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0c55e87d8f
When calculating regions, a lot of temporary allocations are created. For
the array of rects (which is often a short number of them) we can use
stack allocations up to 1 page (256 cairo_rectangle_int_t). For building
a region of rectangles, cairo and pixman are much faster if you have all
of the rectangles up front or else it mallocs quite a bit of temporary
memory.
If we re-use the cairo_rectangle_int_t array we've already allocated (and
preferably on the stack), we can delay the creation of regions until after
the tight loop.
Additionally, it requires fewer allocations to union two cairo_region_t
than to incrementally union the rectangles into the region.
Before (percentages are of total number of allocations)
TOTAL FUNCTION
[ 100.00%] [Everything]
[ 100.00%] [gnome-shell --wayland --display-server]
[ 99.67%] _start
[ 99.67%] __libc_start_main
[ 99.67%] main
[ 98.60%] meta_run
[ 96.90%] g_main_loop_run
[ 96.90%] g_main_context_iterate.isra.0
[ 96.90%] g_main_context_dispatch
[ 90.27%] clutter_clock_dispatch
[ 86.54%] _clutter_stage_do_update
[ 85.00%] clutter_stage_cogl_redraw
[ 84.98%] clutter_stage_cogl_redraw_view
[ 81.09%] cairo_region_union_rectangle
After (overhead has much dropped)
TOTAL FUNCTION
[ 100.00%] [Everything]
[ 99.80%] [gnome-shell --wayland --display-server]
[ 99.48%] _start
[ 99.48%] __libc_start_main
[ 99.48%] main
[ 92.37%] meta_run
[ 81.49%] g_main_loop_run
[ 81.49%] g_main_context_iterate.isra.0
[ 81.43%] g_main_context_dispatch
[ 39.40%] clutter_clock_dispatch
[ 26.93%] _clutter_stage_do_update
[ 25.80%] clutter_stage_cogl_redraw
[ 25.60%] clutter_stage_cogl_redraw_view
https://gitlab.gnome.org/GNOME/mutter/merge_requests/1071
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