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https://issues.apache.org/jira/browse/LUCENE-4889?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13615093#comment-13615093
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Dawid Weiss commented on LUCENE-4889:
-------------------------------------
Just to complete: didn't inspect jit assembly dumps, didn't check for dead code
elimination (although I don't think it should happen here because of how
Caliper is written).
> UnicodeUtil.codePointCount microbenchmarks (wtf)
> ------------------------------------------------
>
> Key: LUCENE-4889
> URL: https://issues.apache.org/jira/browse/LUCENE-4889
> Project: Lucene - Core
> Issue Type: Task
> Reporter: Dawid Weiss
> Assignee: Dawid Weiss
> Priority: Trivial
> Fix For: 5.0
>
>
> This is interesting. I posted a link to a state-machine-based UTF8
> parser/recognizer:
> http://bjoern.hoehrmann.de/utf-8/decoder/dfa/
> I spent some time thinking if the lookup table could be converted into a
> stateless computational function, which would avoid a table lookup (which in
> Java will cause an additional bounds check that will be hard to eliminate I
> think). This didn't turn out to be easy (it boils down to finding a simple
> function that would map a set of integers to its concrete permutation; a
> generalization of minimal perfect hashing).
> But out of curiosity I though it'd be fun to compare how Lucene's codepoint
> counting compares to Java's built-in one (Decoder) and a sequence of if's.
> I've put together a Caliper benchmark that processes 50 million unicode
> codepoints; one only ASCII, one Unicode. The results are interesting. On my
> win/I7:
> {code}
> implementation dataType ns linear runtime
> LUCENE UNICODE 167359502.6 ===============
> LUCENE ASCII 334015746.5 ==============================
> NOLOOKUP_SWITCH UNICODE 154294141.8 =============
> NOLOOKUP_SWITCH ASCII 119500892.8 ==========
> NOLOOKUP_IF UNICODE 90149072.6 ========
> NOLOOKUP_IF ASCII 29151411.4 ==
> {code}
> Disregard the switch lookup -- it's for fun only. But a sequence of if's is
> significantly faster than the current Lucene's table lookup, especially on
> ASCII input. And now compare this to Java's built-in decoder...
> {code}
> JAVA UNICODE 5753930.1 =
> JAVA ASCII 23.8 =
> {code}
> Yes, it's the same benchmark. Wtf? I realize buffers are partially native and
> probably so is utf8 decoder but by so much?! Again, to put it in context:
> {code}
> implementation dataType ns linear runtime
> LUCENE UNICODE 167359502.6 ===============
> LUCENE ASCII 334015746.5 ==============================
> JAVA UNICODE 5753930.1 =
> JAVA ASCII 23.8 =
> NOLOOKUP_IF UNICODE 90149072.6 ========
> NOLOOKUP_IF ASCII 29151411.4 ==
> NOLOOKUP_SWITCH UNICODE 154294141.8 =============
> NOLOOKUP_SWITCH ASCII 119500892.8 ==========
> {code}
> Wtf? The code is here if you want to experiment.
> https://github.com/dweiss/utf8dfa
> I realize the Java version needs to allocate a temporary space buffer but if
> these numbers hold for different VMs it may actually be worth it...
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