[Mesa-dev] [PATCH] intel/compiler: emit actual barriers for working-group level barriers

[Iago Toral Quiroga]

Until now we have assumed that we could skip emitting these barriers
in the general case based on empirical testing and a few assumptions
detailed in a comment in the driver code, however, recent CTS tests
have showed that we actually need them to produce correct behavior.
---
 src/intel/compiler/brw_fs_nir.cpp | 25 ++-----------------------
 1 file changed, 2 insertions(+), 23 deletions(-)

diff --git a/src/intel/compiler/brw_fs_nir.cpp b/src/intel/compiler/brw_fs_nir.cpp
index 0abb4798e70..d0648c89865 100644
--- a/src/intel/compiler/brw_fs_nir.cpp
+++ b/src/intel/compiler/brw_fs_nir.cpp
@@ -3884,6 +3884,8 @@ fs_visitor::nir_emit_intrinsic(const fs_builder &bld, nir_intrinsic_instr *instr
       break;
    }
 
+   case nir_intrinsic_group_memory_barrier:
+   case nir_intrinsic_memory_barrier_shared:
    case nir_intrinsic_memory_barrier_atomic_counter:
    case nir_intrinsic_memory_barrier_buffer:
    case nir_intrinsic_memory_barrier_image:
@@ -3895,29 +3897,6 @@ fs_visitor::nir_emit_intrinsic(const fs_builder &bld, nir_intrinsic_instr *instr
       break;
    }
 
-   case nir_intrinsic_group_memory_barrier:
-   case nir_intrinsic_memory_barrier_shared:
-      /* We treat these workgroup-level barriers as no-ops.  This should be
-       * safe at present and as long as:
-       *
-       *  - Memory access instructions are not subsequently reordered by the
-       *    compiler back-end.
-       *
-       *  - All threads from a given compute shader workgroup fit within a
-       *    single subslice and therefore talk to the same HDC shared unit
-       *    what supposedly guarantees ordering and coherency between threads
-       *    from the same workgroup.  This may change in the future when we
-       *    start splitting workgroups across multiple subslices.
-       *
-       *  - The context is not in fault-and-stream mode, which could cause
-       *    memory transactions (including to SLM) prior to the barrier to be
-       *    replayed after the barrier if a pagefault occurs.  This shouldn't
-       *    be a problem up to and including SKL because fault-and-stream is
-       *    not usable due to hardware issues, but that's likely to change in
-       *    the future.
-       */
-      break;
-
    case nir_intrinsic_shader_clock: {
       /* We cannot do anything if there is an event, so ignore it for now */
       const fs_reg shader_clock = get_timestamp(bld);
-- 
2.14.1