Consider fast-path deserialization logic in JSON source generator

[layomia]

In #51945, we addressed a mode in the JSON source generator that generates optimized serialization logic using Utf8JsonWriter directly. We should consider a similar mode for deserialization using Utf8JsonReader directly.

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Issue Details

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In #51945, we addressed a mode in the JSON source generator that generates optimized serialization logic using Utf8JsonWriter directly. We should consider a similar mode for deserialization using Utf8JsonReader directly.

Author:	layomia
Assignees:	layomia
Labels:	area-System.Text.Json, tenet-performance
Milestone:	7.0.0

[layomia]

From @mrange in #57117:

Hi.
I was asked if I had any further feedback on the JSON Serializer in ticket: #56995

One thing that came to mind is that in my experiments generating a deserialize method can lead to performance improvements.

I suspect you considered it and for some reason not implemented it but in case you are interested my testing shows an increase in performance with ~30% for a simple object when using generated code rather than metadata driven deserializer. In addition it seems to have positive impact on the memory aspect.

In case you are interested here is my experiment: https://github.com/mrange/T4JsonSerializer

[mrange]

Could be worth investigating IMHO but I am sure there subtleties to the deserialization problem that makes it hard to create a one size fits all generated version. Perhaps there is value to generate a version that works in many use cases but also allow the metadata driven version when flexibility is required.

From my testing the overhead when using a static deserializer fell significantly but as the JSON parsing process itself takes sometime the overall performance gain is not as good.