This commit implements the Object Shapes technique in CRuby.

Object Shapes is used for accessing instance variables and representing the
"frozenness" of objects.  Object instances have a "shape" and the shape
represents some attributes of the object (currently which instance variables are
set and the "frozenness").  Shapes form a tree data structure, and when a new
instance variable is set on an object, that object "transitions" to a new shape
in the shape tree.  Each shape has an ID that is used for caching. The shape
structure is independent of class, so objects of different types can have the
same shape.

For example:

```ruby
class Foo
  def initialize
    # Starts with shape id 0
    @a = 1 # transitions to shape id 1
    @b = 1 # transitions to shape id 2
  end
end

class Bar
  def initialize
    # Starts with shape id 0
    @a = 1 # transitions to shape id 1
    @b = 1 # transitions to shape id 2
  end
end

foo = Foo.new # `foo` has shape id 2
bar = Bar.new # `bar` has shape id 2
```

Both `foo` and `bar` instances have the same shape because they both set
instance variables of the same name in the same order.

This technique can help to improve inline cache hits as well as generate more
efficient machine code in JIT compilers.

This commit also adds some methods for debugging shapes on objects.  See
`RubyVM::Shape` for more details.

For more context on Object Shapes, see [Feature: #18776]

Co-Authored-By: Aaron Patterson <tenderlove@ruby-lang.org>
Co-Authored-By: Eileen M. Uchitelle <eileencodes@gmail.com>
Co-Authored-By: John Hawthorn <john@hawthorn.email>

1 files changed, 10 insertions, 2 deletions

diff --git a/yjit/src/cruby.rs b/yjit/src/cruby.rs
index 25149ab730..65f398f075 100644
--- a/yjit/src/cruby.rs
+++ b/yjit/src/cruby.rs
@@ -120,7 +120,7 @@ extern "C" {
         obj: VALUE,
         v: VALUE,
     ) -> bool;
-    pub fn rb_vm_set_ivar_idx(obj: VALUE, idx: u32, val: VALUE) -> VALUE;
+    pub fn rb_vm_set_ivar_id(obj: VALUE, idx: u32, val: VALUE) -> VALUE;
     pub fn rb_vm_setinstancevariable(iseq: IseqPtr, obj: VALUE, id: ID, val: VALUE, ic: IVC);
     pub fn rb_aliased_callable_method_entry(
         me: *const rb_callable_method_entry_t,
@@ -354,18 +354,26 @@ impl VALUE {
 
     /// Read the flags bits from the RBasic object, then return a Ruby type enum (e.g. RUBY_T_ARRAY)
     pub fn builtin_type(self) -> ruby_value_type {
+        (self.builtin_flags() & (RUBY_T_MASK as usize)) as ruby_value_type
+    }
+
+    pub fn builtin_flags(self) -> usize {
         assert!(!self.special_const_p());
 
         let VALUE(cval) = self;
         let rbasic_ptr = cval as *const RBasic;
         let flags_bits: usize = unsafe { (*rbasic_ptr).flags }.as_usize();
-        (flags_bits & (RUBY_T_MASK as usize)) as ruby_value_type
+        return flags_bits;
     }
 
     pub fn class_of(self) -> VALUE {
         unsafe { CLASS_OF(self) }
     }
 
+    pub fn shape_of(self) -> u32 {
+        unsafe { rb_shape_get_shape_id(self) }
+    }
+
     pub fn as_isize(self) -> isize {
         let VALUE(is) = self;
         is as isize