Avoid taking vm barrier in heap_prepare()

[luke-gruber]

We can avoid taking this barrier if we're not incremental marking or lazy sweeping. I found this was taking a significant amount of samples when profiling Psych.load in multiple ractors due to the vm barrier. With this change, we get significant improvements in ractor benchmarks that allocate lots of objects.

-- Psych.load benchmark --

Before:            After:
r:   itr:   time   r:   itr:   time
0    #1:  960ms    0    #1:  943ms
0    #2:  979ms    0    #2:  939ms
0    #3:  968ms    0    #3:  948ms
0    #4:  963ms    0    #4:  946ms
0    #5:  964ms    0    #5:  944ms
1    #1:  947ms    1    #1:  940ms
1    #2:  950ms    1    #2:  947ms
1    #3:  962ms    1    #3:  950ms
1    #4:  947ms    1    #4:  945ms
1    #5:  947ms    1    #5:  943ms
2    #1: 1131ms    2    #1: 1005ms
2    #2: 1153ms    2    #2:  996ms
2    #3: 1155ms    2    #3: 1003ms
2    #4: 1205ms    2    #4: 1012ms
2    #5: 1179ms    2    #5: 1012ms
4    #1: 1555ms    4    #1: 1209ms
4    #2: 1509ms    4    #2: 1244ms
4    #3: 1529ms    4    #3: 1254ms
4    #4: 1512ms    4    #4: 1267ms
4    #5: 1513ms    4    #5: 1245ms
6    #1: 2122ms    6    #1: 1584ms
6    #2: 2080ms    6    #2: 1532ms
6    #3: 2079ms    6    #3: 1476ms
6    #4: 2021ms    6    #4: 1463ms
6    #5: 1999ms    6    #5: 1461ms
8    #1: 2741ms    8    #1: 1630ms
8    #2: 2711ms    8    #2: 1632ms
8    #3: 2688ms    8    #3: 1654ms
8    #4: 2641ms    8    #4: 1684ms
8    #5: 2656ms    8    #5: 1752ms

[etiennebarrie]

What about an early return instead?

Suggested change

	bool needs_gc = is_incremental_marking(objspace) || (heap->free_pages == NULL && is_lazy_sweeping(objspace));
	if (needs_gc) {
	gc_enter(objspace, gc_enter_event_continue, &lock_lev); // takes vm barrier, try to avoid
	}
	bool needs_gc = is_incremental_marking(objspace) || (heap->free_pages == NULL && is_lazy_sweeping(objspace));
	if (!needs_gc) {
	return;
	}

[nobu]

How about extracting a macro:

/* In lazy sweeping or the previous incremental marking finished and
 * did not yield a free page. */
#define needs_continue_sweeping(objspace, heap) \
    ((heap)->free_pages == NULL && is_lazy_sweeping(objspace))

[luke-gruber]

@nobu Thanks, done!