[PATCH] glamor: Don't unbind the GL_ARRAY_BUFFER in glamor_put_vbo_space().

[Kenneth Graunke]

Mesa's implementation of glBindBuffer() checks to see if the supplied
buffer name happens to be already bound.  If so, it returns immediately,
skipping the hash table lookup (mapping the GLuint buffer handle to the
actual struct), the associated locking, and reference counting.

Glamor uses a single VBO for everything - operations reserve space in
the VBO via glamor_get_vbo_space(); if it fills up, we replace it with
a fresh buffer (which is then used for everything).  Ideally, Glamor
would almost always hit the above optimization.

Instead, it never hits it.  Today, glamor_get_vbo_space() binds the VBO,
and glamor_put_vbo_space() explicitly unbinds it.  This means that our
VBO is never bound on a get operation, so we pay the hash table cost
every time we call glamor_get_vbo_space().  Every get/put also alter
reference counts, so we pay that cost twice over.

This patch removes the explicit unbind, which makes us hit the fast path
on every get/put (except the rare ones which create a new buffer).

When running a fullscreen GL application under xf86-video-modesetting
(which doesn't do page flipping yet), glamor_get_vbo_space() accounted
for 4.24% of the CPU instructions in each glamor_copy_fbo_fbo_draw()
operation.  With this patch, it became only 0.87%.

Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
Cc: Eric Anholt <eric at anholt.net>
---
 glamor/glamor_vbo.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/glamor/glamor_vbo.c b/glamor/glamor_vbo.c
index e906101..5d7c55f 100644
--- a/glamor/glamor_vbo.c
+++ b/glamor/glamor_vbo.c
@@ -150,8 +150,6 @@ glamor_put_vbo_space(ScreenPtr screen)
         glBufferData(GL_ARRAY_BUFFER, glamor_priv->vbo_offset,
                      glamor_priv->vb, GL_DYNAMIC_DRAW);
     }
-
-    glBindBuffer(GL_ARRAY_BUFFER, 0);
 }
 
 void
-- 
2.2.1