[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 <ru...@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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