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

[Paolo Bonzini <pbonzini@redhat.com>]

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;
>> }