perf(ts): simple monomorphic message parse

[AaronO]

Not fully monomorphic in the parse path, but we still see a ~10% throughput boost for this simple scoped change

Before

McapStreamReader
        4.47±0.09 op/s  Heap Used: 38.73±6.90 MB/op     Heap Total: 30.21±6.23 MB/op    ArrayBuffers: 105.31±12.03 MB/op

After

McapStreamReader
        4.88±0.10 op/s  Heap Used: 37.84±6.64 MB/op     Heap Total: 29.51±6.05 MB/op    ArrayBuffers: 106.73±12.33 MB/op

[defunctzombie]

What makes this monomorphic? What prevents "parse record" from doing similar checks to achieve the same results?

The name of the new function does not line up with the return variable names. Does the function return message records or chunk records? If chunk records then I think it should be named accordingly (if we can't make parseRecord dispatch better). A better comment for the function describing that behavior and when one might prefer it to parseRecord would help capture some of the learnings as well.

[AaronO]

What makes this monomorphic?

That its inputs & outputs are a single shape across calls.

What prevents "parse record" from doing similar checks to achieve the same results?

parseRecord by construction checks opcode and then dispatches to sub-parsers that parse different kinds of messages with varying shapes, it's "megamorphic"

[defunctzombie]

parseRecord by construction checks opcode and then dispatches to sub-parsers that parse different kinds of messages with varying shapes, it's "megamorphic"

And in our case the input is the same (a Reader) but it is the difference in output (aka return value) that causes deoptimization?

[AaronO]

parseRecord by construction checks opcode and then dispatches to sub-parsers that parse different kinds of messages with varying shapes, it's "megamorphic"

And in our case the input is the same (a Reader) but it is the difference in output (aka return value) that causes deoptimization?

Yes return values matter, outputs of one function are inputs to the other. You broadly want as much of your hot path call tree to be as "monomorphic" / specialized as possible.