Refactor iterate_meta_bits

[wks]

This PR refactors the mechanism for visiting (reading and/or updating) side metadata in bulk, specifically the function SideMetadataSpec::iterate_meta_bits.

The function now uses a single FnMut callback instead of two Fn callbacks. It uses the enum type BitByteRange to distinguish whole byte ranges from bit ranges in a byte. This allows the user to capture variables mutably in the callback. This also removes the Cell used in find_prev_non_zero_value_fast.

The function is made non-recursive to improve the performance. Some test cases are added to test for corner cases.

The function is moved to a dedicated ranges module and renamed to break_bit_range, for several reasons:

• It was a method of SideMetadataSpec, but it does not access any member of SideMetadataSpec.
• It needs a non-trivial amount of testing to get corner cases correct, especially after refactoring into a non-recursive function.
• Related types and functions can be added to the ranges module in the future.
  • Breaking a range of bytes into a range of aligned words and unaligned bytes in the beginning and the end. It will be used by finding VO bits from internal pointers and finding all VO bits in a region (for heap traversal).
  • Breaking a range of bytes at chunk boundaries. It will be used by bulk_update_metadata.

[wks]

Here are some benchmark results.

size	data bytes	meta bytes	master	master no recursion	this PR	memset
line	256	4	6.1574 ns	4.1684 ns	4.0134 ns	1.1416 ns
block	32768	512	6.5757 ns	4.5108 ns	4.5009 ns	1.7800 ns

The benchmarks can be found in benches/bulk_meta/bzero_bset.rs, and can be invoked by

taskset -c 0 cargo bench bzero_bset --features bench

Note: You need taskset only if your CPU has heterogeneous cores (so-called "performance-cores" and "efficient-cores").

This PR makes iterate_meta_bits faster than the master branch. That's partly because the existing iterate_meta_bits function is recursive, although it is guaranteed to terminate at the second level of recursion. This PR replaced the recursion with base cases and reduced the cost. I modified the master branch (see https://github.com/mmtk/mmtk-core/commit/24326a22c4441aae5357208c70b3af7dbe24a90e) to make iterate_meta_bits non-recursive using the same algorithm as this PR, and it's almost as fast as this PR (as seen in the "master no recursion" column in the table above). There may be some subtle difference in the generated code after changing two closures into one closure, but the difference between "master no recusrion" and "this PR" is small.

But neither master nor this PR is as fast as calling memset directly to set the meta bits, and the overhead is huge.

I'll try to rebase https://github.com/mmtk/mmtk-core/pull/1174 on top of this PR and mark both PRs for review if the refactored code is pleasant for implementing heap traversal with.