RFR: 8339492: StackMapDecoder::writeFrames makes lots of allocations [v2]

[Chen Liang]

On Wed, 4 Sep 2024 14:12:06 GMT, David M. Lloyd <duke at openjdk.org> wrote:

>> LGTM
>> 
>> AFAICT before this we'd only call `DirectCodeBuilder::labelToBci` once per label, but now we'll do so roughly `2*n*log(n) + n` times. I would assume getting rid of the `TreeMap` and `Integer` key allocations more than makes up for this, though. Do we have any JMH tests where `writeFrames` is a significant contributor we could check?
>
>> LGTM
>> 
>> AFAICT before this we'd only call `DirectCodeBuilder::labelToBci` once per label, but now we'll do so roughly `2*n*log(n) + n` times. I would assume getting rid of the `TreeMap` and `Integer` key allocations more than makes up for this, though. Do we have any JMH tests where `writeFrames` is a significant contributor we could check?
> 
> My expectation was that the short path of `DirectCodeBuilder` (where `context == this` and thus we directly return `lab.getBCI()`, which is a simple field getter) would be used in most, if not all, cases. If so, then this amounts to a fairly minimal and direct code path, thus I approached this as being an "obvious" (as opposed to theoretical) improvement.

@dmlloyd I think we might need a benchmark that uses `ClassFile.of(StackMapsOption.DROP_STACK_MAPS)` to build a class with method with code, and supply the stack maps to generate. That is the best way to measure the gains from this patch. (Something like what we do in ProxyGenerator, but proxy generation has too many uncertainties)

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PR Comment: https://git.openjdk.org/jdk/pull/20841#issuecomment-2329244951