Re: [Qemu-devel] [PATCH] copy, dd: simplify and optimize NUL bytes detection

[Radim Krčmář]

2015-10-22 18:14+0200, Paolo Bonzini:
> On 22/10/2015 18:02, Eric Blake wrote:
>> I see a bug in there:
> 
> Of course.  You shouldn't have told me what the bug was, I deserved
> to look for it myself. :)

It rather seems that you don't want spoilers, :)

I see two bugs now.

> bool memeqzero4_paolo(const void *data, size_t length)
> {
>     const unsigned char *p = data;
>     unsigned long word;
> 
>     while (__builtin_expect(length & (sizeof(word) - 1), 0)) {
>         if (*p)
>             return false;
>         p++;
>         length--;
>         if (!length)
>             return true;
>     }
> 
>     /* We must always read one byte or word, even if everything is aligned!
>      * Otherwise, memcmp(data, data, length) is trivially true.
>      */
>     for (;;) {
>         memcpy(&word, p, sizeof(word));
>         if (word)
>             return false;
>         if (__builtin_expect(length & (16 - sizeof(word)), 0) == 0)
>             break;
>         p += sizeof(word);
>         length -= sizeof(word);
>         if (!length)
>             return true;
>     }
> 
>      /* Now we know that's zero, memcmp with self. */
>      return memcmp(data, p, length) == 0;
> }