[Bug libc/19920] RFE: test whether a block of memory is all 0

["eblake at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=19920

--- Comment #2 from Eric Blake <eblake at redhat dot com> ---
Without using any processor-specific optimizations,
http://stackoverflow.com/questions/1493936/faster-approach-to-checking-for-an-all-zero-buffer-in-c
has some ideas for optimizations, including one that has a useful performance
ramification: Once you have taken care of any unaligned head and tail, if you
can check that the first aligned cache line is 0, you can then do memcmp(start,
start+cache_line_len, buf_len-cache_line_len) to get the machine-specific
speedups built into memcmp.

But yes, a dedicated interface that directly uses machine-specific tricks where
possible will be faster, and seems useful to have.

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