[Openmp-commits] [PATCH] D65001: [OpenMP][libomptarget] Add support for unified memory for regular maps
Jonas Hahnfeld via Phabricator via Openmp-commits
openmp-commits at lists.llvm.org
Mon Jul 29 12:23:48 PDT 2019
Hahnfeld added inline comments.
Comment at: libomptarget/src/omptarget.cpp:247-249
+ // TODO: Check if this is correct
+ bool IsInUseDevicePtrClause = arg_types[i] & OMP_TGT_MAPTYPE_TARGET_PARAM &&
+ arg_types[i] & OMP_TGT_MAPTYPE_RETURN_PARAM;
> Hahnfeld wrote:
> > gtbercea wrote:
> > > Hahnfeld wrote:
> > > > grokos wrote:
> > > > > This is correct, with one little exception. Although the OpenMP standard does not mandate it, upstream clang supports `use_device_ptr` on pointers which are struct members. Because they are struct members, they are not marked with `TARGET_PARAM` (only the combined entry is considered a target parameter, not the individual members). On the other hand, they are marked with `PTR_AND_OBJ` and have some value in the `MEMBER_OF` bits.
> > > > >
> > > > > Once again, it's a non-standard extension so we are free to decide whether to support it or not in the unified shared memory scenario.
> > > > Can we please first answer my question why we need to care about the existence of `use_device_ptr`? Why does it make a difference for unified shared memory?
> > > Because use_device_ptr implies the use of a true device pointer and again that needs to be respected even unified memory is used.
> > Where is that in the spec?
> The pointers returned by omp_target_alloc and accessed through use_device_ptr are guaranteed to be pointer values that can support pointer arithmetic while still being native device pointers. (Section 2.4 page 61 bottom).
With `unified_shared_memory` we get: (Section 2.4, page 62, lines 4:7)
> Additionally, memory in the device data environment of any device visible to OpenMP, including but not limited to the host, is considered part of the device data environment of all devices accessible through OpenMP except as noted below. Every device address allocated through OpenMP device memory routines is a valid host pointer.
>From my understanding, this implies all shared memory can be referenced by a host pointer. This guarantees pointer arithmetic per the C / C++ standard.
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