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

[Vladimir Ivanov]

On Fri, 5 Mar 2021 21:25:20 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

>> Wang Huang has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   fix bugs
>
> 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())

Another confusing aspect: what happens with `call` node after `replace_box_to_scalar` call is over?

My understanding is you rely on `replace_box_to_scalar` to eliminate all the uses by safepoints and then let  the following code to clean it up:
  bool result_not_used = (callprojs.resproj == NULL || callprojs.resproj->outcnt() == 0);
  if (is_pure_call() && result_not_used) {
    // The call is marked as pure (no important side effects), but result isn't used.
    // It's safe to remove the call.
    GraphKit kit(call->jvms());
    kit.replace_call(call, C->top(), true);

The relation between `is_pure_call()` and `C->eliminate_boxing()`/`call->as_CallStaticJava()->is_boxing_method()` is not clear.

I suggest to refactor `is_pure_call()` and add a special case for `is_boxing_method()` to perform scalarization when there are only debug usages. 

Also, add an assert in/after `replace_box_to_scalar` ensuring there are no users left, so the call can go away.

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