[Mesa-dev] [PATCH 2/2] nir/instr_set: allow rewrite of SSBO loads

[Iago Toral Quiroga]

So we can effectively CSE the ones that are safe to reuse. Makes a shader
such as this:

buffer SSBO {
    mat4 sm4;
};

uniform mat4 um4;

void main() {
    sm4 *= um4;
}

go from 16 SSBO loads to only 4.
---
 src/glsl/nir/nir_instr_set.c | 24 ++++++++++++++++++++----
 1 file changed, 20 insertions(+), 4 deletions(-)

diff --git a/src/glsl/nir/nir_instr_set.c b/src/glsl/nir/nir_instr_set.c
index d3f939f..470683f 100644
--- a/src/glsl/nir/nir_instr_set.c
+++ b/src/glsl/nir/nir_instr_set.c
@@ -398,6 +398,13 @@ dest_is_ssa(nir_dest *dest, void *data)
    return dest->is_ssa;
 }
 
+static bool
+is_ssbo_load(nir_intrinsic_instr *instr)
+{
+   return instr->intrinsic == nir_intrinsic_load_ssbo ||
+          instr->intrinsic == nir_intrinsic_load_ssbo_indirect;
+}
+
 /* This function determines if uses of an instruction can safely be rewritten
  * to use another identical instruction instead. Note that this function must
  * be kept in sync with hash_instr() and nir_instrs_equal() -- only
@@ -428,11 +435,20 @@ instr_can_rewrite(nir_instr *instr)
       return true;
    }
    case nir_instr_type_intrinsic: {
+      nir_intrinsic_instr *intrinsic = nir_instr_as_intrinsic(instr);
       const nir_intrinsic_info *info =
-         &nir_intrinsic_infos[nir_instr_as_intrinsic(instr)->intrinsic];
-      return (info->flags & NIR_INTRINSIC_CAN_ELIMINATE) &&
-             (info->flags & NIR_INTRINSIC_CAN_REORDER) &&
-             info->num_variables == 0; /* not implemented yet */
+         &nir_intrinsic_infos[intrinsic->intrinsic];
+      bool can_eliminate_and_reorder =
+         (info->flags & NIR_INTRINSIC_CAN_ELIMINATE) &&
+         (info->flags & NIR_INTRINSIC_CAN_REORDER) &&
+         info->num_variables == 0; /* not implemented yet */
+
+      /* SSBO loads are special: they can be reordered only in certain
+       * situations, so they don't set NIR_INTRINSIC_CAN_REORDER, however,
+       * we do want to CSE them in the cases where it is safe, so return
+       * true for them here.
+       */
+      return can_eliminate_and_reorder ? true : is_ssbo_load(intrinsic);
    }
    case nir_instr_type_call:
    case nir_instr_type_jump:
-- 
1.9.1