Fix length of Map64::descriptor_map

[wks]

Its length should be MAX_SPACES instead of MAX_CHUNKS.

The size was MAX_SPACES in the JikesRVM MMTk. It may have been a typo when porting JikesRVM MMTk to Rust.

The new Map64::descriptor_map does not deserve the unsafe new_zeroed_vec function which was introduced to solve the slow start-up problem. We keep new_zeroed_vec just in case we may still need it in the future.

[wks]

@wenyuzhao We used vm_layout()::max_chunks() to find the number of chunks, but I didn't find a method in VMLayout that returns the number of spaces. I assume we should still use MAX_SPACES, shouldn't we? Did I miss anything?

[qinsoon]

LGTM

[wenyuzhao]

@wenyuzhao We used vm_layout()::max_chunks() to find the number of chunks, but I didn't find a method in VMLayout that returns the number of spaces. I assume we should still use MAX_SPACES, shouldn't we? Did I miss anything?

Yes I think MAX_SPACES is the correct one to use. Unlike max_chunks which is related to the vm layout defined at runtime, max spaces is a hardcoded constant.