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, 31 insertions, 6 deletions

diff --git a/mjit_c.rb b/mjit_c.rb
index d8e5628bda..fd8991af3f 100644
--- a/mjit_c.rb
+++ b/mjit_c.rb
@@ -5,6 +5,10 @@ module RubyVM::MJIT
   C = Object.new
 
   class << C
+    def SHAPE_BITS
+      RubyVM::Shape::SHAPE_BITS
+    end
+
     def ROBJECT_EMBED_LEN_MAX
       Primitive.cexpr! 'INT2NUM(RBIMPL_EMBED_LEN_MAX_OF(VALUE))'
     end
@@ -165,6 +169,14 @@ module RubyVM::MJIT
     Primitive.cexpr! %q{ INT2NUM(VM_METHOD_TYPE_ISEQ) }
   end
 
+  def C.INVALID_SHAPE_ID
+    Primitive.cexpr! %q{ ULONG2NUM(INVALID_SHAPE_ID) }
+  end
+
+  def C.SHAPE_MASK
+    Primitive.cexpr! %q{ ULONG2NUM(SHAPE_MASK) }
+  end
+
   def C.CALL_DATA
     @CALL_DATA ||= self.rb_call_data
   end
@@ -181,6 +193,10 @@ module RubyVM::MJIT
     @RB_BUILTIN ||= self.rb_builtin_function
   end
 
+  def C.attr_index_t
+    @attr_index_t ||= CType::Immediate.parse("uint32_t")
+  end
+
   def C.compile_branch
     @compile_branch ||= CType::Struct.new(
       "compile_branch", Primitive.cexpr!("SIZEOF(struct compile_branch)"),
@@ -201,7 +217,6 @@ module RubyVM::MJIT
       compiled_id: [CType::Immediate.parse("int"), Primitive.cexpr!("OFFSETOF((*((struct compile_status *)NULL)), compiled_id)")],
       compile_info: [CType::Pointer.new { self.rb_mjit_compile_info }, Primitive.cexpr!("OFFSETOF((*((struct compile_status *)NULL)), compile_info)")],
       merge_ivar_guards_p: [self._Bool, Primitive.cexpr!("OFFSETOF((*((struct compile_status *)NULL)), merge_ivar_guards_p)")],
-      ivar_serial: [self.rb_serial_t, Primitive.cexpr!("OFFSETOF((*((struct compile_status *)NULL)), ivar_serial)")],
       max_ivar_index: [CType::Immediate.parse("size_t"), Primitive.cexpr!("OFFSETOF((*((struct compile_status *)NULL)), max_ivar_index)")],
       inlined_iseqs: [CType::Pointer.new { CType::Pointer.new { self.rb_iseq_constant_body } }, Primitive.cexpr!("OFFSETOF((*((struct compile_status *)NULL)), inlined_iseqs)")],
       inline_context: [self.inlined_call_context, Primitive.cexpr!("OFFSETOF((*((struct compile_status *)NULL)), inline_context)")],
@@ -240,7 +255,9 @@ module RubyVM::MJIT
   def C.iseq_inline_iv_cache_entry
     @iseq_inline_iv_cache_entry ||= CType::Struct.new(
       "iseq_inline_iv_cache_entry", Primitive.cexpr!("SIZEOF(struct iseq_inline_iv_cache_entry)"),
-      entry: [CType::Pointer.new { self.rb_iv_index_tbl_entry }, Primitive.cexpr!("OFFSETOF((*((struct iseq_inline_iv_cache_entry *)NULL)), entry)")],
+      source_shape_id: [self.shape_id_t, Primitive.cexpr!("OFFSETOF((*((struct iseq_inline_iv_cache_entry *)NULL)), source_shape_id)")],
+      dest_shape_id: [self.shape_id_t, Primitive.cexpr!("OFFSETOF((*((struct iseq_inline_iv_cache_entry *)NULL)), dest_shape_id)")],
+      attr_index: [self.attr_index_t, Primitive.cexpr!("OFFSETOF((*((struct iseq_inline_iv_cache_entry *)NULL)), attr_index)")],
     )
   end
 
@@ -313,7 +330,11 @@ module RubyVM::MJIT
       call_: [self.vm_call_handler, Primitive.cexpr!("OFFSETOF((*((struct rb_callcache *)NULL)), call_)")],
       aux_: [CType::Union.new(
         "", Primitive.cexpr!("SIZEOF(((struct rb_callcache *)NULL)->aux_)"),
-        attr_index: CType::Immediate.parse("unsigned int"),
+        attr: CType::Struct.new(
+          "", Primitive.cexpr!("SIZEOF(((struct rb_callcache *)NULL)->aux_.attr)"),
+          index: [self.attr_index_t, Primitive.cexpr!("OFFSETOF(((struct rb_callcache *)NULL)->aux_.attr, index)")],
+          dest_shape_id: [self.shape_id_t, Primitive.cexpr!("OFFSETOF(((struct rb_callcache *)NULL)->aux_.attr, dest_shape_id)")],
+        ),
         method_missing_reason: self.method_missing_reason,
         v: self.VALUE,
       ), Primitive.cexpr!("OFFSETOF((*((struct rb_callcache *)NULL)), aux_)")],
@@ -502,9 +523,9 @@ module RubyVM::MJIT
   def C.rb_iv_index_tbl_entry
     @rb_iv_index_tbl_entry ||= CType::Struct.new(
       "rb_iv_index_tbl_entry", Primitive.cexpr!("SIZEOF(struct rb_iv_index_tbl_entry)"),
-      index: [CType::Immediate.parse("uint32_t"), Primitive.cexpr!("OFFSETOF((*((struct rb_iv_index_tbl_entry *)NULL)), index)")],
-      class_serial: [self.rb_serial_t, Primitive.cexpr!("OFFSETOF((*((struct rb_iv_index_tbl_entry *)NULL)), class_serial)")],
-      class_value: [self.VALUE, Primitive.cexpr!("OFFSETOF((*((struct rb_iv_index_tbl_entry *)NULL)), class_value)")],
+      idx: [CType::Immediate.parse("uint32_t"), Primitive.cexpr!("OFFSETOF((*((struct rb_iv_index_tbl_entry *)NULL)), idx)")],
+      source_shape_id: [self.shape_id_t, Primitive.cexpr!("OFFSETOF((*((struct rb_iv_index_tbl_entry *)NULL)), source_shape_id)")],
+      dest_shape_id: [self.shape_id_t, Primitive.cexpr!("OFFSETOF((*((struct rb_iv_index_tbl_entry *)NULL)), dest_shape_id)")],
     )
   end
 
@@ -577,6 +598,10 @@ module RubyVM::MJIT
     @VALUE ||= CType::Immediate.find(Primitive.cexpr!("SIZEOF(VALUE)"), Primitive.cexpr!("SIGNED_TYPE_P(VALUE)"))
   end
 
+  def C.shape_id_t
+    @shape_id_t ||= CType::Immediate.find(Primitive.cexpr!("SIZEOF(shape_id_t)"), Primitive.cexpr!("SIGNED_TYPE_P(shape_id_t)"))
+  end
+
   def C._Bool
     CType::Bool.new
   end