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Change slot sizes from {40,80,160,320,640} to {64,128,256,512,1024}.
BASE_SLOT_SIZE is now 64 (2^6) and all pool sizes are powers of 2,
enabling bit-shift slot indexing instead of magic number division.
Replace slot_div_magics[] multiply-and-shift with simple right shift.
Simplify heap_add_page() alignment to a single bitmask round-up.
Pool 0 aligns to cache line boundaries and embeds more objects
(strings: 39 vs 15 chars, arrays: 6 vs 3 elements, ivars: 6 vs 3).
On 32-bit, sizeof(VALUE) is 4 so objects are roughly half the size of 64-bit. Use BASE_SLOT_SIZE_LOG2=5 (32 bytes) instead of 6 (64) to keep slot sizes proportional to pointer width.
RFLOAT mostly
because BASE_SLOT_SIZE is now 32 bytes, it's no longer suitable for use in tests that use it to assume the size of most RVALUE objects, like strings
When RVALUE_OVERHEAD is large (debug builds with RACTOR_CHECK_MODE + GC_DEBUG), the smallest size pool's usable size can be less than sizeof(struct RBasic). The capacity calculation underflows: (8 - 16) / 8 → 0xFFFF (via size_t wraparound, truncated to uint16_t) Since shape_grow_capa iterates capacities from index 0, the garbage 65535 at capacities[0] poisons all ivar capacity growth, causing a buffer overflow in the RUBY_DEBUG assertion that fills unused capacity with Qundef.
When RVALUE_OVERHEAD > 0 (GC_DEBUG, RACTOR_CHECK_MODE), heap[0]'s usable space can equal sizeof(struct RBasic), leaving zero bytes for instance variables. The capacity was incorrectly set to 1, allowing the shape system to embed an IV that overflows into the overhead area. Change the fallback capacity to 0 and switch shape_grow_capa to count-based iteration so that a zero capacity is not confused with the array sentinel.
rb_obj_embedded_size(0) returned sizeof(struct RBasic), which is too small on builds with RVALUE_OVERHEAD (GC_DEBUG) where heap[0] has no usable space beyond RBasic. The as.heap variant needs at least one VALUE of space for the external IV pointer. Clamp the minimum fields_count to 1 so T_OBJECT allocations always request enough space for the as union.
strings (except shared ones) 32 bytes
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pm_parse_process initializes the index_lookup_table but nothing seems to
use it after it has been allocated. However, pm_compile_scope_node will
overwrite the index_lookup_table and cause it to leak memory. This can
be seen during bootup with the following memory leaks reported by ASAN:
#0 0x60dba31b7af3 in malloc
#1 0x60dba32e0718 in rb_gc_impl_malloc gc/default/default.c:8287:5
#2 0x60dba32c7aa7 in ruby_xmalloc_body gc.c:5373:12
#3 0x60dba32c4a54 in ruby_xmalloc gc.c:5355:34
ruby#4 0x60dba3260314 in pm_index_lookup_table_init_heap prism_compile.h:89:29
ruby#5 0x60dba3209388 in pm_parse_process prism_compile.c:11366:5
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