[clangd-dev] Moving clangd/Trace over to LLVM

Ilya Biryukov via clangd-dev clangd-dev at lists.llvm.org
Mon Aug 13 08:05:47 PDT 2018

Hi Alex, Sam,

I have a few comments regarding the producer part of the API and how it
interacts with Context.

If I were to move the Tracing API out of clangd, I would probably decouple
it from the Context first and leave the Context private to clangd.
There are ways to do that, e.g. by 1) introducing explicit API to allow
keeping the traces alive after the corresponding RAII "Span" dies, 2) using
this API in clangd to propagate traces across thread boundaries using
clangd's Context.

Context has all the same problems that global variables have and I
personally find it's too easy to misuse (specifically since it propagates
across threads!).

On Sat, Aug 11, 2018 at 1:23 AM Sam McCall via clangd-dev <
clangd-dev at lists.llvm.org> wrote:

> Hey Alex,
> In principle no objections, but some design concerns/questions for a
> larger scope.
> - the global registration of the EventTracer is awkward and may not be the
> right design for all environments who might otherwise want to consume
> events. Didn't spend a lot of time digging for alternatives here since it
> was sufficient for clangd.
> - tracer depends on Context to handle cross-thread interactions, which is
> another possibly-controversial piece. (I'm more confident the design of
> Context is right though)
> - contexts need to be propagated across thread boundaries when work is
> transferred. Clangd's TUScheduler does this, but the wider LLVM universe
> does not. (More generally: these APIs may not be very useful if they're not
> used consistently)
> - both tracer and context propagation were designed as extension points
> for weird clangd embedders, and we have nontrivial uses of these (happy to
> give details). So if these APIs change a lot to make them more suitable for
> LLVM, we need to make sure they still work for those original purposes.
> On Fri, Aug 10, 2018, 22:20 Alex L via clangd-dev <
> clangd-dev at lists.llvm.org> wrote:
>> Hi,
>> I would like to move the Tracing support from Clangd over to LLVM, as I
>> would like to add some tracing support to Clang itself as well. Are there
>> any objections to this from the Clangd side? I will make a follow-up post
>> on llvm-dev with a patch if there are no objections.
>> Cheers,
>> Alex
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Ilya Biryukov
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