https://godbolt.org/z/9h7Ex1Ych

vector_bool_index.S

f(std::vector<bool, std::allocator<bool>> const&, unsigned long):
        mov     rax, rdi                    ; rax = v
        mov     rdi, rsi                    ; rdi = i
        mov     ecx, esi			        ; rcx = i
        mov     rdx, QWORD PTR [rax]	    ; rdx = *v
        shr     rdi, 6				        ; rdi >>= 6
        mov     eax, 1				        ; rax = 1
        sal     rax, cl				        ; rax <<= (rcx & 0xFF)
        and     rax, QWORD PTR [rdx+rdi*8]	; rax &= rdx[rdi]
        setne   al				            ; al = (rax != 0)
        ret

; rax = (1 << (i & 0xFF)) & v[i >> 6]