On 2023/7/11 2:33 AM, Chung-Lin Tang via Gcc-patches wrote:
> As we discussed earlier, the work for actually linking this to middle-end
> points-to analysis is a somewhat non-trivial issue. This first patch allows
> the language feature to be used in OpenACC directives first (with no effect for now).
> The middle-end changes are probably going to be a later patch.

This second patch tries to link the readonly modifier to points-to analysis.

There already exists SSA_NAME_POINTS_TO_READONLY_MEMORY and it's support in the
alias oracle routines in tree-ssa-alias.cc, so basically what this patch does is
try to make the variables holding the array section base pointers to have this
flag set.

There is an another OMP_CLAUSE_MAP_POINTS_TO_READONLY set by front-ends on the
associated pointer clauses if OMP_CLAUSE_MAP_READONLY is set.
Also a DECL_POINTS_TO_READONLY flag is set for VAR_DECLs when creating the tmp
vars carrying these receiver references on the offloaded side. These
eventually get translated to SSA_NAME_POINTS_TO_READONLY_MEMORY.

This still doesn't always work as expected in terms of optimization:
struct pointer fields and Fortran arrays (kind of like C structs) which have
several accesses to create the pointer access on the receive/offloaded side,
and SRA appears to not work on these sequences, so gets in the way of much
redundancy elimination.

Currently have one testcase where we can demonstrate 'readonly' can avoid
a clobber by function call. Tested on powerpc64le-linux/nvptx.

Note this patch is create a-top of the front-end patch.
(will respond to the other front-end patch comments later)

Thanks,
Chung-Lin

2023-07-25  Chung-Lin Tang  <cltang@codesourcery.com>

gcc/c/ChangeLog:

	* c-typeck.cc (handle_omp_array_sections):
	Set OMP_CLAUSE_MAP_POINTS_TO_READONLY on pointer clause.

gcc/cp/ChangeLog:

	* semantics.cc (handle_omp_array_sections):
	Set OMP_CLAUSE_MAP_POINTS_TO_READONLY on pointer clause.

gcc/fortran/ChangeLog:

	* trans-openmp.cc (gfc_trans_omp_array_section):
	Set OMP_CLAUSE_MAP_POINTS_TO_READONLY on pointer clause.

gcc/ChangeLog:

	* gimple-expr.cc (copy_var_decl): Copy DECL_POINTS_TO_READONLY
	for VAR_DECLs.
	* gimplify.cc (struct gimplify_omp_ctx):
	Add 'hash_set<tree_operand_hash> *pt_readonly_ptrs' field.
	(internal_get_tmp_var):	Set
	DECL_POINTS_TO_READONLY/SSA_NAME_POINTS_TO_READONLY_MEMORY for
	new temp vars.
	(build_omp_struct_comp_nodes):
	Set OMP_CLAUSE_MAP_POINTS_TO_READONLY on pointer clause.
	(gimplify_scan_omp_clauses): Collect OMP_CLAUSE_MAP_POINTS_TO_READONLY
	to ctx->pt_readonly_ptrs.
	* omp-low.cc (lower_omp_target): Set DECL_POINTS_TO_READONLY for
	variables of receiver refs.
	* tree-pretty-print.cc (dump_omp_clause):
	Print OMP_CLAUSE_MAP_POINTS_TO_READONLY.
	(dump_generic_node): Print SSA_NAME_POINTS_TO_READONLY_MEMORY.
	* tree.h (DECL_POINTS_TO_READONLY): New macro.
	(OMP_CLAUSE_MAP_POINTS_TO_READONLY): New macro.

gcc/testsuite/ChangeLog:

	* c-c++-common/goacc/readonly-1.c: Adjust testcase.
	* c-c++-common/goacc/readonly-2.c: New testcase.
	* gfortran.dg/goacc/readonly-1.f90: Adjust testcase.