[cfe-dev] Diagnostic IDs, parsing speed and how to generate lookup tables

[Manuel Klimek]

Hi clangers,

while doing some parsing speed investigation I noted that a lot of
diagnostics stuff is in the very hot path for parsing time. One thing that
fell out of that is Benjamin's recent patch that speeds up getting the
diagnostic info. But, there's more to be gotten - with a simple cache for
the diagnotic classes, I was able to get another 1.5% parsing speedup
(benchmarked over all google code).

Unfortunately the patch in its current state is not thread safe; the
obvious solution to that problem would be to generate the diagnostic class
table statically instead of writing to a cache at runtime.

This turns out to be surprisingly hard though - the diagnostic id start
values are defined in an enum, while the actual diagnostics come from the
.td files.

A first idea was that we might define the start values inside the .td
files, and create the enum from that, instead of the other way around.

That led to the question posed by Dmitri on irc why there are start ranges
in the first place - we could instead tablegen all diagnostics at once and
let the tablegen take care of generating the appropriate enum values for
the start of certain ranges.

So if anybody has better ideas for how to solve the lookup problem, or
knows why the diagnostic ids have fixed start values, I'd be very
interested to learn more about it :)

Thanks!
/Manuel
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