Converting a Go string to a string suitable use a specialized function,
UTF16Encode, that can encode the string directly to a malloc'ed buffer. That
way, only two copies are made when strings are passed from Go to Java; once
for UTF-8 to UTF-16 encoding and once for the creation of the Java String.
This CL implements the same optimization in the other direction, with a
UTF-16 to UTF-8 decoder implemented in C. Unfortunately, while calling into a
Go decoder also saves the extra copy, the Cgo overhead makes the calls much
slower for short strings.
To alleviate the risk of introducing decoding bugs, I've added the tests from
the encoding/utf16 package to SeqTest.
As a sideeffect, both Java and ObjC now always copy strings, regardless of
the argument mode. The cpy argument can therefore be removed from the string
conversion functions. Furthermore, the modeRetained and modeReturned modes
can be collapsed into just one.
While we're here, delete a leftover function from seq/strings.go that
wasn't removed when the old seq buffers went away.
Benchmarks, as compared with benchstat over 5 runs:
name old time/op new time/op delta
JavaStringShort 11.4µs ±13% 11.6µs ± 4% ~ (p=0.859 n=10+5)
JavaStringShortDirect 19.5µs ± 9% 20.3µs ± 2% +3.68% (p=0.019 n=9+5)
JavaStringLong 103µs ± 8% 24µs ± 4% -77.13% (p=0.001 n=9+5)
JavaStringLongDirect 113µs ± 9% 32µs ± 7% -71.63% (p=0.001 n=9+5)
JavaStringShortUnicode 11.1µs ±16% 10.7µs ± 5% ~ (p=0.190 n=9+5)
JavaStringShortUnicodeDirect 19.6µs ± 7% 20.2µs ± 1% +2.78% (p=0.029 n=9+5)
JavaStringLongUnicode 97.1µs ± 9% 28.0µs ± 5% -71.17% (p=0.001 n=9+5)
JavaStringLongUnicodeDirect 105µs ±10% 34µs ± 5% -67.23% (p=0.002 n=8+5)
JavaStringRetShort 14.2µs ± 2% 13.9µs ± 1% -2.15% (p=0.006 n=8+5)
JavaStringRetShortDirect 20.8µs ± 2% 20.4µs ± 2% ~ (p=0.065 n=8+5)
JavaStringRetLong 42.2µs ± 9% 42.4µs ± 3% ~ (p=0.190 n=9+5)
JavaStringRetLongDirect 51.2µs ±21% 50.8µs ± 8% ~ (p=0.518 n=9+5)
GoStringShort 23.4µs ± 7% 22.5µs ± 3% -3.55% (p=0.019 n=9+5)
GoStringLong 51.9µs ± 9% 53.1µs ± 3% ~ (p=0.240 n=9+5)
GoStringShortUnicode 24.2µs ± 6% 22.8µs ± 1% -5.54% (p=0.002 n=9+5)
GoStringLongUnicode 58.6µs ± 8% 57.6µs ± 3% ~ (p=0.518 n=9+5)
GoStringRetShort 27.6µs ± 1% 23.2µs ± 2% -15.87% (p=0.003 n=7+5)
GoStringRetLong 129µs ±12% 33µs ± 2% -74.03% (p=0.001 n=10+5)
Change-Id: Icb9481981493ffca8defed9fb80a9433d6048937
Reviewed-on: https://go-review.googlesource.com/20250
Reviewed-by: David Crawshaw <crawshaw@golang.org>
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