
	(gang_private_shared_hmap): Likewise.
	(nvptx_option_override): Initialize these.
	(nvptx_file_end): Output gang_private_shared_sym.
	(nvptx_goacc_adjust_private_decl, nvptx_goacc_expand_var_decl):
	New functions.
	(nvptx_set_current_function): Clear gang_private_shared_hmap.
	(TARGET_GOACC_ADJUST_PRIVATE_DECL): Define hook.
	(TARGET_GOACC_EXPAND_VAR_DECL): Likewise.
	libgomp/
	PR middle-end/90115
	* testsuite/libgomp.oacc-c-c++-common/private-atomic-1-gang.c: New
	test.
	* testsuite/libgomp.oacc-fortran/private-atomic-1-gang.f90:
	Likewise.
	* testsuite/libgomp.oacc-fortran/private-atomic-1-worker.f90:
	Likewise.

Co-Authored-By: Chung-Lin Tang <cltang@codesourcery.com>
Co-Authored-By: Thomas Schwinge <thomas@codesourcery.com>