String encoding in json.c

[Philipp Stephani <p.stephani2@gmail.com>]

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Hi,

I've benchmarked serialization and parsing of JSON with and without
explicit encoding. I've found that leaving out the coding makes both
operations significantly faster – from a speedup of a factor of 1.11 ± 0.06
for parsing canada.json to 1.57 ± 0.08 for serializing twitter.json. Other
speedups are in between, but the speedup is always significant (to at least
one standard deviation). All unit tests pass when leaving out the coding
steps – which isn't surprising given that currently the coding operations
are expensive no-ops. Therefore I'd suggest to document the internal string
encoding in lisp.h or character.h and remove the explicit coding in json.c
and emacs-module.c. It's very unlikely that the internal string encoding
will change frequently, and if so, the unit tests should catch potential
issues caused by that.

Philipp

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