RFR: 8261137: Optimization of Box nodes in uncommon_trap [v6]

[Vladimir Ivanov]

On Sat, 27 Feb 2021 07:17:22 GMT, Wang Huang <whuang at openjdk.org> wrote:

>> JDK-8075052 has  removed useless autobox. However, in some cases, the box is still saved. For instance:
>> @Benchmark
>> public void testMethod(Blackhole bh) {
>>   int sum = 0;
>>   for (int i = 0; i < data.length; i++) {
>>       Integer ii = Integer.valueOf(data[i]);
>>       if (i < data.length) {
>>           sum += ii.intValue();
>>       }
>>   }
>>   bh.consume(sum);
>> }
>> Although the variable ii is only used at ii.intValue(), it cannot be eliminated as a result of being used by a hidden uncommon_trap.
>> The uncommon_trap is generated by the optimized "if", because its condition is always true.
>> 
>> We can postpone box in uncommon_trap in this situation. We treat box as a scalarized object by adding a SafePointScalarObjectNode in the uncommon_trap node,
>> and deleting the use of box:
>> 
>> There is no additional fail/error(s) of jtreg after this patch.
>> 
>> I adjust my codes and add a new benchmark
>> 
>> public class MyBenchmark {
>> 
>>     static int[] data = new int[10000];
>> 
>>     static {
>>         for(int i = 0; i < data.length; ++i) {
>>             data[i] = i * 1337 % 7331;
>>         }
>>     }
>> 
>>     @Benchmark
>>     public void testMethod(Blackhole bh) {
>>       int sum = 0;
>>       for (int i = 0; i < data.length; i++) {
>>           Integer ii = Integer.valueOf(data[i]);
>>           black();
>>           if (i < 100000) {
>>               sum += ii.intValue();
>>           }
>>       }
>>       bh.consume(sum);
>>     }
>> 
>>     public void black(){}
>> }
>> 
>> 
>> aarch64:  
>> base line：
>> Benchmark                     Mode  Samples   Score  Score error  Units
>> o.s.MyBenchmark.testMethod    avgt       30  88.513        1.111  us/op
>> 
>> opt：
>> Benchmark                     Mode  Samples   Score  Score error  Units
>> o.s.MyBenchmark.testMethod    avgt       30  52.776        0.096  us/op
>> 
>> x86:  
>> base line：
>> Benchmark                     Mode  Samples   Score  Score error  Units
>> o.s.MyBenchmark.testMethod    avgt       30  81.066        3.156  us/op
>> 
>> opt:
>> Benchmark                     Mode  Samples   Score  Score error  Units
>> o.s.MyBenchmark.testMethod    avgt       30  55.596        0.775  us/op
>
> Wang Huang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   fix bugs

test/hotspot/jtreg/compiler/c2/TestEliminateBoxInDebugInfo.java line 49:

> 47:             "-XX:CompileCommand=compileonly,compiler/c2/TestEliminateBoxInDebugInfo$Test.foo",
> 48:             "-XX:CompileCommand=dontinline,compiler/c2/TestEliminateBoxInDebugInfo$Test.black",
> 49:             "-XX:+PrintEliminateAllocations",

Add `-Xbatch` to ensure the compilation is complete by the time the test finishes.

test/hotspot/jtreg/compiler/c2/TestIdentityWithEliminateBoxInDebugInfo.java line 31:

> 29:  * @library /test/lib
> 30:  *
> 31:  * @run main/othervm compiler.c2.TestIdentityWithEliminateBoxInDebugInfo

Same here: add `-Xbatch`.

src/hotspot/share/opto/callGenerator.cpp line 659:

> 657: 
> 658:   if (C->eliminate_boxing()) {
> 659:     replace_box_to_scalar(call, callprojs.resproj);

I suggest to lift the checks from `replace_box_to_scalar` to here. Otherwise, it's confusing to apply the method to an arbitrary call.

if (resproj != nullptr && call->is_CallStaticJava() &&
       call->as_CallStaticJava()->is_boxing_method())

src/hotspot/share/opto/callGenerator.cpp line 573:

> 571: 
> 572: #ifndef PRODUCT
> 573:     if (PrintEliminateAllocations && safepoints.size() > 0) {

Strictly speaking, `PrintEliminateAllocations` is EA-specific flag.
Not sure it has to cover `EliminateAutoBox`/`AggressiveUnboxing`.

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

PR: https://git.openjdk.java.net/jdk/pull/2401