RFR: 8273585: String.charAt performance degrades due to JDK-8268698 [v2]

[Yi Yang]

> String.charAt shows significant performance regression due to [JDK-8268698](https://bugs.openjdk.java.net/browse/JDK-8268698), which replaces index bound checking with Preconditions.checkIndex intrinsic method.
> 
> The result of "time linux-x86_64-server-release/images/jdk/bin/java Test":
> 
> 
> Before JDK-8268698
>     real 0m8.369s
>     user 0m8.386s
>     sys 0m0.019s
> 
> After JDK-8268698,
>     real 0m19.722s
>     user 0m19.748s
>     sys 0m0.013s
> 
> 
> The reason is Preconditions.checkIndex generates a CastII for index node as index is now known to be >= 0 and < length.:
> 
> https://github.com/openjdk/jdk/blob/5dab76b939e381312ce5c89b9aebca628238a387/src/hotspot/share/opto/library_call.cpp#L1077-L1083
> 
> CastII can not be recognized as a parallel induction variable because AddNode's input must be the PhiNode:
> 
> https://github.com/openjdk/jdk/blob/5dab76b939e381312ce5c89b9aebca628238a387/src/hotspot/share/opto/loopnode.cpp#L3177-L3184
> 
> It seems this prevents further loop unrolling. I think we can relax this constraint, i.e CastII can be the input of AddNode if its input is PhiNode. After applying this patch, performance regression disappears:
> 
> 
> $time ./test.sh 
> 
> real    0m9.514s
> user    0m10.310s
> sys     0m0.155s
> 
> This is likely the reason for [JDK-8272493](https://bugs.openjdk.java.net/browse/JDK-8272493). Please help review it. Thanks!

Yi Yang has updated the pull request incrementally with one additional commit since the last revision:

  use uncast

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6096/files
  - new: https://git.openjdk.java.net/jdk/pull/6096/files/8029f673..992906df

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6096&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6096&range=00-01

  Stats: 11 lines in 1 file changed: 1 ins; 9 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6096.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6096/head:pull/6096

PR: https://git.openjdk.java.net/jdk/pull/6096