[mutter/wip/carlosg/unassorted-perf-fixes: 3/6] cogl: Ensure to only clear the depth buffer if depth testing is enabled

[Carlos Garnacho <carlosg src gnome org>]

commit 4139907a7e614c8ff4943c3bd50a55b1287b4761
Author: Carlos Garnacho <carlosg gnome org>
Date:   Mon May 8 15:10:58 2017 +0200

    cogl: Ensure to only clear the depth buffer if depth testing is enabled
    
    The depth buffer is marked as invalid when 1) the framebuffer is just created,
    and 2) whenever GL_DEPTH_TEST is enabled on it. This will ensure the
    framebuffers attached depth buffer (if any) is properly cleared before it's
    actually used, while saving needless clears while depth testing is disabled
    (the default).
    
    https://bugzilla.gnome.org/show_bug.cgi?id=782344

 cogl/cogl/cogl-framebuffer-private.h       |  5 +++++
 cogl/cogl/cogl-framebuffer.c               | 11 +++++++++++
 cogl/cogl/driver/gl/cogl-pipeline-opengl.c |  6 +++++-
 3 files changed, 21 insertions(+), 1 deletion(-)
---
diff --git a/cogl/cogl/cogl-framebuffer-private.h b/cogl/cogl/cogl-framebuffer-private.h
index d78bd408d..46a1d146d 100644
--- a/cogl/cogl/cogl-framebuffer-private.h
+++ b/cogl/cogl/cogl-framebuffer-private.h
@@ -192,6 +192,11 @@ struct _CoglFramebuffer
   CoglFramebufferBits bits;
 
   int                 samples_per_pixel;
+
+  /* Whether the depth buffer was enabled for this framebuffer,
+   * usually means it needs to be cleared before being reused next.
+   */
+  CoglBool            depth_buffer_clear_needed;
 };
 
 typedef enum {
diff --git a/cogl/cogl/cogl-framebuffer.c b/cogl/cogl/cogl-framebuffer.c
index f2df9c229..8660f910a 100644
--- a/cogl/cogl/cogl-framebuffer.c
+++ b/cogl/cogl/cogl-framebuffer.c
@@ -116,6 +116,7 @@ _cogl_framebuffer_init (CoglFramebuffer *framebuffer,
   framebuffer->viewport_age_for_scissor_workaround = -1;
   framebuffer->dither_enabled = TRUE;
   framebuffer->depth_writing_enabled = TRUE;
+  framebuffer->depth_buffer_clear_needed = TRUE;
 
   framebuffer->modelview_stack = cogl_matrix_stack_new (ctx);
   framebuffer->projection_stack = cogl_matrix_stack_new (ctx);
@@ -267,6 +268,13 @@ cogl_framebuffer_clear4f (CoglFramebuffer *framebuffer,
   int scissor_y1;
   CoglBool saved_viewport_scissor_workaround;
 
+  if (!framebuffer->depth_buffer_clear_needed &&
+      (buffers & COGL_BUFFER_BIT_DEPTH))
+    buffers &= ~(COGL_BUFFER_BIT_DEPTH);
+
+  if (buffers == 0)
+    return;
+
   _cogl_clip_stack_get_bounds (clip_stack,
                                &scissor_x0, &scissor_y0,
                                &scissor_x1, &scissor_y1);
@@ -414,6 +422,9 @@ cleared:
   _cogl_framebuffer_mark_mid_scene (framebuffer);
   _cogl_framebuffer_mark_clear_clip_dirty (framebuffer);
 
+  if (buffers & COGL_BUFFER_BIT_DEPTH)
+    framebuffer->depth_buffer_clear_needed = FALSE;
+
   if (buffers & COGL_BUFFER_BIT_COLOR && buffers & COGL_BUFFER_BIT_DEPTH)
     {
       /* For our fast-path for reading back a single pixel of simple
diff --git a/cogl/cogl/driver/gl/cogl-pipeline-opengl.c b/cogl/cogl/driver/gl/cogl-pipeline-opengl.c
index 0e8f7c698..1de4b65eb 100644
--- a/cogl/cogl/driver/gl/cogl-pipeline-opengl.c
+++ b/cogl/cogl/driver/gl/cogl-pipeline-opengl.c
@@ -378,7 +378,11 @@ flush_depth_state (CoglContext *ctx,
   if (ctx->depth_test_enabled_cache != depth_state->test_enabled)
     {
       if (depth_state->test_enabled == TRUE)
-        GE (ctx, glEnable (GL_DEPTH_TEST));
+        {
+          GE (ctx, glEnable (GL_DEPTH_TEST));
+          if (ctx->current_draw_buffer)
+            ctx->current_draw_buffer->depth_buffer_clear_needed = TRUE;
+        }
       else
         GE (ctx, glDisable (GL_DEPTH_TEST));
       ctx->depth_test_enabled_cache = depth_state->test_enabled;