This commit changes the shape id comparisons to use a 32 bit comparison
rather than 64 bit. That means we don't need to load the shape id to a
register on x86 machines.
Given the following program:
```ruby
class Foo
def initialize
@foo = 1
@bar = 1
end
def read
[@foo, @bar]
end
end
foo = Foo.new
foo.read
foo.read
foo.read
foo.read
foo.read
puts RubyVM::YJIT.disasm(Foo.instance_method(:read))
```
The machine code we generated _before_ this change is like this:
```
== BLOCK 1/4, ISEQ RANGE [0,3), 65 bytes ======================
# getinstancevariable
0x559a18623023: mov rax, qword ptr [r13 + 0x18]
# guard object is heap
0x559a18623027: test al, 7
0x559a1862302a: jne 0x559a1862502d
0x559a18623030: cmp rax, 4
0x559a18623034: jbe 0x559a1862502d
# guard shape, embedded, and T_OBJECT
0x559a1862303a: mov rcx, qword ptr [rax]
0x559a1862303d: movabs r11, 0xffff00000000201f
0x559a18623047: and rcx, r11
0x559a1862304a: movabs r11, 0xb000000002001
0x559a18623054: cmp rcx, r11
0x559a18623057: jne 0x559a18625046
0x559a1862305d: mov rax, qword ptr [rax + 0x18]
0x559a18623061: mov qword ptr [rbx], rax
== BLOCK 2/4, ISEQ RANGE [3,6), 0 bytes =======================
== BLOCK 3/4, ISEQ RANGE [3,6), 47 bytes ======================
# gen_direct_jmp: fallthrough
# getinstancevariable
# regenerate_branch
# getinstancevariable
# regenerate_branch
0x559a18623064: mov rax, qword ptr [r13 + 0x18]
# guard shape, embedded, and T_OBJECT
0x559a18623068: mov rcx, qword ptr [rax]
0x559a1862306b: movabs r11, 0xffff00000000201f
0x559a18623075: and rcx, r11
0x559a18623078: movabs r11, 0xb000000002001
0x559a18623082: cmp rcx, r11
0x559a18623085: jne 0x559a18625099
0x559a1862308b: mov rax, qword ptr [rax + 0x20]
0x559a1862308f: mov qword ptr [rbx + 8], rax
```
After this change, it's like this:
```
== BLOCK 1/4, ISEQ RANGE [0,3), 41 bytes ======================
# getinstancevariable
0x5560c986d023: mov rax, qword ptr [r13 + 0x18]
# guard object is heap
0x5560c986d027: test al, 7
0x5560c986d02a: jne 0x5560c986f02d
0x5560c986d030: cmp rax, 4
0x5560c986d034: jbe 0x5560c986f02d
# guard shape
0x5560c986d03a: cmp word ptr [rax + 6], 0x19
0x5560c986d03f: jne 0x5560c986f046
0x5560c986d045: mov rax, qword ptr [rax + 0x10]
0x5560c986d049: mov qword ptr [rbx], rax
== BLOCK 2/4, ISEQ RANGE [3,6), 0 bytes =======================
== BLOCK 3/4, ISEQ RANGE [3,6), 23 bytes ======================
# gen_direct_jmp: fallthrough
# getinstancevariable
# regenerate_branch
# getinstancevariable
# regenerate_branch
0x5560c986d04c: mov rax, qword ptr [r13 + 0x18]
# guard shape
0x5560c986d050: cmp word ptr [rax + 6], 0x19
0x5560c986d055: jne 0x5560c986f099
0x5560c986d05b: mov rax, qword ptr [rax + 0x18]
0x5560c986d05f: mov qword ptr [rbx + 8], rax
```
The first ivar read is a bit more complex, but the second ivar read is
much simpler. I think eventually we could teach the context about the
shape, then emit only one shape guard.