RFR: 8278831: Use table lookup for the last two bytes in Integer.getChars

[Claes Redestad]

During TemplatedStrings work Jim has noticed that we could probably profit from reusing the lookup tables also for the 1 or 2 leftmost bytes:

        // We know there are at most two digits left at this point.
        buf[--charPos] = DigitOnes[-i];
        if (i < -9) {
            buf[--charPos] = DigitTens[-i];
        }

This avoids some arithmetic, and on average achieves a small speed-up since the lookup tables are likely cached are likely to be in cache. Small improvements on a few affected microbenchmarks, including the new Integers.toStringTiny, can be observed. 

Baseline:

Benchmark              (size)  Mode  Cnt   Score   Error  Units
Integers.toStringTiny     500  avgt   50  21.862 ± 0.211  us/op


Patch:

Benchmark              (size)  Mode  Cnt   Score   Error  Units
Integers.toStringTiny     500  avgt   50  20.619 ± 0.330  us/op

-------------

Commit messages:
 - 8278831: Use table lookup for the last two bytes in Integer.getChars

Changes: https://git.openjdk.java.net/jdk/pull/6854/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6854&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278831
  Stats: 42 lines in 4 files changed: 11 ins; 16 del; 15 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6854.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6854/head:pull/6854

PR: https://git.openjdk.java.net/jdk/pull/6854