[Mesa-dev] [PATCH 1/2] i965/gs: Move vertex_count != 0 check up a level; skip one caller.

[Kenneth Graunke]

Paul's original code had emit_control_data_bits() skip the URB write if
vertex_count was 0.  This meant wrapping every control data write in a
conditional write.

We accumulate control data bits in a single UD (32-bit) register.  For
simple shaders that don't emit many vertices, the control data header
will be <= 32-bits long, so we only need to write it once at the end of
the shader.

For shaders with larger headers, we write out batches of control data
bits at EmitVertex(), when (vertex_count * bits_per_vertex) % 32 == 0.
On the first EmitVertex() call, the above expression will evaluate to
true simply because vertex_count == 0.  But we want to avoid emitting
the control data bits, because we haven't accumulated 32-bits worth yet.

In other words, the vertex_count != 0 check is really only necessary in
the EmitVertex() batching case, not the end-of-thread case.

This saves a CMP/IF/ENDIF in every shader that uses EndPrimitive() or
multiple streams.  The only downside is that a shader which emits no
vertices at all will execute an additional URB write---but such shaders
are pointless and not worth optimizing.

Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
---
 src/mesa/drivers/dri/i965/brw_vec4_gs_visitor.cpp | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/src/mesa/drivers/dri/i965/brw_vec4_gs_visitor.cpp b/src/mesa/drivers/dri/i965/brw_vec4_gs_visitor.cpp
index 2f948ee..55408eb 100644
--- a/src/mesa/drivers/dri/i965/brw_vec4_gs_visitor.cpp
+++ b/src/mesa/drivers/dri/i965/brw_vec4_gs_visitor.cpp
@@ -348,11 +348,6 @@ vec4_gs_visitor::emit_control_data_bits()
    if (c->control_data_header_size_bits > 128)
       urb_write_flags = urb_write_flags | BRW_URB_WRITE_PER_SLOT_OFFSET;
 
-   /* If vertex_count is 0, then no control data bits have been accumulated
-    * yet, so we should do nothing.
-    */
-   emit(CMP(dst_null_d(), this->vertex_count, 0u, BRW_CONDITIONAL_NEQ));
-   emit(IF(BRW_PREDICATE_NORMAL));
    {
       /* If we are using either channel masks or a per-slot offset, then we
        * need to figure out which DWORD we are trying to write to, using the
@@ -431,7 +426,6 @@ vec4_gs_visitor::emit_control_data_bits()
       inst->base_mrf = base_mrf;
       inst->mlen = 2;
    }
-   emit(BRW_OPCODE_ENDIF);
 }
 
 void
@@ -531,9 +525,17 @@ vec4_gs_visitor::visit(ir_emit_vertex *ir)
             emit(AND(dst_null_d(), this->vertex_count,
                      (uint32_t) (32 / c->control_data_bits_per_vertex - 1)));
          inst->conditional_mod = BRW_CONDITIONAL_Z;
+
          emit(IF(BRW_PREDICATE_NORMAL));
          {
+            /* If vertex_count is 0, then no control data bits have been
+             * accumulated yet, so we skip emitting them.
+             */
+            emit(CMP(dst_null_d(), this->vertex_count, 0u,
+                     BRW_CONDITIONAL_NEQ));
+            emit(IF(BRW_PREDICATE_NORMAL));
             emit_control_data_bits();
+            emit(BRW_OPCODE_ENDIF);
 
             /* Reset control_data_bits to 0 so we can start accumulating a new
              * batch.
-- 
2.4.4