Now that we've inlined the eden_heap into the size_pool, we should
rename the size_pool to heap. So that Ruby contains multiple heaps, with
different sized objects.
The term heap as a collection of memory pages is more in memory
management nomenclature, whereas size_pool was a name chosen out of
necessity during the development of the Variable Width Allocation
features of Ruby.
The concept of size pools was introduced in order to facilitate
different sized objects (other than the default 40 bytes). They wrapped
the eden heap and the tomb heap, and some related state, and provided a
reasonably simple way of duplicating all related concerns, to provide
multiple pools that all shared the same structure but held different
objects.
Since then various changes have happend in Ruby's memory layout:
* The concept of tomb heaps has been replaced by a global free pages list,
with each page having it's slot size reconfigured at the point when it
is resurrected
* the eden heap has been inlined into the size pool itself, so that now
the size pool directly controls the free_pages list, the sweeping
page, the compaction cursor and the other state that was previously
being managed by the eden heap.
Now that there is no need for a heap wrapper, we should refer to the
collection of pages containing Ruby objects as a heap again rather than
a size pool
Using gc_impl.h inside of gc/gc.h will cause gc/gc.h to use the functions
in gc/default.c when builing with shared GC support because gc/gc.h is
included into gc.c before the rb_gc_impl functions are overridden by the
preprocessor.
gc.c mistakenly defined GC_ASSERT as blank, which caused it to be a
no-op. This caused all assertions in gc.c and gc/default.c to not do
anything. This commit fixes it by moving the definition of GC_ASSERT
to gc/gc.h.
We discovered that having gc.o and gc_impl.o in separate translation
units diminishes codegen quality with GCC 11 on x86-64. This commit
solves that problem by including default/gc.c into gc.c, letting the
optimizer have visibility into the body of functions again in builds
not using link-time optimization, which are common.
This effectively restores things to the way they were before
[Feature #20470] from the optimizer's perspective while maintaining the
ability to build gc/default.c as a DSO.
There were a few functions duplicated across gc.c and gc/default.c.
Extract them and put them into gc/gc.h.
