[PATCH] D39086: Performance tracing facility for clangd.

[Sam McCall via Phabricator via cfe-commits]

sammccall created this revision.
Herald added a subscriber: mgorny.

This lets you visualize clangd's activity on different threads over time,
and understand critical paths of requests and object lifetimes.
The data produced can be visualized in Chrome (at chrome://tracing), or
in a standalone copy of catapult (http://github.com/catapult-project/catapult)

This patch consists of:

- a command line flag "-trace" that causes clangd to emit JSON trace data
- an API (in Trace.h) allowing clangd code to easily add events to the stream
- several initial uses of this API to capture JSON-RPC requests, builds, logs

Example result: https://photos.app.goo.gl/12L9swaz5REGQ1rm1

Caveats:

- JSON serialization is ad-hoc (isn't it everywhere?) so the API is limited to naming events rather than attaching arbitrary metadata. I'd like to fix this (I think we could use a JSON-object abstraction).
- The recording is very naive: events are written immediately by locking a mutex. Contention on the mutex might disturb performance.
- For now it just traces instants or spans on the current thread. There are other things that make sense to show (cross-thread flows, non-thread resources such as ASTs). But we have to start somewhere.


https://reviews.llvm.org/D39086

Files:
  clangd/CMakeLists.txt
  clangd/ClangdServer.cpp
  clangd/ClangdUnit.cpp
  clangd/JSONRPCDispatcher.cpp
  clangd/ProtocolHandlers.cpp
  clangd/Trace.cpp
  clangd/Trace.h
  clangd/tool/ClangdMain.cpp
  test/clangd/trace.test
  unittests/clangd/CMakeLists.txt
  unittests/clangd/TraceTests.cpp

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