On 22/10/2015 19:39, Radim Krčmář wrote:
> 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.
Me too. :) But Rusty surely has some testcases in case he wants to
adopt some of the ideas here. O:-)
Paolo
>> 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;
>> }
