RFR: 8246338: Reduce overhead of normalizing file paths

[Claes Redestad]

Hi,

this patch simplifies and improves performance of
UnixFileSystem.normalize and prefixLength, which are used when opening
files. The Windows implementation is left untouched, since it's a bit
more complicated.

Bug:    https://bugs.openjdk.java.net/browse/JDK-8246338
Webrev: http://cr.openjdk.java.net/~redestad/8246338/open.00/

On the provided microbenchmark, we see a small (5-10%) speed-up
in peak performance, while the speed-up is quite large when
interpreting, especially in the case where the file path is already
normalized (2.1x)[1]. Since this is the normal case, the patch has a
small but measurable effect on some startup tests.

Testing: tier1-2

Thanks!

/Claes

[1]
Before:
Benchmark               Mode  Cnt  Score   Error  Units
FileOpen.mix            avgt   10  0.246 ± 0.005  us/op
FileOpen.normalized     avgt   10  0.022 ± 0.001  us/op
FileOpen.notNormalized  avgt   10  0.080 ± 0.002  us/op
FileOpen.trailingSlash  avgt   10  0.084 ± 0.003  us/op

After:
Benchmark               Mode  Cnt  Score   Error  Units
FileOpen.mix            avgt   10  0.237 ± 0.004  us/op
FileOpen.normalized     avgt   10  0.019 ± 0.001  us/op
FileOpen.notNormalized  avgt   10  0.077 ± 0.002  us/op
FileOpen.trailingSlash  avgt   10  0.078 ± 0.002  us/op


Before - -Xint:
Benchmark               Mode  Cnt   Score   Error  Units
FileOpen.mix            avgt   10  33.573 ± 0.348  us/op
FileOpen.normalized     avgt   10   6.219 ± 0.074  us/op
FileOpen.notNormalized  avgt   10  12.973 ± 0.084  us/op
FileOpen.trailingSlash  avgt   10  13.018 ± 0.066  us/op

After - -Xint:

Benchmark               Mode  Cnt   Score   Error  Units
FileOpen.mix            avgt   10  27.084 ± 0.554  us/op
FileOpen.normalized     avgt   10   2.954 ± 0.005  us/op
FileOpen.notNormalized  avgt   10  10.225 ± 0.061  us/op
FileOpen.trailingSlash  avgt   10  10.211 ± 0.075  us/op