[dev.garbage] runtime: allocate directly from GC mark bits
Instead of building a freelist from the mark bits generated by the GC this CL allocates directly from the mark bits. The approach moves the mark bits from the pointer/no pointer heap structures into their own per span data structures. The mark/allocation vectors consist of a single mark bit per object. Two vectors are maintained, one for allocation and one for the GC's mark phase. During the GC cycle's sweep phase the interpretation of the vectors is swapped. The mark vector becomes the allocation vector and the old allocation vector is cleared and becomes the mark vector that the next GC cycle will use. Marked entries in the allocation vector indicate that the object is not free. Each allocation vector maintains a boundary between areas of the span already allocated from and areas not yet allocated from. As objects are allocated this boundary is moved until it reaches the end of the span. At this point further allocations will be done from another span. Since we no longer sweep a span inspecting each freed object the responsibility for maintaining pointer/scalar bits in the heapBitMap containing is now the responsibility of the the routines doing the actual allocation. This CL is functionally complete and ready for performance tuning. Change-Id: I336e0fc21eef1066e0b68c7067cc71b9f3d50e04 Reviewed-on: https://go-review.googlesource.com/19470Reviewed-by:Austin Clements <austin@google.com>
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