encoding/json: do all reflect work before decoding

[josharian]

@crawshaw commented in CL 24472:

With more time and care, I believe more of the JSON decoder's work could be shifted so it is done before decoding, and independent of the number of bytes processed. Maybe someone could explore that for Go 1.8.

This issue is a reminder, because CLs get forgotten.

[josharian]

Hey @sctb, I promised to pester you when the 1.8 tree opened up again, and offer some more bad advice about what issues to work on. This one is pretty vague and likely to be a bit gnarly, so...a perfect terrible recommendation for you. :) Happy to suggest others as well, depending on what part of the world you care about this month...

[sctb]

@josharian Sounds just right, I'll take a look!

[mvdan]

I've closed the broad json performance issue #5683 in favor of more specific performance issues about the encoder and decoder. This is the obvious first choice - the decoder is way slower than the encoder, and reflection is the obvious culprit.

I imagine that doing this, especially for structs, would give a noticeable speed-up. I might give this a look during the current 1.12 cycle.

[mvdan]

I no longer think this should be the priority to speed up JSON decoding. I think we should consider #28923 first, since the scanner takes vastly more time than any decoding work involving reflection.

I still think we should do this at some point, though.