Cache PolymorphicSerializer in compiler-generated serializer

[qwwdfsad]

Currently, for the serializable class with polymorphic property, e.g.

@Serializable
data class PolymorphicWrapper(val i: @Polymorphic Poly)

nested write$self contains the following snippet:

output.encodeSerializableElement(serialDesc, 0, new PolymorphicSerializer(Reflection.getOrCreateKotlinClass(PurePolymorphismOverheadBenchmark.Poly.class)), self.i);

The problem is that PolymorphicSerializer instantiated every time deserialize is invoked instead of being cached in an outer field.

It leads to a serious performance impact because PolymorphicSerializer has non-trivial cost in its constructor: descriptor instantiation is a dominating factor in a polymorphic benchmark, see screenshot attached:

[qwwdfsad]

I believe this is third or fourth time we have a problem with non-cacheable serializer. @sandwwraith maybe it's time to revisit internal code-gen and just prohibit any non-cacheable serializers on encode/decode code paths, as profiling and eliminating each individual one is quite cumbersome.

Additionally, we'll reduce number of allocations this way, which usually worth it

[sandwwraith]

I think we have some caching by default, we just need to find out lay outs

[shanshin]

Expected in 1.8.20