[OpenMP][FIX] Avoid use of stack allocations in asynchronous calls

[jdoerfert]

Summary

As reported by Guilherme Valarini [0], we used to pass stack allocations
to calls that can nowadays be asynchronous. This is arguably a problem
and it will inevitably result in UB. To remedy the situation we
allocate the locations as part of the AsyncInfoTy object. The lifetime
of that object matches what we need for now. If the synchronization is
not tied to the AsyncInfoTy object anymore we might need to have a
different buffer construct in global space.

This should be back-ported to LLVM 12 but needs slight modifications as
it is based on refactoring patches we do not need to backport.

[0] https://lists.llvm.org/pipermail/openmp-dev/2021-February/003867.html

[lebedev.ri]

openmp/libomptarget/src/omptarget.cpp

35

?

[JonChesterfield]

LG. Why deque over list?

[tianshilei1992]

openmp/libomptarget/include/omptarget.h

162

I'd go with a more generic method:

std::vector<std::unique_ptr<char[]>> BufferLocations;
template <typename T>
T *getPtr() {
  BufferLocations.emplace_back(std::make_unique<char[]>(sizeof(T)));
  return reinterpret_cast<T *>(BufferLocations.back().get());
}

[jdoerfert]

In D96667#2562413, @JonChesterfield wrote:

LG. Why deque over list?

Some small memory overhead sounds better than linear allocation cost.

@tianshilei1992 Can you back port a variation of this to LLVM 12?

openmp/libomptarget/include/omptarget.h

162

That seems overkill as long as we only get void* out of this. My method avoids one level of indirection and the unique pointer has the same lifetime as the vector anyway.

[jdoerfert]

Minor update

[tianshilei1992]

In D96667#2562557, @jdoerfert wrote:

In D96667#2562413, @JonChesterfield wrote:

LG. Why deque over list?

Some small memory overhead sounds better than linear allocation cost.

@tianshilei1992 Can you back port a variation of this to LLVM 12?

I suggest to land all related patches because they're all dependent. Having a variant is like doing all these things again.

openmp/libomptarget/include/omptarget.h

162

I'm fine with your method. It's just too "tricky". Having a queue of 4B/8B objects is not so "natural".

openmp/libomptarget/include/omptarget.h

 //===-------- omptarget.h - Target independent OpenMP target RTL -- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // Interface to be used by Clang during the codegen of a
 // target region.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef _OMPTARGET_H_
 #define _OMPTARGET_H_
 
+#include <deque>
 #include <stddef.h>
 #include <stdint.h>
 
 #include <SourceInfo.h>
 
 #define OFFLOAD_SUCCESS (0)
 #define OFFLOAD_FAIL (~0)
 

 };
 
 struct DeviceTy;
 
 /// The libomptarget wrapper around a __tgt_async_info object directly
 /// associated with a libomptarget layer device. RAII semantics to avoid
 /// mistakes.
 class AsyncInfoTy {
+  /// Locations we used in (potentially) asynchronous calls which should live
+  /// as long as this AsyncInfoTy object.
+  std::deque<void *> BufferLocations;
+
   __tgt_async_info AsyncInfo;
   DeviceTy &Device;
 
 public:
   AsyncInfoTy(DeviceTy &Device) : Device(Device) {}
   ~AsyncInfoTy() { synchronize(); }
 
   /// Implicit conversion to the __tgt_async_info which is used in the
   /// plugin interface.
   operator __tgt_async_info *() { return &AsyncInfo; }
 
   /// Synchronize all pending actions.
   ///
   /// \returns OFFLOAD_FAIL or OFFLOAD_SUCCESS appropriately.
   int synchronize();
+
+  /// Return a void* reference with a lifetime that is at least as long as this
+  /// AsyncInfoTy object. The location can be used as intermediate buffer.
+  void *&getVoidPtrLocation();
 };
 
 /// This struct is a record of non-contiguous information
 struct __tgt_target_non_contig {
   uint64_t Offset;
   uint64_t Count;
   uint64_t Stride;
 };

openmp/libomptarget/src/omptarget.cpp

if (AsyncInfo.Queue) {
     Result = Device.synchronize(*this);
     assert(AsyncInfo.Queue == nullptr &&
            "The device plugin should have nulled the queue to indicate there "
            "are no outstanding actions!");
   }
   return Result;
 }
 
+void *&AsyncInfoTy::getVoidPtrLocation() {
+  BufferLocations.push_back(0);
+  return BufferLocations.back();
+}
+
 /* All begin addresses for partially mapped structs must be 8-aligned in order
  * to ensure proper alignment of members. E.g.
  *
  * struct S {
  *   int a;   // 4-aligned
  *   int b;   // 4-aligned
  *   int *p;  // 8-aligned
  * } s1;
if (arg_types[i] & OMP_TGT_MAPTYPE_TO) {
         }
       }
     }
 
     if (arg_types[i] & OMP_TGT_MAPTYPE_PTR_AND_OBJ && !IsHostPtr) {
       DP("Update pointer (" DPxMOD ") -> [" DPxMOD "]\n",
          DPxPTR(PointerTgtPtrBegin), DPxPTR(TgtPtrBegin));
       uint64_t Delta = (uint64_t)HstPtrBegin - (uint64_t)HstPtrBase;
-      void *TgtPtrBase = (void *)((uint64_t)TgtPtrBegin - Delta);
+      void *&TgtPtrBase = AsyncInfo.getVoidPtrLocation();
+      TgtPtrBase = (void *)((uint64_t)TgtPtrBegin - Delta);
       int rt = Device.submitData(PointerTgtPtrBegin, &TgtPtrBase,
                                  sizeof(void *), AsyncInfo);
       if (rt != OFFLOAD_SUCCESS) {
         REPORT("Copying data to device failed.\n");
         return OFFLOAD_FAIL;
       }
       // create shadow pointers for this entry
       Device.ShadowMtx.lock();
if (!(ArgTypes[I] & OMP_TGT_MAPTYPE_TARGET_PARAM)) {
         void *HstPtrBegin = ArgBases[I];
         void *HstPtrBase = Args[Idx];
         bool IsLast, IsHostPtr; // unused.
         void *TgtPtrBase =
             (void *)((intptr_t)TgtArgs[TgtIdx] + TgtOffsets[TgtIdx]);
         DP("Parent lambda base " DPxMOD "\n", DPxPTR(TgtPtrBase));
         uint64_t Delta = (uint64_t)HstPtrBegin - (uint64_t)HstPtrBase;
         void *TgtPtrBegin = (void *)((uintptr_t)TgtPtrBase + Delta);
-        void *PointerTgtPtrBegin = Device.getTgtPtrBegin(
-            HstPtrVal, ArgSizes[I], IsLast, false, IsHostPtr);
+        void *&PointerTgtPtrBegin = AsyncInfo.getVoidPtrLocation();
+        PointerTgtPtrBegin = Device.getTgtPtrBegin(HstPtrVal, ArgSizes[I],
+                                                   IsLast, false, IsHostPtr);
         if (!PointerTgtPtrBegin) {
           DP("No lambda captured variable mapped (" DPxMOD ") - ignored\n",
              DPxPTR(HstPtrVal));
           continue;
         }
         if (PM->RTLs.RequiresFlags & OMP_REQ_UNIFIED_SHARED_MEMORY &&
             TgtPtrBegin == HstPtrBegin) {
           DP("Unified memory is active, no need to map lambda captured"