This integrates the functionality for `parallel_for_weighted` from 9a3ceb79de
into `parallel_for`. This reduces the number of entry points to the threading
API and also makes it easier to build higher level threading primitives. For
example, `IndexMask.foreach_*` may use `parallel_for` if a `GrainSize` is
provided, but can't use `parallel_for_weighted` easily without duplicating a
fair amount of code.
The default behavior of `parallel_for` does not change. However, now one can
optionally pass in `TaskSizeHints` as the last parameter. This can be used to
specify the size of individual tasks relative to each other and relative to the
grain size. This helps scheduling more equally sized tasks which generally
improves performance because threads are used more effectively.
One generally does not construct `TaskSizeHints` manually, but calls either
`threading::individual_task_sizes` or `threading::accumulated_task_sizes`. Both
allow specifying individual task sizes, but the latter should be used when the
combined size of consecutive tasks can be computed in O(1) time. This allows
splitting up the work more efficiently. It can often be used in conjunction with
`OffsetIndices`.
Pull Request: https://projects.blender.org/blender/blender/pulls/121127
8d13a9608b