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Subject: Re: [boost] RFC: interest in Unicode codecs?
From: Phil Endecott (spam_from_boost_dev_at_[hidden])
Date: 2009-07-18 06:02:31

Cory Nelson wrote:
> I finally found some time to do some optimizations of my own and have
> had some good progress using a small lookup table, a switch, and
> slightly deducing branches. See line 318:
> Despite these efforts, Windows 7 still decodes UTF-8 three times
> faster (~750MiB/s vs ~240MiB/s on my Core 2. I assume they are either
> using some gigantic look up tables or SSE.

Hi Cory,

What is your test input?

When the input is largely ASCII, a worthwhile optimisation is to cast
groups of 4 (or 8) characters to ints and & with 0x80808080; if the
answer is zero, no further conversion is needed.

In general I'm unsure of the performance issues of lookup tables
compared to explicit bit-manipulation. Cache effects may be
significant, and a benchmark will tend to warm up the cache better than
a real application might.

I can't see how SSE could be applied to this problem, but it's not
something I know much about.

I don't have much time to work on this right now, but if the algorithm
plus test harness and test data were bundled up into something that I
can just "make", I will try to compare it with my version.

Regards, Phil.

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