RFR: 8286104: use aggressive liveness for unstable_if traps [v11]

[Vladimir Kozlov]

On Sun, 5 Jun 2022 22:59:13 GMT, Xin Liu <xliu at openjdk.org> wrote:

>> I found that some phi nodes are useful because its uses are uncommon_trap nodes. In worse case, it would hinder boxing object eliminating and scalar replacement. Test.java of JDK-8286104 reveals this issue. This patch allows c2 parser to collect liveness based on next bci for unstable_if traps.  In most cases, next bci is closer to exits, so live locals are diminishing.  It helps to reduce the number of inputs of unstable_if traps. 
>> 
>> This is not a REDO of Optimization of Box nodes in uncommon_trap(JDK-8261137). Two patches are orthogonal.  I adapt test from [TestEliminateBoxInDebugInfo.java](https://github.com/openjdk/jdk/pull/2401/files#diff-49b2e38825aa4c28ca196bdc70c3cbecc2e835c2899f4f393527df4796b177ea), so part of credit goes to the original author.  I found that Scalar replacement can take care of the object `Integer ii = Integer.valueOf(value)` in original test even it can't be removed by later inliner.  I tweak the profiling data of Integer.valueOf() to hinder scalar replacement. 
>> 
>> This patch can cover the problem discovered by JDK-8261137. I ran the microbench and got 9x speedup on x86_64. It's almost same as JDK-8261137. 
>> 
>> Before:  
>> 
>> Benchmark               Mode  Cnt   Score   Error  Units
>> MyBenchmark.testMethod  avgt   10  32.776 ± 0.075  us/op
>> 
>> After: 
>> 
>> Benchmark               Mode  Cnt   Score   Error  Units
>> MyBenchmark.testMethod  avgt   10  3.656 ± 0.006  us/op
>>  ```
>> 
>> Testing
>> I ran tier1 test with and without `-XX:+DeoptimizeALot`. No regression has been found yet.
>
> Xin Liu has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Bail out if fold-compares sees that a unstable_if trap has modified.
>   
>   Also add a regression test

Update is good. I agree with avoiding folding if unc trap info was modified.
I have few comments.

src/hotspot/share/opto/compile.cpp line 1911:

> 1909: 
> 1910: void Compile::preprocess_unstable_if_traps() {
> 1911: #ifndef PRODUCT

You can use `#ifndef PRODUCT` around all this code and use `PRODUCT_RETURN` macro in header file:
`void preprocess_unstable_if_traps() PRODUCT_RETURN;` to het the same effect.

src/hotspot/share/opto/compile.cpp line 1952:

> 1950:       const MethodLivenessResult& live_locals = method->liveness_at_bci(next_bci);
> 1951:       assert(live_locals.is_valid(), "broken liveness info");
> 1952:       bool changed = false;

Rename `changed` -> `modified`

src/hotspot/share/opto/compile.cpp line 1961:

> 1959:           uint idx = jvms->locoff() + i;
> 1960: #ifndef PRODUCT
> 1961:           if (Verbose) {

`Verbose` is debug (develop) flag. Use `#ifdef ASSERT` here.

src/hotspot/share/opto/compile.hpp line 811:

> 809:                                              _dead_node_count = 0;
> 810:                                            }
> 811:   void         record_unstable_if(UnstableIfTrap* trap);

You missed to rename method to record_unstable_if_trap().
The placement of declaration is strange. Can you move to other `unstable_if_trap` declared methods?

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PR: https://git.openjdk.java.net/jdk/pull/8545