RFR: 8261137: Optimization of Box nodes in uncommon_trap [v2]

[Vladimir Kozlov]

On Fri, 5 Feb 2021 07:29:00 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.
>
> Wang Huang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   refactor codes

I am a little concern about stretching uses of input value outside scope where it is created (for example, loop's variable or a value depending on it).
This optimization may work only because boxed values are immutable.
I will run our tests with this changes.

src/hotspot/share/opto/callGenerator.cpp line 582:

> 580:         Node* uncommon_trap_node = delay_boxes.pop();
> 581:         int in_edge = uncommon_trap_node->find_edge(res);
> 582:         assert(in_edge > 0, "sanity");

If there are several references you need to replace all of them.

src/hotspot/share/opto/callGenerator.cpp line 591:

> 589:         Node* sobj = new SafePointScalarObjectNode(gvn.type(res)->isa_oopptr(),
> 590: #ifdef ASSERT
> 591:                                                   NULL,

I would suggest to record `call` node here treating it as allocation.

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