[Openmp-commits] [PATCH] D65001: [OpenMP][libomptarget] Add support for unified memory for regular maps

Gheorghe-Teodor Bercea via Phabricator via Openmp-commits openmp-commits at lists.llvm.org
Wed Jul 31 13:12:00 PDT 2019

gtbercea marked an inline comment as done.
gtbercea added inline comments.

Comment at: libomptarget/src/api.cpp:118-123
+  // Under unified memory the host pointer can be returned by the
+  // getTgtPtrBegin() function which means that there is no device
+  // corresponding point for ptr. This function should return false
+  // in that situation.
+  if (Device.RTLRequiresFlags & OMP_REQ_UNIFIED_SHARED_MEMORY)
+    rc = (TgtPtr != ptr);
gtbercea wrote:
> Hahnfeld wrote:
> > gtbercea wrote:
> > > Hahnfeld wrote:
> > > > gtbercea wrote:
> > > > > Hahnfeld wrote:
> > > > > > gtbercea wrote:
> > > > > > > Hahnfeld wrote:
> > > > > > > > The spec says `This routine returns non-zero if the specified pointer would be found present on device device_num by a map clause; otherwise, it returns zero.` Programs with `unified_shared_memory` could imply that all pointers are present on all devices, so I would have expected the routine to always return true. What's the reasoning for checking that the pointers differ?
> > > > > > > I'm checking if the pointer is on the actual device. If unified memory is used then the pointers will match and the device present test will return false.
> > > > > > > I have now refactored this check to make it more precise: if the host pointer is used then we have a flag for that.
> > > > > > Okay, I think I get what you're saying: You want to check that this particular pointer has been `map`ped to that specific `device_num`, right?
> > > > > > 
> > > > > > But with unified shared memory, this shouldn't matter, right? Because all pointers can be accessed from all devices, no?
> > > > > Employing the API functions allows the user to bypass the unified memory behaviour and these functions allow the user to manage device pointers explicitly.
> > > > I'm still not sure about this, what do others think?
> > > See Alex's answer below!
> > This doesn't even mention `omp_target_is_present`, so this is no answer.
> So the problem is that if I return true all the time for the presence test when unified memory is on, then there's no way to distinguish the case where I use omp_target_alloc()  but I haven't associated the host pointer with the device pointer yet.
> So in other words the presence test is always an "honest" device presence test: it will only return true when there is a host version and corresponding device duplicate version. I think that case is the more meaningful to cover since for the other cases you don't really need a presence test.
I think your confusion is based on the fact that you assume that the presence test is for checking if a future usage of a variable in a device target region leads to a seg fault or not. And I agree that in the non-unified case this intent coincides with that of a map existing.
In the unified case that's not necessarily the case: you can use a variable on the device by referring to its host version without the need of a map existing. The presence test in that case will return false and that's fine.

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