[lworld] RFR: 8253113: [lworld] [lw3] C1 should avoid copying element of flattened arrays when reading a sub-element

Frederic Parain fparain at openjdk.java.net
Mon Sep 14 14:01:50 UTC 2020

Please review these changes optimizing access to flattened arrays in C1.

The optimization avoids unnecessary copies of intermediate values when the code accesses a sub-element of a flattened

For instance, with the following code:

inline class Point { int x = 0, y = 0; }
void foo() {
    Point[] array = new Point[10];
    int i = array[1].x;

C1 used to create a new instance of Point, copy the content from the array, then reads the x field from this new
instance. With the optimization, C1 directly accesses the x field from the flattened array, and makes no heap

A simple benchmark on the "int i = array[1].x;" line gives these results:

Benchmark                          Mode  Samples  Score  Score error  Units
o.s.MyBenchmark.testArrayReads     avgt      200  4.250        0.011  ns/op

Benchmark                          Mode  Samples  Score  Score error  Units
o.s.MyBenchmark.testArrayReads     avgt      200  2.228        0.004  ns/op

Thank you,



Commit messages:
 - Remove debug message in unit test
 - Renaming and cleanup
 - Fix flattened sub-element access, add unit tests
 - Fix white space
 - Extend CFG format with information related to sub-element accesses
 - Remove debugging code
 - First implementation

Changes: https://git.openjdk.java.net/valhalla/pull/187/files
 Webrev: https://webrevs.openjdk.java.net/?repo=valhalla&pr=187&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253113
  Stats: 361 lines in 7 files changed: 316 ins; 2 del; 43 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/187.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/187/head:pull/187

PR: https://git.openjdk.java.net/valhalla/pull/187

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