[SYCL][CUDA] Add missing barrier to collectives

SYCL sub-group and group functions should act as synchronization points.
Group collectives need a barrier at the end to ensure that back-to-back
collectives do not lead to a race condition.

Note that the barrier at the beginning of each collective occurs after
each work-item writes its partial results to the scratch space.  This is
assumed safe because only the collective functions can access the space,
and collective functions must be encountered in uniform control flow; any
work-item encountering a collective function can assume it is safe to use
the scratch space, because all work-items in the same work-group must have
either executed no collective functions or the barrier at the end of a previous
collective function.

Signed-off-by: John Pennycook <[email protected]>

Author: Pennycook

libclc/ptx-nvidiacl/libspirv/group/collectives.cl

@@ -264,6 +264,7 @@ __CLC_SUBGROUP_COLLECTIVE(FMax, __CLC_MAX, double, -DBL_MAX)
         result = OP(sg_x, scratch[sg_id - 1]);                                 \
       }                                                                        \
     }                                                                          \
+    __spirv_ControlBarrier(Workgroup, 0, 0);                                   \
     return result;                                                             \
   }