RFR: 8305995: Use full dominant search for regions

[Vladimir Kozlov]

On Thu, 13 Apr 2023 01:02:08 GMT, Kirill A. Korinsky <duke at openjdk.org> wrote:

> This is a fix for the regression introduced by da43cb5e463069cf4dafb262664f0d3d7c2e0eac in fix 8224957.
> 
> This regression was found while attempting to migrate an application from JDK 1.8 to JDK 17, by running internal benchmarks, and while investigating abnormal memory usage for about 4 times more from one of them.
> 
> The regression appears in the JMH benchmark, which builds a huge tree which contains boxed integers from 0 to a few thousand. A tree has very complex structure and the same objects are reused a lot.
> 
> When an `integer` is found it's collected as `Integer` and unboxed inside the collector callback.
> 
> This benchmark was run with `ParallelGC` on different JVMs: `JDK 1.8.0_362`, `JDK 11.0.18`, `JDK 13.0.13`, `JDK 15.0.9`, `JDK 17.0.6`, `JDK 19.0.2` and `JDK 20`. This allows to see that something has changed between 13 and 15, and that the memory footprint for this code has increased from `3152` to `11828` bytes per operation.
> 
> After that I've done a `git bisect` which allows me to locate the introducer.
> 
> So the current fix reduces the memory footprint on the local root 425ef0685c584abec80454fbcccdcc6db6558f93 to `2960` bytes per operation.

First, please update PR's title, it should match JBS's title.
Second, @catap you referenced JMH benchmark. Is it already in `test/micro/org/openjdk/bench/` or it is new? Please add it if it is new.
 
So before 8224957 code looks through in(1) for req() > 3 (and == 2). After 8224957 we bailout for >3 inputs.
This fix removed this restriction and we now go through all inputs for case > 3 inputs.
It may increase compilation time for such cases significantly but on other hand it may allow reduce IR.

I assume, based on description, x4 increase is in Java heap due to not unboxing some Java objects. Or is it memory consumed by C2 Jit compiler during benchmark execution?

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

PR Comment: https://git.openjdk.org/jdk/pull/13453#issuecomment-1508893706