[PATCH] libgomp, OpenMP, nvptx: Low-latency memory allocator

[Tom de Vries]

On 1/6/22 10:29, Tom de Vries wrote:
> At first glance, the above behaviour doesn't look like a too short timeout.

Using patch below, this passes for me, I'm currently doing a full build 
and test to confirm.

Looks like it has to do with:
...
For sm_6x and earlier architectures, atom operations on .shared state 
space do not guarantee atomicity with respect to normal store 
instructions to the same address. It is the programmer's responsibility 
to guarantee correctness of programs that use shared memory
atomic instructions, e.g., by inserting barriers between normal stores 
and atomic operations to a common address, or by using atom.exch to 
store to locations accessed by other atomic operations.
...

My current understanding is that this is a backend problem, and needs to 
be fixed by defining atomic_store patterns which take care of this 
peculiarity.

Thanks,
- Tom

diff --git a/libgomp/config/nvptx/allocator.c 
b/libgomp/config/nvptx/allocator.c
index 6bc2ea48043..4524122b3e7 100644
--- a/libgomp/config/nvptx/allocator.c
+++ b/libgomp/config/nvptx/allocator.c
@@ -122,7 +122,8 @@ nvptx_memspace_alloc (omp_memspace_handle_t 
memspace, size_t size)

         }

        /* Update the free chain root and release the lock.  */
-      __atomic_store_n (&__nvptx_lowlat_heap_root, root.raw, 
MEMMODEL_RELEASE);
+      __atomic_exchange_n (&__nvptx_lowlat_heap_root,
+                          root.raw, MEMMODEL_RELEASE);
        return result;
      }
    else-
@@ -221,7 +222,8 @@ nvptx_memspace_free (omp_memspace_handle_t memspace, 
void *addr, s
ize_t size)
         root.raw = newfreechunk.raw;

        /* Update the free chain root and release the lock.  */
-      __atomic_store_n (&__nvptx_lowlat_heap_root, root.raw, 
MEMMODEL_RELEASE);
+      __atomic_exchange_n (&__nvptx_lowlat_heap_root,
+                          root.raw, MEMMODEL_RELEASE);
      }
    else
      free (addr);
@@ -331,7 +333,8 @@ nvptx_memspace_realloc (omp_memspace_handle_t 
memspace, void *addr
,
        /* Else realloc in-place has failed and result remains NULL.  */

        /* Update the free chain root and release the lock.  */
-      __atomic_store_n (&__nvptx_lowlat_heap_root, root.raw, 
MEMMODEL_RELEASE);
+      __atomic_exchange_n (&__nvptx_lowlat_heap_root,
+                          root.raw, MEMMODEL_RELEASE);

        if (result == NULL)
         {