RFR: JDK-8074579 Use more efficient and readable way of checking PKZIP signatures

[Martin Buchholz]

On Fri, Mar 6, 2015 at 1:37 PM, Xueming Shen <xueming.shen at oracle.com>
wrote:

> On 03/06/2015 12:46 PM, Martin Buchholz wrote:
>
>> Hi jar/zip maintainers,
>>
>> Here's an easy improvement.
>>
>> http://cr.openjdk.java.net/~martin/webrevs/openjdk9/PKsig/ <
>> http://cr.openjdk.java.net/%7Emartin/webrevs/openjdk9/PKsig/>
>> https://bugs.openjdk.java.net/browse/JDK-8074579
>>
> Looks fine, assume the statement of "byte-byte-byte comparing is more
> efficient" is true
> In most cases, the signature comparing is just a "confirm", should not
> miss though.
>

Reading the whole signature with one read might be more efficient if
unaligned 4-byte reads were available and being used (which they're not).
IIRC we experimented with that on x86 but found no improvement (perhaps
because many fields are 2-byte).  Anyways, it should still be a win because
the individual bytes aren't being assembled/shifted into an int, and the
byte comparisons can use "immediate" instructions.

(a reason to worry just occurred to me - my change introduces more
branches, which might be slower when lacking branch prediction... hmmmm ...)

Ahhh, of course, we can remove the branches by doing:

#define PKZIP_SIGNATURE_AT(p, b2, b3) \
  ((((p)[0] - 'P') | ((p)[1] - 'K') | ((p)[2] - b2) | ((p)[3] - b3)) == 0)

Well, now that looks like much less of a win, performance wise... but still
worth doing.
We haz new webrev.