RFR: 8246451: Reduce overhead of normalizing file paths with trailing slash

[Claes Redestad]

Hi,

I'd like to fix a bug in the microbenchmark I pushed for JDK-8246338
where the FileOpen.trailingSlash variant doesn't do what it intends to.
There's also a profitable low-hanging optimization down that path to
substring rather than take the detour via StringBuilder[1]

Bug:    https://bugs.openjdk.java.net/browse/JDK-8246451
Webrev: http://cr.openjdk.java.net/~redestad/8246451/open.00/

Testing: tier1

Thanks!

/Claes

[1]
Benchmark                                                Mode  Cnt 
Score     Error   Units
FileOpen.trailingSlash                                   avgt    5 
0.054 ±   0.010   us/op
FileOpen.trailingSlash:·gc.alloc.rate.norm               avgt    5 
160.013 ±   0.003    B/op

FileOpen.trailingSlash                                   avgt    5 
0.036 ±   0.003   us/op
FileOpen.trailingSlash:·gc.alloc.rate.norm               avgt    5 
96.008 ±   0.002    B/op