[Openmp-commits] [PATCH] D107925: [OpenMP] Use IsHostPtr where needed for targetDataEnd

Joel E. Denny via Phabricator via Openmp-commits openmp-commits at lists.llvm.org
Wed Aug 11 13:01:26 PDT 2021

jdenny created this revision.
jdenny added reviewers: grokos, tianshilei1992, RaviNarayanaswamy, jdoerfert.
jdenny added a project: OpenMP.
Herald added subscribers: guansong, yaxunl.
jdenny requested review of this revision.
Herald added a subscriber: sstefan1.

As discussed in D105990 <https://reviews.llvm.org/D105990>, without this patch, `targetDataEnd`
determines whether to transfer data or delete a device mapping (as 
opposed to assuming it's in shared memory) using two different
conditions, each of which are broken for some cases:

1. `!(UNIFIED_SHARED_MEMORY && TgtPtrBegin == HstPtrBegin)`: The broken case is rare: the device and host might happen to use the same address for their mapped allocations.  I don't know how to write a test that's likely to reveal this case, but this patch does fix it, as discussed below.
2. `!UNIFIED_SHARED_MEMORY || HasCloseModifier`: There are at least two broken cases:
  1. The `close` modifier might have been specified on an `omp target enter data` but not the corresponding `omp target exit data`, which thus might falsely assume a mapping is in shared memory.  The test `unified_shared_memory/close_enter_exit.c` already has a missing deletion as a result, and this patch adds a check for that.  This patch also adds the new test `close_member.c` to reveal a missing transfer and deletion.
  2. Use of discrete memory might have been forced by `omp_target_associate_ptr`, as in the test `unified_shared_memory/api.c`.  In the current `targetDataEnd` implementation, this condition turns out not be used for this case: because the reference count is infinite, a transfer is possible only with an `always` modifier, and this condition is never used in that case.  To ensure it's never used for that case in the future, this patch adds the test `unified_shared_memory/associate_ptr.c`.

Fortunately, `DeviceTy::getTgtPtrBegin` already has a solution: it
reports whether the allocation was found in shared memory via the 
variable `IsHostPtr`.

After this patch, `HasCloseModifier` is no longer used in
`targetDataEnd`, and I wonder if the `close` modifier is ever useful
on an `omp target data end`.

  rG LLVM Github Monorepo



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