On 9/3/18 7:50 PM, Julian Brown wrote:
On Fri, 31 Aug 2018 16:20:08 +0200
Jakub Jelinek <ja...@redhat.com> wrote:
On Fri, Aug 31, 2018 at 10:04:07AM -0400, Nathan Sidwell wrote:
On 08/30/2018 04:27 PM, Jason Merrill wrote:
On Thu, Aug 30, 2018 at 3:31 PM, Julian Brown
<jul...@codesourcery.com> wrote:
"Apart from parsing, it's necessary to prevent the "cannot take
the address of 'this', which is an rvalue expression" error
from appearing
Breaking a rather fundamental language attribute does not seem wise.
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.
Yes, transferring the this pointer is very unlikely to be what the
user wants -- the object being referred to contains the data. It
might be wise to look at the DR's and changes relating to lambdas
and this capture. Those changes now make it much harder to simply
capture the pointer unintentionally.
Yeah, I agree we shouldn't try to make this addressable.
Does OpenACC try to map the base of the array section (rather than
what e.g. OpenMP does, privatize the pointer base instead and assign
the pointer the new value inside of the region)?
Even if it is mapped, can't it be mapped by taking an address of a
temporary initialized from this?
For OpenACC, two mappings are created for an array section: one for the
data (to, from, tofrom, etc.) and a firstprivate pointer with a bias to
locate the (possibly virtual) zero'th element of the array. I think
that's the same as OpenMP.
For the test case given, it's sufficient to merely allow "this" to be
used as the base pointer for an array section. That usage doesn't
require "this" to be made addressable.
The this[0:1] syntax is accepted by PGI
(https://www.pgroup.com/resources/docs/18.4/x86/openacc-gs/index.htm,
2.4 C++ Classes in OpenACC) -- in order to copy "the class itself" to
the accelerator.
Referring to class member variables in OpenACC clauses (as the example
in that section does also) is still problematic in GCC, though.
PGI also allows the user to specify just "this" in OpenACC clauses,
which presumably does the same thing as specifying this[0:1]. For PGI,
but not for OpenACC <= 2.5, that seems to follow a general case for
pointers to structs (2.3. C Structs in OpenACC), "A pointer to a scalar
struct is treated as a one-element array, and should be shaped as
r[0:1]". That's notably different from OpenMP 4.5, which treats plain
mapped pointers as zero-length array sections, and also differs from
the current behaviour of GCC (which bizarrely, IIUC, is to copy the
bits of the host pointer verbatim to the target). OpenACC 2.5 arguably
leaves the behaviour unspecified for pointers without an explicit array
section.
The attached patch allows basic class-wrapping-an-array kinds of
usages, anyway. Re-tested with offloading to nvptx and bootstrapped. OK
to apply?
Thanks,
Julian
2018-09-03 Joseph Myers <jos...@codesourcery.com>
Julian Brown <jul...@codesourcery.com>
PR C++/66053
* semantics.c (handle_omp_array_sections_1): Allow array
sections with "this" pointer for OpenACC.
OK.
Jason
