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

[Eric Blake <eblake@redhat.com>]

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[adding qemu]

On 10/22/2015 08:00 AM, Pádraig Brady wrote:
> * src/system.h (is_nul): Reimplement with a version
> that doesn't require a sentinel after the buffer,
> and which calls down to (the system optimized) memcmp.
> Performance analyzed at http://rusty.ozlabs.org/?p=560

>  /* Return whether the buffer consists entirely of NULs.
> -   Note the word after the buffer must be non NUL. */
> +   From CCAN by Rusty Russell <rusty@rustcorp.com.au>
> +   released under CC0 (Public domain).  */
>  
>  static inline bool _GL_ATTRIBUTE_PURE
>  is_nul (void const *buf, size_t bufsize)
>  {

> +  const unsigned char *p = buf;
> +  size_t len;
> +
> +  /* Check first 16 bytes manually.  */
> +  for (len = 0; len < 16; len++)
> +    {
> +      if (! bufsize)
> +        return true;
> +      if (*p)
> +        return false;
> +      p++;
> +      bufsize--;
> +    }
> +
> +  /* Now we know that's zero, memcmp with self.  */
> +  return memcmp (buf, p, bufsize) == 0;
>  }

Cool trick of using a suitably-aligned overlap-to-self check to then
trigger platform-specific speedups without having to rewrite them by
hand!  qemu is doing a similar check in util/cutils.c:buffer_is_zero()
that could probably benefit from the same idea.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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