two end nursery GC

So things have been kind of complicated in the past four weeks.
And my original project changed to a new one as someone in pypy have been working on  the original project(add pinning to GC) for months, but none of my mentors know until two weeks passed.

So my new project is :
two end nursery GC.
As mentioned in previous post, current incremen
tal minimark GC in pypy has two generations: nursery generation and old generation. There is also a external_malloc() to raw-allocate objects(these objects are out of GC control)

The current nursery allocation logic is clear: there is three pointers :nursery-free/nursery-top/nursery-real-top, between nursery-free and nursery-top there is a piece of zeroed memory (grey area below)
,between nursery-top and nursery-real-top  there is non-zeroed( contains random garbage) memory( blue area below).

The nursery is cleaned up step by step, when the nursery-free reaches nursery-top, the nursery-top will move forward and zero the memory for one more step(defined by nursery-cleanup).

From this mechanism  we can see every object in allocation in memory that is allocated in memory full of zeros.

This is very useful for some objects.  Take an object that contains a field which is a GC pointer.  It needs to be zero-initialized, because otherwise, if the program doesn't quickly fill the pointer, garbage is left in this field.  At the next minor or major collection, trying to follow this garbage-valued pointer would

crash.

However, not all objects need such zero-allocation.  For any object not containing any GC pointers, it is not useful from the point of view of the GC.   The point is that we would save part of the time needed to zero out the nursery if we could choose to allocate zeroed or non-zeroed objects.

So the idea of the project is to allocation objects(don't have GC pointers) from nursery-real-top and grows towards nursery-top. So the nursery can allocate objects from two end.

These will save the time that instead of there are two writes(zeroing and allocation) for every object there will be just one write for some of the objects.

The current allocation function are malloc_fixedsize_clear(), malloc_varsize_clear(), I added two new pointers nursery-second-part-free(grows from nursery-real-top), nursery-second-part-end(points to nursery-top).

I also implemented malloc_fixedsize() as the third allocation function in GC to allocate objects in opposite direction from the other end of nursery.

 The next step will be modify the GCtransformer to replace malloc_fixedsize_clear() with malloc_fixedsize() when appropriate.