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.

2 files changed, 7 insertions, 2 deletions

diff --git a/include/ruby/backward.h b/include/ruby/backward.h
index f804c2c36e..f5662f13d5 100644
--- a/include/ruby/backward.h
+++ b/include/ruby/backward.h
@@ -15,8 +15,6 @@
 #define RBIMPL_ATTR_DEPRECATED_INTERNAL(ver) RBIMPL_ATTR_DEPRECATED(("since "#ver", also internal"))
 #define RBIMPL_ATTR_DEPRECATED_INTERNAL_ONLY() RBIMPL_ATTR_DEPRECATED(("only for internal use"))
 
-RBIMPL_ATTR_DEPRECATED_INTERNAL_ONLY() void rb_clear_constant_cache(void);
-
 /* from version.c */
 #if defined(RUBY_SHOW_COPYRIGHT_TO_DIE) && !!(RUBY_SHOW_COPYRIGHT_TO_DIE+0)
 # error RUBY_SHOW_COPYRIGHT_TO_DIE is deprecated
diff --git a/include/ruby/internal/intern/vm.h b/include/ruby/internal/intern/vm.h
index eb53c7a356..76af796b54 100644
--- a/include/ruby/internal/intern/vm.h
+++ b/include/ruby/internal/intern/vm.h
@@ -253,6 +253,13 @@ void rb_undef_alloc_func(VALUE klass);
 rb_alloc_func_t rb_get_alloc_func(VALUE klass);
 
 /**
+ * Clears the inline constant caches associated with a particular ID. Extension
+ * libraries should not bother with such things. Just forget about this API (or
+ * even, the presence of constant caches).
+ */
+void rb_clear_constant_cache_for_id(ID id);
+
+/**
  * Resembles `alias`.
  *
  * @param[out]  klass            Where to define an alias.