C++ 中文周刊 第74期

/*
divide_by_32_no_assume(int):
        lea     eax, [rdi + 31]
        test    edi, edi
        cmovns  eax, edi
        sar     eax, 5
        ret
*/
auto divide_by_32_no_assume(int x) {
    return x / 32;
}


/*
divide_by_32_assume(int):
        mov     eax, edi
        shr     eax, 5
        ret
*/
auto divide_by_32_assume(int x) {
    __builtin_assume(x >= 0);
    return x / 32;
}

template<class It>
bool is_palindrome(It first, It last) {
    while (first != last) {
        --last;
        if (first == last) break;
        if (*first != *last) {
            return false;
        }
        ++first;
    }
    return true;
}

template<class It>
bool is_palindrome(It first, It last) {
    return std::equal(
        first, last,
        std::reverse_iterator(last),
        std::reverse_iterator(first)
    );
}

std::string s = "foo";
assert(!is_palindrome(s.begin(), s.end()));
assert(is_palindrome(s.begin()+1, s.end()));
assert(!is_palindrome(s.rbegin(), s.rend()));
assert(is_palindrome(s.rbegin(), s.rend()-1));  // Fails! Oof!

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