RFR: 8273585: String.charAt performance degrades due to JDK-8268698
Yi Yang
yyang at openjdk.java.net
Mon Oct 25 05:59:17 UTC 2021
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!
-------------
Commit messages:
- 8273585: String.charAt performance degrades due to JDK-8268698
Changes: https://git.openjdk.java.net/jdk/pull/6096/files
Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6096&range=00
Issue: https://bugs.openjdk.java.net/browse/JDK-8273585
Stats: 11 lines in 1 file changed: 9 ins; 1 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
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