strlen example, source code:

    #include <string.h>

    int main()
    {
        return strlen("Hello world!");
    }

strlen example, assembly code at -O3:

    movl    $12, %eax
    ret

wcslen example, source code:

    #include <wchar.h>

    int main()
    {
        return wcslen(L"Hello world!");
    }

wcslen example, assembly code at -O3:

    subq    $8, %rsp
    .cfi_def_cfa_offset 16
    leaq    .LC0(%rip), %rdi
    call    wcslen@PLT
    addq    $8, %rsp
    .cfi_def_cfa_offset 8
    ret

Interestingly, Clang produces identical assembly code for both examples, which is what I expected GCC to do.