mesa: Add a new GetTransformFeedbackVertexCount() driver hook.

DrawTransformFeedback() needs to obtain the number of vertices written
to a particular stream during the last Begin/EndTransformFeedback block.
The new driver hook returns exactly that information.

Gallium drivers already implement this by passing the transform feedback
object to the drawing function, counting the number of vertices written
on the GPU, and using draw indirect.  This is efficient, but doesn't
always work:

If vertex data comes from user arrays, then the VBO module needs to
know how many vertices to upload, so we need to synchronously count.
Gallium drivers are currently broken in this case.

It also doesn't work if primitive restart is done in software.  For
normal drawing, vbo_draw_arrays() performs software primitive restart,
splitting the draw call in two.  vbo_draw_transform_feedback() currently
doesn't because it has no idea how many vertices need to be drawn.

The new driver hook gives it that information, allowing us to reuse
the existing vbo_draw_arrays() code to do everything right.

On Intel hardware (at least Ivybridge), using the draw indirect approach
is difficult since the hardware counts primitives, rather than vertices,
which requires doing some simple math.  So we always use this hook.

Gallium drivers will likely want to use this hook in some cases, but
want to use the existing draw indirect approach where possible.  Hence,
I've added a flag to allow drivers to opt-in to this call.

v2: Make it possible to implement this hook but only use this path
    when necessary (suggested by Marek).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

diff --git a/src/mesa/drivers/dri/i965/brw_context.c b/src/mesa/drivers/dri/i965/brw_context.c
index 90d9be449c1115eefcbcb50b917a584f38529b54..623273c260f84afd2e59e9bb66774a097de4f5b5 100644 (file)
--- a/src/mesa/drivers/dri/i965/brw_context.c
+++ b/src/mesa/drivers/dri/i965/brw_context.c
@@ -329,6 +329,8 @@ brw_initialize_context_constants(struct brw_context *brw)
    ctx->Const.MaxTransformFeedbackSeparateComponents =
       BRW_MAX_SOL_BINDINGS / BRW_MAX_SOL_BUFFERS;
 
+   ctx->Const.AlwaysUseGetTransformFeedbackVertexCount = true;
+
    if (brw->gen == 6) {
       ctx->Const.MaxSamples = 4;
       ctx->Const.MaxColorTextureSamples = 4;

diff --git a/src/mesa/main/dd.h b/src/mesa/main/dd.h
index 501192199064fc21288054c275168409c37ef437..d7c43271355609b679d5cfebd7e32853c5c878df 100644 (file)
--- a/src/mesa/main/dd.h
+++ b/src/mesa/main/dd.h
@@ -842,6 +842,14 @@ struct dd_function_table {
    void (*ResumeTransformFeedback)(struct gl_context *ctx,
                                    struct gl_transform_feedback_object *obj);
 
+   /**
+    * Return the number of vertices written to a stream during the last
+    * Begin/EndTransformFeedback block.
+    */
+   GLsizei (*GetTransformFeedbackVertexCount)(struct gl_context *ctx,
+                                              struct gl_transform_feedback_object *obj,
+                                              GLuint stream);
+
    /**
     * \name GL_NV_texture_barrier interface
     */

diff --git a/src/mesa/main/mtypes.h b/src/mesa/main/mtypes.h
index a1a5eb4bf137b1f8a0622715c42dd213d4fd005c..087bc3727071f22cab56c666167b3468b42d8f4f 100644 (file)
--- a/src/mesa/main/mtypes.h
+++ b/src/mesa/main/mtypes.h
@@ -3156,6 +3156,12 @@ struct gl_constants
     */
    GLboolean PrimitiveRestartInSoftware;
 
+   /**
+    * Always use the GetTransformFeedbackVertexCount() driver hook, rather
+    * than passing the transform feedback object to the drawing function.
+    */
+   GLboolean AlwaysUseGetTransformFeedbackVertexCount;
+
    /** GL_ARB_map_buffer_alignment */
    GLuint MinMapBufferAlignment;
 

diff --git a/src/mesa/vbo/vbo_exec_array.c b/src/mesa/vbo/vbo_exec_array.c
index 1670409d48b28ae53538b00230a24b15d1d50742..f25a9dec308257bf884bea6cbfdcc081c9bd8ee3 100644 (file)
--- a/src/mesa/vbo/vbo_exec_array.c
+++ b/src/mesa/vbo/vbo_exec_array.c
@@ -1464,6 +1464,16 @@ vbo_draw_transform_feedback(struct gl_context *ctx, GLenum mode,
       return;
    }
 
+   if (ctx->Driver.GetTransformFeedbackVertexCount &&
+       (ctx->Const.AlwaysUseGetTransformFeedbackVertexCount ||
+        (ctx->Const.PrimitiveRestartInSoftware &&
+         ctx->Array._PrimitiveRestart) ||
+        !vbo_all_varyings_in_vbos(exec->array.inputs))) {
+      GLsizei n = ctx->Driver.GetTransformFeedbackVertexCount(ctx, obj, stream);
+      vbo_draw_arrays(ctx, mode, 0, n, numInstances, 0);
+      return;
+   }
+
    vbo_bind_arrays(ctx);
 
    /* init most fields to zero */