Performance tracing facility for clangd.

Summary:
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.

Reviewers: ioeric, ilya-biryukov

Subscribers: cfe-commits, mgorny

Differential Revision: https://reviews.llvm.org/D39086

1 files changed, 1 insertions, 0 deletions

diff --git a/clang-tools-extra/unittests/clangd/CMakeLists.txt b/clang-tools-extra/unittests/clangd/CMakeLists.txt
index 38ae2803fd2..f45bc712381 100644
--- a/clang-tools-extra/unittests/clangd/CMakeLists.txt
+++ b/clang-tools-extra/unittests/clangd/CMakeLists.txt
@@ -10,6 +10,7 @@ include_directories(
 
 add_extra_unittest(ClangdTests
   ClangdTests.cpp
+  TraceTests.cpp
   )
 
 target_link_libraries(ClangdTests