Finer-grained inline constant cache invalidation

Current behavior - caches depend on a global counter. All constant mutations cause caches to be invalidated.

```ruby
class A
  B = 1
end

def foo
  A::B # inline cache depends on global counter
end

foo # populate inline cache
foo # hit inline cache

C = 1 # global counter increments, all caches are invalidated

foo # misses inline cache due to `C = 1`
```

Proposed behavior - caches depend on name components. Only constant mutations with corresponding names will invalidate the cache.

```ruby
class A
  B = 1
end

def foo
  A::B # inline cache depends constants named "A" and "B"
end

foo # populate inline cache
foo # hit inline cache

C = 1 # caches that depend on the name "C" are invalidated

foo # hits inline cache because IC only depends on "A" and "B"
```

Examples of breaking the new cache:

```ruby
module C
  # Breaks `foo` cache because "A" constant is set and the cache in foo depends
  # on "A" and "B"
  class A; end
end

B = 1
```

We expect the new cache scheme to be invalidated less often because names aren't frequently reused. With the cache being invalidated less, we can rely on its stability more to keep our constant references fast and reduce the need to throw away generated code in YJIT.

1 files changed, 3 insertions, 0 deletions

diff --git a/iseq.h b/iseq.h
index f90b0be7ab..a7d542fa09 100644
--- a/iseq.h
+++ b/iseq.h
@@ -182,6 +182,9 @@ void rb_iseq_build_from_ary(rb_iseq_t *iseq, VALUE misc,
 void rb_iseq_mark_insn_storage(struct iseq_compile_data_storage *arena);
 
 /* iseq.c */
+typedef bool rb_iseq_each_i(VALUE *code, VALUE insn, size_t index, void *data);
+void rb_iseq_each(const rb_iseq_t *iseq, size_t start_index, rb_iseq_each_i iterator, void *data);
+
 VALUE rb_iseq_load(VALUE data, VALUE parent, VALUE opt);
 VALUE rb_iseq_parameters(const rb_iseq_t *iseq, int is_proc);
 unsigned int rb_iseq_line_no(const rb_iseq_t *iseq, size_t pos);