clang++ -O0
        .text
        .globl  _ZN3KJS8ValueImp12setGcAllowedEv # -- Begin function _ZN3KJS8ValueImp12setGcAllowedEv
        .p2align        4, 0x90
        .type   _ZN3KJS8ValueImp12setGcAllowedEv,@function
_ZN3KJS8ValueImp12setGcAllowedEv:       # @_ZN3KJS8ValueImp12setGcAllowedEv
        .cfi_startproc
# %bb.0:
        pushq   %rbp
        .cfi_def_cfa_offset 16
        .cfi_offset %rbp, -16
        movq    %rsp, %rbp
        .cfi_def_cfa_register %rbp
        subq    $16, %rsp
        movq    %rdi, -8(%rbp)
        movq    -8(%rbp), %rdi
        movq    %rdi, -16(%rbp)                 # 8-byte Spill
        callq   _ZN3KJS12SimpleNumber2isEPKNS_8ValueImpE@PLT
        testb   $1, %al
        jne     .LBB6_2
# %bb.1:
        movq    -16(%rbp), %rax                 # 8-byte Reload
        movzwl  10(%rax), %ecx
        orl     $2, %ecx
                                        # kill: def $cx killed $cx killed $ecx
        movw    %cx, 10(%rax)
.LBB6_2:
        addq    $16, %rsp
        popq    %rbp
        .cfi_def_cfa %rsp, 8
        retq
.Lfunc_end6:
        .size   _ZN3KJS8ValueImp12setGcAllowedEv, .Lfunc_end6-_ZN3KJS8ValueImp12setGcAllowedEv
        .cfi_endproc
                                        # -- End function

clang++ -O2
        .size   _ZNK3KJS8ValueImp6markedEv, .-_ZNK3KJS8ValueImp6markedEv
        .align 2
        .p2align 4
        .globl  _ZN3KJS8ValueImp12setGcAllowedEv
        .type   _ZN3KJS8ValueImp12setGcAllowedEv, @function
_ZN3KJS8ValueImp12setGcAllowedEv:
.LFB2472:
        .cfi_startproc
        movq    %rdi, %rax
        andl    $3, %eax
        cmpq    $1, %rax
        je      .L10
        orw     $2, 10(%rdi)
.L10:
        ret
        .cfi_endproc