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;
> }
