TruffleRuby uses an optimisation called object shapes to optimise Ruby. It automatically learns and understands the layout and types, or the shape, of your objects as your code is running and optimises code to work better with those shapes. As the community tries to make MRI faster, it could be time to adopt object shapes there as well. We’ll talk about what TruffleRuby does, how it does it, and the benefits it achieves in practice.

1. @
Kaigi The Future Shape of Ruby Objects 202:1

2. name: ‘Chris’
age: 36
company: ‘Shopify’
home: ‘Cheshire’

3. struct RObject {
VALUE klass;
int numiv;
VALUE[] ivptr;
};
struct RClass {
struct st_table *iv_index_tbl; // Hash of Symbol -> index
};

4. VALUE vm_getivar(VALUE obj, IVC inline_cache) {
if (inline_cache && inline_cache->class_serial == obj->klass->class_serial) {
int index = inline_cache->index;
if (obj->type == T_OBJECT) && index < obj->numiv) {
return obj->ivptr[index];
} else {
// slow path
}
} else {
// slow path
}
}
VALUE vm_setivar(VALUE obj, VALUE val, IVC inline_cache) {
if (inline_cache && inline_cache->class_serial == obj->klass->class_serial) {
int index = inline_cache->index;
if (obj->type == T_OBJECT) && index < obj->numiv) {
obj->ivptr[index] = val;
} else {
// slow path
} } else {
// slow path
}
}

5. def vm_getivar(cache, obj)
if cache.serial == obj.klass.serial && cache.index < obj.numiv
obj.ivptr[cache.index]
else
# slow path
end
end
def vm_setivar(cache, obj, val)
if cache.serial == obj.klass.serial && cache.index < obj.numiv
obj.ivptr[cache.index] = val
else
# slow path
end
end

6. class TwoVariablesObject {
Object var0;
Object var1;
}
def vm_getivar(cache, obj)
if cache.id == obj.getMetaClass().getRealClass().id
obj[cache.index]
else
# slow path
end
end

12. name: 0
home: 1
company: 2
age: 3
klass
shape
‘Chris’
‘Cheshire’
‘Shopify’
36

13. index = obj.shape[:name]
obj[index]

14. index = obj.shape[:name]
obj[index] = value

15. name: 0
home: 1
company: 2
age: 3
klass
shape
‘Chris’
‘Cheshire’
‘Shopify’
36
181 name: 0
home: 1
company: 2
age: 3
height: 4
+ height

16. new_shape = obj.shape.transitions[:add_height]
index = new_shape[:name]
obj.shape = new_shape
obj.resize
obj[index] = value

17. slow_path unless obj.shape == 0x12345678
obj[4]

19. def getter
@ivar
End
0x1242ec600: cmp dword ptr [rax*8 + 0xc], expected_shape
0x1242ec60b: jne slow_path
0x1242ec611: mov r10d, dword ptr [rax*8 + index]

20. def setter(value)
@ivar = value
end
0x11fd28a00: cmp dword ptr [rsi*8 + 0xc], expected_shape
0x11fd28a0b: jne slow_path
0x11fd28a1f: mov qword ptr [rsi*8 + 0x20], r10

22. def add
@a + @b
end
0x12258d380: cmp dword ptr [rax*8 + 0xc], expected_shape
0x12258d38b: jne slow_path
0x12258d391: mov esi, dword ptr [rax*8 + index_a]
0x12258d398: mov eax, dword ptr [rax*8 + index_b]
0x12258d39f: mov r10d, eax
0x12258d3a2: add r10d, esi
0x12258d3a5: jo overflow

23. name: 0
home: 1
company: 2
age: 3
klass
shape
‘Chris’
‘Cheshire’
‘Shopify’
36
181 name: 0
home: 1
company: 2
age: 3
(frozen)
freeze

24. 0x11fd28a00: cmp dword ptr [rsi*8 + 0xc], expected_shape
0x11fd28a0b: jne slow_path
0x11fd28a11: mov eax, dword ptr [rdx + index]

25. @chrisgseaton