large longs to string

[Brett Okken]

Is there interest in optimizing StringLatin1.getChars(long, int, byte[])
for large (larger than int) long values[1]?
We can change this to work with 8 digits at a time, which reduces the
amount of 64 bit arithmetic required.

if (i <= -1_000_000_000) {

long q = i / 100_000_000;

charPos -= 8;

write4DigitPairs(buf, charPos, (int) ((q * 100_000_000) - i));

i = q;

if (i <= -1_000_000_000) {

q = i / 100_000_000;

charPos -= 8;

write4DigitPairs(buf, charPos, (int) ((q * 100_000_000) - i));

i = q;

}

}


A simple implementation of write4DigitPairs would just call the existing
writeDigitPair method 4 times:


private static void write4DigitPairs(byte[] buf, int idx, int value) {

int v = value;

int v2 = v / 100;

writeDigitPair(buf, idx + 6, v - (v2 * 100));

v = v2;


v2 = v / 100;

writeDigitPair(buf, idx + 4, v - (v2 * 100));

v = v2;


v2 = v / 100;

writeDigitPair(buf, idx + 2, v - (v2 * 100));

v = v2;


v2 = v / 100;

writeDigitPair(buf, idx, v - (v2 * 100));

}

There is the option to OR the 4 short values together into a long and
leverage a ByteArrayLittleEndian.setLong call, but I see that the previous
usage of ByteArrayLittleEndian.setShort was removed[2].

A small benchmark of longs which would qualify shows up to 20% improvement.

Presumably a similar change could make sense for StringUTF16, but I have
not spent any time benchmarking it.

Brett

[1] -
https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/lang/StringLatin1.java#L163-L168
[2] -
https://github.com/openjdk/jdk/commit/913e43fea995b746fb9e1b25587d254396c7c3c9
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