[xlib] Use server-side gradients.

We can offload creation of gradients to server that support RENDER 0.10
and later. This greatly reduces the amount of traffic we need to send over
our display connection as the gradient patterns are much smaller than the
full image. Even if the server fallbacks to using pixman, performance
should be improved by the reduced transport overhead. Furthermore this is a
requisite to enable hardware accelerated gradients with the xlib backend.

Running cairo-perf-trace on tiny, Celeron/i915:

  before: firefox-20090601 211.585
   after: firefox-20090601 270.939

and on tiger, CoreDuo/nvidia:

  before: firefox-20090601 70.143
   after: firefox-20090601 87.326

where linear gradients are used extensively throughout the GTK+ theme.
Not quite the result I was expecting!

In particular, looking at tiny:

 xlib-rgba paint-with-alpha_linear-rgba_over-512   47.11 (47.16 0.05%) -> 123.42 (123.72 0.13%):  2.62x slowdown
█▋
 xlib-rgba paint-with-alpha_linear3-rgba_over-512   47.27 (47.32 0.04%) -> 123.78 (124.04 0.13%):  2.62x slowdown
█▋
 xlib-rgba paint-with-alpha_linear-rgb_over-512   47.19 (47.21 0.02%) -> 123.37 (123.70 0.13%):  2.61x slowdown
█▋
 xlib-rgba paint-with-alpha_linear3-rgb_over-512   47.30 (47.31 0.04%) -> 123.52 (123.62 0.09%):  2.61x slowdown
█▋
 xlib-rgba     paint_linear3-rgb_over-512   47.29 (47.32 0.05%) -> 118.95 (119.60 0.29%):  2.52x slowdown
█▌
 xlib-rgba     paint_linear-rgba_over-512   47.14 (47.17 0.06%) -> 116.76 (117.06 0.16%):  2.48x slowdown
█▌
 xlib-rgba    paint_linear3-rgba_over-512   47.32 (47.34 0.04%) -> 116.85 (116.98 0.05%):  2.47x slowdown
█▌
 xlib-rgba      paint_linear-rgb_over-512   47.15 (47.19 0.03%) -> 114.08 (114.55 0.20%):  2.42x slowdown
█▍
 xlib-rgba paint-with-alpha_radial-rgb_over-512  117.25 (119.43 1.21%) -> 194.36 (194.73 0.09%):  1.66x slowdown
▋
 xlib-rgba paint-with-alpha_radial-rgba_over-512  117.22 (117.26 0.02%) -> 193.81 (194.17 0.11%):  1.65x slowdown
▋
 xlib-rgba     paint_radial-rgba_over-512  117.23 (117.26 0.02%) -> 186.35 (186.41 0.03%):  1.59x slowdown
▋
 xlib-rgba      paint_radial-rgb_over-512  117.23 (117.27 0.02%) -> 184.14 (184.62 1.51%):  1.57x slowdown
▋

Before 1.10, we may choose to disable server-side gradients for the
current crop of Xorg servers, similar to the extended repeat modes.

[Updated by Chris Wilson. All bugs are his.]

1 files changed, 28 insertions, 0 deletions

diff --git a/NEWS b/NEWS
index a561a3f9..9c798dd2 100644
--- a/NEWS
+++ b/NEWS
@@ -26,6 +26,34 @@ New utilities:
       Further minimisation of the fail trace using "delta debugging".
       More control over test/reference targets.
 
+Backend improvements:
+
+  xlib
+
+     Server-side gradients. The theory is that we can offload computation
+     of gradients to the GPU and avoid pushing large images over the
+     connection. Even if the driver has to fallback and use pixman to render
+     a temporary source, it should be able to do so in a more efficient manner
+     than Cairo itself. However, cairo-perf suggests otherwise:
+
+     On tiny, Celeron/i915:
+
+      before: firefox-20090601 211.585
+       after: firefox-20090601 270.939
+
+     and on tiger, CoreDuo/nvidia:
+
+      before: firefox-20090601 70.143
+       after: firefox-20090601 87.326
+
+     In particular, looking at tiny:
+
+     xlib-rgba paint-with-alpha_linear-rgba_over-512   47.11 (47.16 0.05%) -> 123.42 (123.72 0.13%):  2.62x slowdown
+     █▋
+     xlib-rgba paint-with-alpha_linear3-rgba_over-512   47.27 (47.32 0.04%) -> 123.78 (124.04 0.13%):  2.62x slowdown
+     █▋
+
+
 New experimental backends:
 
    QT