Re: [PATCH, OpenACC] Support C++ "this" in OpenACC directives (PR66053)

[Jason Merrill <jason@redhat.com>]

On Thu, Aug 30, 2018 at 3:31 PM, Julian Brown <julian@codesourcery.com> wrote:
> This patch (by Joseph) allows "this" to be used in OpenACC directives,
> following -- IIUC -- the behaviour of other compilers. The standard (as
> of OpenACC 2.5) does not appear to have explicit language either
> permitting or forbidding such usage.
>
> Joseph's original commentary is in the bug report (ca. 2015,
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66053):
>
> "This patch, for gomp-4_0-branch, adds support for C++ "this" in
> OpenACC directives.  (This patch does not do anything to handle OpenMP
> differently from OpenACC; that - bug 66053 - will need to be resolved
> for mainline, either deciding these cases should be accepted for
> OpenMP or making the parsing only accept them in OpenACC directives
> and not OpenMP ones.)
>
> "Apart from parsing, it's necessary to prevent the "cannot take the
> address of 'this', which is an rvalue expression" error from appearing
> when "this" is used in such contexts.  This patch duly adds a new
> argument to cxx_mark_addressable (default false so callers don't all
> need to change) to allow disabling that error, passing that argument
> in all calls that seem relevant to OpenACC directives."
>
> AFAICT though, this attached version of the patch does still forbid
> "this" in OpenMP directives (apart from "declare simd"). I couldn't see
> that there was any change to the OpenMP spec language in 4.5.
>
> Tested with offloading to NVPTX and bootstrapped.
>
> OK to apply?
>
> Julian
>
> ChangeLog
>
> 20xx-xx-xx  Joseph Myers  <joseph@codesourcery.com>
>
>         PR C++/66053
>
>         gcc/cp/
>         * cp-tree.h (enum cxx_mark_addressable_flags): New.
>         (cxx_mark_addressable): Use it.  Adjust users.
>         * parser.c (cp_parser_omp_var_list_no_open): Handle RID_THIS.
>         * semantics.c (handle_omp_array_sections_1)
>         (handle_omp_array_sections, finish_omp_reduction_clause)
>         (finish_omp_clauses): Pass CXX_MARK_ADDRESSABLE_FLAGS_ALLOW_THIS
>         to cxx_mark_addressable.  Enforce "this" usage limitation only
>         for OpenMP.
>         * typeck.c (cp_build_array_ref): Adjust cxx_mark_addressble
>         call. (cxx_mark_addressable): Handle
>         CXX_MARK_ADDRESSABLE_FLAGS_ALLOW_THIS.
>
>         libgomp/
>         * testsuite/libgomp.oacc-c++/this.C: New test.

Why does referring to this[0:1] require making 'this' addressable?
Surely what we're interested in is the value of 'this', not the
address.

Jason