runtime: use 2-bit heap bitmap (in place of 4-bit)
Previous CLs changed the representation of the non-heap type bitmaps to be 1-bit bitmaps (pointer or not). Before this CL, the heap bitmap stored a 2-bit type for each word and a mark bit and checkmark bit for the first word of the object. (There used to be additional per-word bits.) Reduce heap bitmap to 2-bit, with 1 dedicated to pointer or not, and the other used for mark, checkmark, and "keep scanning forward to find pointers in this object." See comments for details. This CL replaces heapBitsSetType with very slow but obviously correct code. A followup CL will optimize it. (Spoiler: the new code is faster than Go 1.4 was.) Change-Id: I999577a133f3cfecacebdec9cdc3573c235c7fb9 Reviewed-on: https://go-review.googlesource.com/9703Reviewed-by:Rick Hudson <rlh@golang.org> Reviewed-by:
Austin Clements <austin@google.com>
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