Time profiler: optimize json output time

[anton-afanasyev]

Summary

Use llvm::json::Array.reserve() to optimize json output time. Here is motivation:
https://reviews.llvm.org/D60609#1468941. In short: for the json array
with ~32K entries, pushing back each entry takes ~4% of whole time compared
to the method of preliminary memory reservation: (3995-3845)/3995 = 3.75%.

[anton-afanasyev]

Updated, clang-format

[anton-afanasyev]

Updated

[lebedev.ri]

LG, thank you.

llvm/lib/Support/TimeProfiler.cpp

92 ↗

(On Diff #195441)

Just do

const size_t ExpectedEntryCount = Entries.size() + CountAndTotalPerName.size() + 1;
Events.reserve(ExpectedEntryCount);
...
assert(Events.size() == ExpectedEntryCount && "Size prediction failed");

else they could drift apart.

[anton-afanasyev]

llvm/lib/Support/TimeProfiler.cpp

92 ↗

(On Diff #195441)

Thanks, good point!

[anton-afanasyev]

Updated assertion

[anton-afanasyev]

Update

llvm/trunk/lib/Support/TimeProfiler.cpp

void end() {
     Stack.pop_back();
   }
 
   void Write(raw_pwrite_stream &OS) {
     assert(Stack.empty() &&
            "All profiler sections should be ended when calling Write");
 
     json::Array Events;
+    const size_t ExpectedEntryCount =
+        Entries.size() + CountAndTotalPerName.size() + 1;
+    Events.reserve(ExpectedEntryCount);
 
     // Emit all events for the main flame graph.
     for (const auto &E : Entries) {
       auto StartUs = duration_cast<microseconds>(E.Start - StartTime).count();
       auto DurUs = duration_cast<microseconds>(E.Duration).count();
 
       Events.emplace_back(json::Object{
           {"pid", 1},
Events.emplace_back(json::Object{
         {"pid", 1},
         {"tid", 0},
         {"ts", 0},
         {"ph", "M"},
         {"name", "process_name"},
         {"args", json::Object{{"name", "clang"}}},
     });
 
+    assert(Events.size() == ExpectedEntryCount && "Size prediction failed!");
+
     OS << formatv("{0:2}", json::Value(json::Object(
                                {{"traceEvents", std::move(Events)}})));
   }
 
   SmallVector<Entry, 16> Stack;
   SmallVector<Entry, 128> Entries;
   StringMap<CountAndDurationType> CountAndTotalPerName;
   time_point<steady_clock> StartTime;