nir/opt_barriers: Add a default callback

Absent any knowledge about the hardware, if the backend wants to combine
barriers we should try to combine all barriers.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23181>

src/compiler/nir/nir_opt_barriers.c

@@ -23,6 +23,20 @@
 
 #include "nir.h"
 
+static bool
+combine_all_barriers(nir_intrinsic_instr *a, nir_intrinsic_instr *b, void *_)
+{
+   nir_intrinsic_set_memory_modes(
+      a, nir_intrinsic_memory_modes(a) | nir_intrinsic_memory_modes(b));
+   nir_intrinsic_set_memory_semantics(
+      a, nir_intrinsic_memory_semantics(a) | nir_intrinsic_memory_semantics(b));
+   nir_intrinsic_set_memory_scope(
+      a, MAX2(nir_intrinsic_memory_scope(a), nir_intrinsic_memory_scope(b)));
+   nir_intrinsic_set_execution_scope(
+      a, MAX2(nir_intrinsic_execution_scope(a), nir_intrinsic_execution_scope(b)));
+   return true;
+}
+
 static bool
 nir_opt_combine_barriers_impl(
    nir_function_impl *impl, nir_combine_barrier_cb combine_cb, void *data)
@@ -69,7 +83,9 @@ bool
 nir_opt_combine_barriers(
    nir_shader *shader, nir_combine_barrier_cb combine_cb, void *data)
 {
-   assert(combine_cb);
+   /* Default to combining everything. Only some backends can do better. */
+   if (!combine_cb)
+      combine_cb = combine_all_barriers;
 
    bool progress = false;