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, 187 insertions, 0 deletions

diff --git a/bootstraptest/test_constant_cache.rb b/bootstraptest/test_constant_cache.rb
new file mode 100644
index 0000000000..1fa83256ed
--- /dev/null
+++ b/bootstraptest/test_constant_cache.rb
@@ -0,0 +1,187 @@
+# Constant lookup is cached.
+assert_equal '1', %q{
+  CONST = 1
+
+  def const
+    CONST
+  end
+
+  const
+  const
+}
+
+# Invalidate when a constant is set.
+assert_equal '2', %q{
+  CONST = 1
+
+  def const
+    CONST
+  end
+
+  const
+
+  CONST = 2
+
+  const
+}
+
+# Invalidate when a constant of the same name is set.
+assert_equal '1', %q{
+  CONST = 1
+
+  def const
+    CONST
+  end
+
+  const
+
+  class Container
+    CONST = 2
+  end
+
+  const
+}
+
+# Invalidate when a constant is removed.
+assert_equal 'missing', %q{
+  class Container
+    CONST = 1
+
+    def const
+      CONST
+    end
+
+    def self.const_missing(name)
+      'missing'
+    end
+
+    new.const
+    remove_const :CONST
+  end
+
+  Container.new.const
+}
+
+# Invalidate when a constant's visibility changes.
+assert_equal 'missing', %q{
+  class Container
+    CONST = 1
+
+    def self.const_missing(name)
+      'missing'
+    end
+  end
+
+  def const
+    Container::CONST
+  end
+
+  const
+
+  Container.private_constant :CONST
+
+  const
+}
+
+# Invalidate when a constant's visibility changes even if the call to the
+# visibility change method fails.
+assert_equal 'missing', %q{
+  class Container
+    CONST1 = 1
+
+    def self.const_missing(name)
+      'missing'
+    end
+  end
+
+  def const1
+    Container::CONST1
+  end
+
+  const1
+
+  begin
+    Container.private_constant :CONST1, :CONST2
+  rescue NameError
+  end
+
+  const1
+}
+
+# Invalidate when a module is included.
+assert_equal 'INCLUDE', %q{
+  module Include
+    CONST = :INCLUDE
+  end
+
+  class Parent
+    CONST = :PARENT
+  end
+
+  class Child < Parent
+    def const
+      CONST
+    end
+
+    new.const
+
+    include Include
+  end
+
+  Child.new.const
+}
+
+# Invalidate when const_missing is hit.
+assert_equal '2', %q{
+  module Container
+    Foo = 1
+    Bar = 2
+
+    class << self
+      attr_accessor :count
+
+      def const_missing(name)
+        @count += 1
+        @count == 1 ? Foo : Bar
+      end
+    end
+
+    @count = 0
+  end
+
+  def const
+    Container::Baz
+  end
+
+  const
+  const
+}
+
+# Invalidate when the iseq gets cleaned up.
+assert_equal '2', %q{
+  CONSTANT = 1
+
+  iseq = RubyVM::InstructionSequence.compile(<<~RUBY)
+    CONSTANT
+  RUBY
+
+  iseq.eval
+  iseq = nil
+
+  GC.start
+  CONSTANT = 2
+}
+
+# Invalidate when the iseq gets cleaned up even if it was never in the cache.
+assert_equal '2', %q{
+  CONSTANT = 1
+
+  iseq = RubyVM::InstructionSequence.compile(<<~RUBY)
+    CONSTANT
+  RUBY
+
+  iseq = nil
+
+  GC.start
+  CONSTANT = 2
+}