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//===--- Trace.cpp - Performance tracing facilities -----------------------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
#include "Trace.h"
#include "Context.h"
#include "Function.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/ScopeExit.h"
#include "llvm/Support/Chrono.h"
#include "llvm/Support/FormatProviders.h"
#include "llvm/Support/FormatVariadic.h"
#include "llvm/Support/Threading.h"
#include <atomic>
#include <mutex>
namespace clang {
namespace clangd {
namespace trace {
namespace {
// The current implementation is naive: each thread writes to Out guarded by Mu.
// Perhaps we should replace this by something that disturbs performance less.
class JSONTracer : public EventTracer {
JSONTracer(llvm::raw_ostream &OS, bool Pretty)
: Out(OS, Pretty ? 2 : 0), Start(std::chrono::system_clock::now()) {
// The displayTimeUnit must be ns to avoid low-precision overlap
// calculations!
Out.attribute("displayTimeUnit", "ns");
rawEvent("M", llvm::json::Object{
{"name", "process_name"},
{"args", llvm::json::Object{{"name", "clangd"}}},
~JSONTracer() {
// We stash a Span object in the context. It will record the start/end,
// and this also allows us to look up the parent Span's information.
Context beginSpan(llvm::StringRef Name, llvm::json::Object *Args) override {
return Context::current().derive(
SpanKey, llvm::make_unique<JSONSpan>(this, Name, Args));
// Trace viewer requires each thread to properly stack events.
// So we need to mark only duration that the span was active on the thread.
// (Hopefully any off-thread activity will be connected by a flow event).
// Record the end time here, but don't write the event: Args aren't ready yet.
void endSpan() override {
void instant(llvm::StringRef Name, llvm::json::Object &&Args) override {
llvm::json::Object{{"name", Name}, {"args", std::move(Args)}});
// Record an event on the current thread. ph, pid, tid, ts are set.
// Contents must be a list of the other JSON key/values.
void jsonEvent(llvm::StringRef Phase, llvm::json::Object &&Contents,
uint64_t TID = llvm::get_threadid(), double Timestamp = 0) {
Contents["ts"] = Timestamp ? Timestamp : timestamp();
Contents["tid"] = int64_t(TID);
std::lock_guard<std::mutex> Lock(Mu);
rawEvent(Phase, Contents);
class JSONSpan {
JSONSpan(JSONTracer *Tracer, llvm::StringRef Name, llvm::json::Object *Args)
: StartTime(Tracer->timestamp()), EndTime(0), Name(Name),
TID(llvm::get_threadid()), Tracer(Tracer), Args(Args) {
// ~JSONSpan() may run in a different thread, so we need to capture now.
// We don't record begin events here (and end events in the destructor)
// because B/E pairs have to appear in the right order, which is awkward.
// Instead we send the complete (X) event in the destructor.
// If our parent was on a different thread, add an arrow to this span.
auto *Parent = Context::current().get(SpanKey);
if (Parent && *Parent && (*Parent)->TID != TID) {
// If the parent span ended already, then show this as "following" it.
// Otherwise show us as "parallel".
double OriginTime = (*Parent)->EndTime;
if (!OriginTime)
OriginTime = (*Parent)->StartTime;
auto FlowID = nextID();
llvm::json::Object{{"id", FlowID},
{"name", "Context crosses threads"},
{"cat", "dummy"}},
(*Parent)->TID, (*Parent)->StartTime);
llvm::json::Object{{"id", FlowID},
{"bp", "e"},
{"name", "Context crosses threads"},
{"cat", "dummy"}},
~JSONSpan() {
// Finally, record the event (ending at EndTime, not timestamp())!
llvm::json::Object{{"name", std::move(Name)},
{"args", std::move(*Args)},
{"dur", EndTime - StartTime}},
TID, StartTime);
// May be called by any thread.
void markEnded() { EndTime = Tracer->timestamp(); }
static int64_t nextID() {
static std::atomic<int64_t> Next = {0};
return Next++;
double StartTime;
std::atomic<double> EndTime; // Filled in by markEnded().
std::string Name;
uint64_t TID;
JSONTracer *Tracer;
llvm::json::Object *Args;
static Key<std::unique_ptr<JSONSpan>> SpanKey;
// Record an event. ph and pid are set.
// Contents must be a list of the other JSON key/values.
void rawEvent(llvm::StringRef Phase,
const llvm::json::Object &Event) /*REQUIRES(Mu)*/ {
// PID 0 represents the clangd process.
Out.attribute("pid", 0);
Out.attribute("ph", Phase);
for (const auto& KV : Event)
Out.attribute(KV.first, KV.second);
// If we haven't already, emit metadata describing this thread.
void captureThreadMetadata() {
uint64_t TID = llvm::get_threadid();
std::lock_guard<std::mutex> Lock(Mu);
if (ThreadsWithMD.insert(TID).second) {
llvm::SmallString<32> Name;
if (!Name.empty()) {
rawEvent("M", llvm::json::Object{
{"tid", int64_t(TID)},
{"name", "thread_name"},
{"args", llvm::json::Object{{"name", Name}}},
double timestamp() {
using namespace std::chrono;
return duration<double, std::micro>(system_clock::now() - Start).count();
std::mutex Mu;
llvm::json::OStream Out /*GUARDED_BY(Mu)*/;
llvm::DenseSet<uint64_t> ThreadsWithMD /*GUARDED_BY(Mu)*/;
const llvm::sys::TimePoint<> Start;
Key<std::unique_ptr<JSONTracer::JSONSpan>> JSONTracer::SpanKey;
EventTracer *T = nullptr;
} // namespace
Session::Session(EventTracer &Tracer) {
assert(!T && "Resetting global tracer is not allowed.");
T = &Tracer;
Session::~Session() { T = nullptr; }
std::unique_ptr<EventTracer> createJSONTracer(llvm::raw_ostream &OS,
bool Pretty) {
return llvm::make_unique<JSONTracer>(OS, Pretty);
void log(const llvm::Twine &Message) {
if (!T)
T->instant("Log", llvm::json::Object{{"Message", Message.str()}});
// Returned context owns Args.
static Context makeSpanContext(llvm::Twine Name, llvm::json::Object *Args) {
if (!T)
return Context::current().clone();
WithContextValue WithArgs{std::unique_ptr<llvm::json::Object>(Args)};
return T->beginSpan(Name.isSingleStringRef() ? Name.getSingleStringRef()
: llvm::StringRef(Name.str()),
// Span keeps a non-owning pointer to the args, which is how users access them.
// The args are owned by the context though. They stick around until the
// beginSpan() context is destroyed, when the tracing engine will consume them.
Span::Span(llvm::Twine Name)
: Args(T ? new llvm::json::Object() : nullptr),
RestoreCtx(makeSpanContext(Name, Args)) {}
Span::~Span() {
if (T)
} // namespace trace
} // namespace clangd
} // namespace clang