Introduce RVALUE_SIZE GC constant

Add GC::INTERNAL_CONSTANTS[:RVALUE_SIZE] to store the usable size
(excluding debug overhead) of the smallest pool that can hold a standard
RVALUE.

Author: eightbitraptor

gc/default/default.c

@@ -9591,6 +9591,12 @@ rb_gc_impl_init(void)
 {
     VALUE gc_constants = rb_hash_new();
     rb_hash_aset(gc_constants, ID2SYM(rb_intern("DEBUG")), GC_DEBUG ? Qtrue : Qfalse);
+    /* Minimum slot size that fits a standard RVALUE */
+    size_t rvalue_pool = 0;
+    for (size_t i = 0; i < HEAP_COUNT; i++) {
+        if (pool_slot_sizes[i] >= RVALUE_SLOT_SIZE) { rvalue_pool = pool_slot_sizes[i]; break; }
+    }
+    rb_hash_aset(gc_constants, ID2SYM(rb_intern("RVALUE_SIZE")), SIZET2NUM(rvalue_pool - RVALUE_OVERHEAD));
     rb_hash_aset(gc_constants, ID2SYM(rb_intern("RBASIC_SIZE")), SIZET2NUM(sizeof(struct RBasic)));
     rb_hash_aset(gc_constants, ID2SYM(rb_intern("RVALUE_OVERHEAD")), SIZET2NUM(RVALUE_OVERHEAD));
     rb_hash_aset(gc_constants, ID2SYM(rb_intern("HEAP_PAGE_BITMAP_SIZE")), SIZET2NUM(HEAP_PAGE_BITMAP_SIZE));

gc/mmtk/mmtk.c

@@ -653,6 +653,7 @@ void
 rb_gc_impl_init(void)
 {
     VALUE gc_constants = rb_hash_new();
+    rb_hash_aset(gc_constants, ID2SYM(rb_intern("RVALUE_SIZE")), SIZET2NUM(SIZEOF_VALUE >= 8 ? 64 : 32));
     rb_hash_aset(gc_constants, ID2SYM(rb_intern("RBASIC_SIZE")), SIZET2NUM(sizeof(struct RBasic)));
     rb_hash_aset(gc_constants, ID2SYM(rb_intern("RVALUE_OVERHEAD")), INT2NUM(0));
     rb_hash_aset(gc_constants, ID2SYM(rb_intern("RVARGC_MAX_ALLOCATE_SIZE")), LONG2FIX(MMTK_MAX_OBJ_SIZE));