这是优化者的怪癖还是禁止优化的语言规则的结果？

我正在使用编译器资源管理器,发现这两个函数在gcc和clang中生成不同的程序集.我预计在内联后它们会产生相同的表达树,因此会产生相同和最佳的装配.

constexpr bool is_nonzero_decimal_digit(char const c) noexcept
{
    return c == '1' || c == '2' || c == '3' || c == '4' || c == '5'
        || c == '6' || c == '7' || c == '8' || c == '9';
}

bool is_decimal_digit_v1(char const c) noexcept
{
    return c == '0' || is_nonzero_decimal_digit(c);
}

bool is_decimal_digit_v2(char const c) noexcept
{
    return c == '0' || c == '1' || c == '2' || c == '3' || c == '4' 
        || c == '5' || c == '6' || c == '7' || c == '8' || c == '9';
}

Clang 3.9.1 -std = c ++ 1z -O3结果

is_decimal_digit_v1(char):
    cmp     dil, 48
    sete    cl
    add     dil, -49
    cmp     dil, 9
    setb    al
    or      al, cl
    ret

is_decimal_digit_v2(char):
    add     dil, -48
    cmp     dil, 10
    setb    al
    ret

gcc 6.3 -std = c ++ 1z -O3结果

is_decimal_digit_v1(char):
    cmp     dil, 48
    je      .L3
    sub     edi, 49
    cmp     dil, 8
    setbe   al
    ret
.L3:
    mov     eax, 1
    ret

is_decimal_digit_v2(char):
    sub     edi, 48
    cmp     dil, 9
    setbe   al
    ret

那么,这是优化者的怪癖还是禁止优化的语言规则的结果？