Struct: keep direct reference to IMEMO/fields when space allows

It's not rare for structs to have additional ivars, hence are one
of the most common, if not the most common type in the `gen_fields_tbl`.

This can cause Ractor contention, but even in single ractor mode
means having to do a hash lookup to access the ivars, and increase
GC work.

Instead, unless the struct is perfectly right sized, we can store
a reference to the associated IMEMO/fields object right after the
last struct member.

```
compare-ruby: ruby 3.5.0dev (2025-08-06T12:50:36Z struct-ivar-fields-2 9a30d141a1) +PRISM [arm64-darwin24]
built-ruby: ruby 3.5.0dev (2025-08-06T12:57:59Z struct-ivar-fields-2 2ff3ec237f) +PRISM [arm64-darwin24]
warming up.....

|                      |compare-ruby|built-ruby|
|:---------------------|-----------:|---------:|
|member_reader         |    590.317k|  579.246k|
|                      |       1.02x|         -|
|member_writer         |    543.963k|  527.104k|
|                      |       1.03x|         -|
|member_reader_method  |    213.540k|  213.004k|
|                      |       1.00x|         -|
|member_writer_method  |    192.657k|  191.491k|
|                      |       1.01x|         -|
|ivar_reader           |    403.993k|  569.915k|
|                      |           -|     1.41x|
```

Co-Authored-By: Étienne Barrié <etienne.barrie@gmail.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/depend b/depend
index ec8c2771c9..ecaf33b1c7 100644
--- a/depend
+++ b/depend
@@ -6065,6 +6065,7 @@ hash.$(OBJEXT): $(top_srcdir)/internal/set_table.h
 hash.$(OBJEXT): $(top_srcdir)/internal/st.h
 hash.$(OBJEXT): $(top_srcdir)/internal/static_assert.h
 hash.$(OBJEXT): $(top_srcdir)/internal/string.h
+hash.$(OBJEXT): $(top_srcdir)/internal/struct.h
 hash.$(OBJEXT): $(top_srcdir)/internal/symbol.h
 hash.$(OBJEXT): $(top_srcdir)/internal/thread.h
 hash.$(OBJEXT): $(top_srcdir)/internal/time.h
@@ -6288,6 +6289,7 @@ hash.$(OBJEXT): {$(VPATH)}symbol.h
 hash.$(OBJEXT): {$(VPATH)}thread_$(THREAD_MODEL).h
 hash.$(OBJEXT): {$(VPATH)}thread_native.h
 hash.$(OBJEXT): {$(VPATH)}util.h
+hash.$(OBJEXT): {$(VPATH)}variable.h
 hash.$(OBJEXT): {$(VPATH)}vm_core.h
 hash.$(OBJEXT): {$(VPATH)}vm_debug.h
 hash.$(OBJEXT): {$(VPATH)}vm_opts.h
@@ -12926,6 +12928,7 @@ range.$(OBJEXT): $(top_srcdir)/internal/enumerator.h
 range.$(OBJEXT): $(top_srcdir)/internal/error.h
 range.$(OBJEXT): $(top_srcdir)/internal/fixnum.h
 range.$(OBJEXT): $(top_srcdir)/internal/gc.h
+range.$(OBJEXT): $(top_srcdir)/internal/imemo.h
 range.$(OBJEXT): $(top_srcdir)/internal/numeric.h
 range.$(OBJEXT): $(top_srcdir)/internal/range.h
 range.$(OBJEXT): $(top_srcdir)/internal/serial.h
@@ -12948,6 +12951,7 @@ range.$(OBJEXT): {$(VPATH)}config.h
 range.$(OBJEXT): {$(VPATH)}defines.h
 range.$(OBJEXT): {$(VPATH)}encoding.h
 range.$(OBJEXT): {$(VPATH)}id.h
+range.$(OBJEXT): {$(VPATH)}id_table.h
 range.$(OBJEXT): {$(VPATH)}intern.h
 range.$(OBJEXT): {$(VPATH)}internal.h
 range.$(OBJEXT): {$(VPATH)}internal/abi.h
@@ -15580,6 +15584,7 @@ shape.$(OBJEXT): $(top_srcdir)/internal/serial.h
 shape.$(OBJEXT): $(top_srcdir)/internal/set_table.h
 shape.$(OBJEXT): $(top_srcdir)/internal/static_assert.h
 shape.$(OBJEXT): $(top_srcdir)/internal/string.h
+shape.$(OBJEXT): $(top_srcdir)/internal/struct.h
 shape.$(OBJEXT): $(top_srcdir)/internal/symbol.h
 shape.$(OBJEXT): $(top_srcdir)/internal/variable.h
 shape.$(OBJEXT): $(top_srcdir)/internal/vm.h
@@ -16569,6 +16574,7 @@ string.$(OBJEXT): $(top_srcdir)/internal/serial.h
 string.$(OBJEXT): $(top_srcdir)/internal/set_table.h
 string.$(OBJEXT): $(top_srcdir)/internal/static_assert.h
 string.$(OBJEXT): $(top_srcdir)/internal/string.h
+string.$(OBJEXT): $(top_srcdir)/internal/struct.h
 string.$(OBJEXT): $(top_srcdir)/internal/transcode.h
 string.$(OBJEXT): $(top_srcdir)/internal/variable.h
 string.$(OBJEXT): $(top_srcdir)/internal/vm.h
@@ -16766,6 +16772,7 @@ string.$(OBJEXT): {$(VPATH)}thread.h
 string.$(OBJEXT): {$(VPATH)}thread_$(THREAD_MODEL).h
 string.$(OBJEXT): {$(VPATH)}thread_native.h
 string.$(OBJEXT): {$(VPATH)}util.h
+string.$(OBJEXT): {$(VPATH)}variable.h
 string.$(OBJEXT): {$(VPATH)}vm_core.h
 string.$(OBJEXT): {$(VPATH)}vm_debug.h
 string.$(OBJEXT): {$(VPATH)}vm_opts.h
@@ -18103,6 +18110,7 @@ variable.$(OBJEXT): $(top_srcdir)/internal/serial.h
 variable.$(OBJEXT): $(top_srcdir)/internal/set_table.h
 variable.$(OBJEXT): $(top_srcdir)/internal/static_assert.h
 variable.$(OBJEXT): $(top_srcdir)/internal/string.h
+variable.$(OBJEXT): $(top_srcdir)/internal/struct.h
 variable.$(OBJEXT): $(top_srcdir)/internal/symbol.h
 variable.$(OBJEXT): $(top_srcdir)/internal/thread.h
 variable.$(OBJEXT): $(top_srcdir)/internal/variable.h