Malloc API

[qinsoon]

This PR adds a set of malloc related API, and another set of malloc API that will count allocation size into MMTk heap under the feature malloc_counted_size. This PR adds things described in #572 (comment).

The changes in this PR:

• tidy up the malloc library import
• provide a set of standard malloc functions (malloc, calloc, realloc and free)
• provide a set of malloc functions that will count the malloc size into MMTk heap under the feature malloc_counted_size
• provide gc_poll() that can be used to check GC when a binding uses counted malloc functions. Small refactoring to allow poll() without a space reference.

[wks]

Mostly OK. I only have one question and one suggestion.

[wks]

This chain of && means "if space is None, then the next GC will not be full-heap". Is this intentional? This means memory_manager::gc_poll will never trigger full-heap GC directly.

If you mean, "if space_full is true and space is None, then trigger full-heap immediately", it should be space_full && (!space.is_some() || space.unwrap().common().descriptor != self.nursery.common().descriptor).

[qinsoon]

You are right. That was a mistake. I will change the code to this:

        // Is the GC triggered by nursery?
        // - if space is none, it is not. Return false immediately.
        // - if space is some, we further check its descriptor.
        let is_triggered_by_nursery = space.map_or(false, |s| s.common().descriptor == self.nursery.common().descriptor);
        // If space is full and the GC is not triggered by nursery, next GC will be full heap GC.
        if space_full && !is_triggered_by_nursery {
            self.next_gc_full_heap.store(true, Ordering::SeqCst);
        }

[wks]

If a fixture is serial, then neither AtomicRefCell nor Once are necessary. It is easier to create a dedicated SerialFixture type, and wrap both Option<Box<T>> and an initialized flag into a big Mutex.

[qinsoon]

I have added a SerialFixture.

[wks]

LGTM