Blender Git Loki

BLI_heap: optimize heap_swap, heap_down and heap_up.

The index field of nodes is supposed to be its actual index, so
there is no need to read it in swap. On a 64-bit processor i and
j are already in registers, so this removes two memory reads.

In addition, cache the tree pointer, use branch hints, and
put the most frequently accessed 'value' field at 0 offset.

Produced a 20% FPS improvement for a 50% heap-heavy workload.