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

["peter.maydell at linaro dot org" <sourceware-bugzilla at sourceware dot org>]

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

Peter Maydell <peter.maydell at linaro dot org> changed:

           What    |Removed                     |Added
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                 CC|                            |peter.maydell at linaro dot org

--- Comment #1 from Peter Maydell <peter.maydell at linaro dot org> ---
(In reply to Dr. David Alan Gilbert from comment #0)
>   The current qemu implementation is:
>     size_t buffer_find_nonzero_offset(const void *buf, size_t len)
> 
>     that returns an index to the first !0 byte;

This isn't quite right -- it returns an index which is definitely
earlier than the first nonzero byte, and this is actually where
it gets a lot of the speedup from: you test the buffer 64 bytes
at a time (say), and your loop body only needs to check "is any of
these 64 bytes nonzero?" rather than "find the first", so it
can just do a SIMD logical OR of everything together (and then
if something came out nonzero you return the index of the start
of this 64-byte chunk).

> however all of our current
> uses are just for testing whether the whole buffer is zero.

A straightforward boolean return is probably a cleaner API in
any case, though.

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