Mesa (master): i965/nir/fs: Add comment for no-op memory barrier functions
Jordan Justen
jljusten at kemper.freedesktop.org
Fri Nov 6 21:58:50 UTC 2015
Module: Mesa
Branch: master
Commit: 51694072218b5ae84b5d8f98ee2172d7c5d61b31
URL: http://cgit.freedesktop.org/mesa/mesa/commit/?id=51694072218b5ae84b5d8f98ee2172d7c5d61b31
Author: Francisco Jerez <currojerez at riseup.net>
Date: Fri Nov 6 13:19:56 2015 -0800
i965/nir/fs: Add comment for no-op memory barrier functions
Reviewed-by: Jordan Justen <jordan.l.justen at intel.com>
---
src/mesa/drivers/dri/i965/brw_fs_nir.cpp | 19 +++++++++++++++++++
1 file changed, 19 insertions(+)
diff --git a/src/mesa/drivers/dri/i965/brw_fs_nir.cpp b/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
index 5d2dd18..02b9f5b 100644
--- a/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
@@ -1709,6 +1709,25 @@ fs_visitor::nir_emit_intrinsic(const fs_builder &bld, nir_intrinsic_instr *instr
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: {
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