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From: =?UTF-8?Q?P=c3=a1draig_Brady?= <P@draigBrady.com>
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Date: Sat, 24 Oct 2015 03:24:29 +0100
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Subject: Re: [Qemu-devel] [PATCH] copy,
 dd: simplify and optimize NUL bytes detection
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To: Paolo Bonzini <pbonzini@redhat.com>, =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>, coreutils@gnu.org, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>

On 23/10/15 12:15, Pádraig Brady wrote:
> On 22/10/15 20:47, Paolo Bonzini wrote:
>>
>>
>> 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:-)
> 
> For completeness this should address the bugs I think?
> 
> bool memeqzero4_paolo(const void *data, size_t length)
> {
>     const unsigned char *p = data;
>     unsigned long word;

Note the original coreutils code accessed a word at a time,
but only on aligned buffers. The code below may generate
unaligned accesses for unaligned sizes or buffers.
You can avoid that by auto degenerating back to byte
at a time access using:

#if _STRING_ARCH_unaligned
  unsigned long word;
#else
  unsigned char word;
#endif

> 
>     if (!length)
>         return true;
> 
>     /* Check len bytes not aligned on a word.  */
>     while (__builtin_expect(length & (sizeof(word) - 1), 0)) {
>         if (*p)
>             return false;
>         p++;
>         length--;
>         if (!length)
>             return true;
>     }
> 
>     /* Check up to 16 bytes a word at a time.  */
>     for (;;) {
>         memcpy(&word, p, sizeof(word));
>         if (word)
>             return false;
>         p += sizeof(word);
>         length -= sizeof(word);
>         if (!length)
>             return true;
>         if (__builtin_expect(length & 15, 0) == 0)
>             break;
>     }
> 
>      /* Now we know that's zero, memcmp with self. */
>      return memcmp(data, p, length) == 0;
> }

cheers,
Pádraig.