[OpenMP][libomptarget] Add support for declare target to clause under unified memory

[gtbercea]

Summary

This patch adds support for handling variables under the:

#pragma omp declare target to()

clause when the

#pragma omp requires unified_shared_memory

is used.

The address of the host variable is copied into the device pointer just like for the declare target link case.

[gtbercea]

ping

[grokos]

libomptarget/plugins/cuda/src/rtl.cpp

452–453

Maybe this OR is redundant; it always evaluates to true. Since the symbol we are processing has size!=0 it is a variable and variables are always either "to" or "link", there are no other possibilities (at least now, maybe in the future more attributes will be added).

[gtbercea]

libomptarget/plugins/cuda/src/rtl.cpp

452–453

So I could remove the attribute check and just have:

if (DeviceInfo.RequiresFlags & OMP_REQ_UNIFIED_SHARED_MEMORY) {..}

Would that work?

The reason I included this explicit check (even if it evaluates to true always) is that if in the future this is extended with other attributes then this check will not automatically work for that. I think this disambiguates the cases that are supported today and guard against this path being taken unintentionally.

[grokos]

LGTM

libomptarget/plugins/cuda/src/rtl.cpp

452–453

Yes, that's what I thought as well. Let's leave the check there, it makes the implementation more future-proof.

[gtbercea]

libomptarget/plugins/cuda/src/rtl.cpp

452–453

Awesome. I'll leave it as is for now then.

[jcownie]

libomptarget/plugins/cuda/src/rtl.cpp

452–453

The code which is checking the flags looks a bit weird. One test is done with & (suggesting these are bit flags which could be or-ed together), the other with == suggesting this is a field full of mutually exclusive enumerations.

[grokos]

libomptarget/plugins/cuda/src/rtl.cpp

452–453

This is because actually "TO" is not a flag, it's the complete absence of flags (0x00). If something has been "declared target" and is not a ctor/dtor or a linked variable then it is "TO".

I don't like this fact either - if we want to be 100% correct we should introduce a dedicated flag for "TO" but that involves changes in the compiler/library interface and is definitely not part of this patch. We can pursue such a patch anyway if there are strong arguments that warrant the effort.

[gtbercea]

• Fix condition.

[gtbercea]

libomptarget/plugins/cuda/src/rtl.cpp

452–453

I've move the checks into a comment and simplified the condition.

[grokos]

libomptarget/plugins/cuda/src/rtl.cpp

452–453

This is what I had in mind when I posted the first comment. It looks better now!

[gtbercea]

libomptarget/plugins/cuda/src/rtl.cpp

452–453

Awesome I'll commit this version!

libomptarget/include/omptarget.h

enum tgt_map_type {
   OMP_TGT_MAPTYPE_LITERAL         = 0x100,
   // mapping is implicit
   OMP_TGT_MAPTYPE_IMPLICIT        = 0x200,
   // member of struct, member given by [16 MSBs] - 1
   OMP_TGT_MAPTYPE_MEMBER_OF       = 0xffff000000000000
 };
 
 enum OpenMPOffloadingDeclareTargetFlags {
+  /// Mark the entry as having a 'to' attribute.
+  OMP_DECLARE_TARGET_TO   = 0x00,
   /// Mark the entry as having a 'link' attribute.
   OMP_DECLARE_TARGET_LINK = 0x01,
   /// Mark the entry as being a global constructor.
   OMP_DECLARE_TARGET_CTOR = 0x02,
   /// Mark the entry as being a global destructor.
   OMP_DECLARE_TARGET_DTOR = 0x04
 };
 

libomptarget/plugins/cuda/src/rtl.cpp

if (e->size) {
         return NULL;
       }
 
       DP("Entry point " DPxMOD " maps to global %s (" DPxMOD ")\n",
           DPxPTR(e - HostBegin), e->name, DPxPTR(cuptr));
       entry.addr = (void *)cuptr;
 
       if (DeviceInfo.RequiresFlags & OMP_REQ_UNIFIED_SHARED_MEMORY &&
-          e->flags & OMP_DECLARE_TARGET_LINK) {
+          (e->flags & OMP_DECLARE_TARGET_LINK ||
+           e->flags == OMP_DECLARE_TARGET_TO)) {
         // If unified memory is present any target link variables
         // can access host addresses directly. There is no longer a
         // need for device copies.
         cuMemcpyHtoD(cuptr, e->addr, sizeof(void *));
         DP("Copy linked variable host address (" DPxMOD ")"
            "to device address (" DPxMOD ")\n",
           DPxPTR(*((void**)e->addr)), DPxPTR(cuptr));
       }