Fix JSON formatting when converting to trace event format

[dberris]

LGTM

llvm/test/tools/llvm-xray/X86/convert-fdr-to-traceevent.txt

+; RUN: llvm-xray convert %S/Inputs/fdr-log-version-1.xray -f=trace_event -o - \
+; RUN:   | %python -c 'import json, sys; json.dump(json.loads(sys.stdin.read()), sys.stdout, sort_keys=True, indent=2)' \
+; RUN:   | FileCheck %s
+
+; CHECK: "traceEvents": [
+; Check fields for an event are present
+; CHECK: "name":
+; CHECK-NEXT: "ph":
+; CHECK-NEXT: "pid":
+; CHECK-NEXT: "sf":
+; CHECK-NEXT: "tid":
+; CHECK-NEXT: "ts":

llvm/test/tools/llvm-xray/X86/convert-traceevent-special-events.txt

+# RUN: llvm-xray convert %s -f=trace_event -o - \
+# RUN:   | %python -c 'import json, sys; json.dump(json.loads(sys.stdin.read()), sys.stdout, sort_keys=True, indent=2)' \
+# RUN:   | FileCheck %s
+---
+header:
+  version: 1
+  type: 0
+  constant-tsc: true
+  nonstop-tsc: true
+  cycle-frequency: 2601000000
+records:
+  - { type: 0, func-id: 1, cpu: 1, thread: 111, kind: function-enter, tsc: 10001 }
+  - { type: 0, func-id: 1, function: 'x', cpu: 1, thread: 111, process: 111, kind: custom-event, tsc: 2000, data: "\x03\0\0\0" }
+  - { type: 0, func-id: 1, cpu: 1, thread: 111, kind: function-exit, tsc: 10100 }
+...
+
+# CHECK: "traceEvents": [
+# Check fields for an event are present
+# CHECK: "name":
+# CHECK-NEXT: "ph":
+# CHECK-NEXT: "pid":
+# CHECK-NEXT: "sf":
+# CHECK-NEXT: "tid":
+# CHECK-NEXT: "ts":

llvm/tools/llvm-xray/xray-converter.cpp

 
 void TraceConverter::exportAsChromeTraceEventFormat(const Trace &Records,
                                                     raw_ostream &OS) {
   const auto &FH = Records.getFileHeader();
   auto Version = FH.Version;
   auto CycleFreq = FH.CycleFrequency;
 
   unsigned id_counter = 0;
+  int NumOutputRecords = 0;
 
-  OS << "{\n  \"traceEvents\": [";
+  OS << "{\n  \"traceEvents\": [\n";
   DenseMap<uint32_t, StackTrieNode *> StackCursorByThreadId{};
   DenseMap<uint32_t, SmallVector<StackTrieNode *, 4>> StackRootsByThreadId{};
   DenseMap<unsigned, StackTrieNode *> StacksByStackId{};
   std::forward_list<StackTrieNode> NodeStore{};
-  int loop_count = 0;
   for (const auto &R : Records) {
-    if (loop_count++ == 0)
-      OS << "\n";
-    else
-      OS << ",\n";
-
     // Chrome trace event format always wants data in micros.
     // CyclesPerMicro = CycleHertz / 10^6
     // TSC / CyclesPerMicro == TSC * 10^6 / CycleHertz == MicroTimestamp
     // Could lose some precision here by converting the TSC to a double to
     // multiply by the period in micros. 52 bit mantissa is a good start though.
     // TODO: Make feature request to Chrome Trace viewer to accept ticks and a
     // frequency or do some more involved calculation to avoid dangers of
     // conversion.
     double EventTimestampUs = double(1000000) / CycleFreq * double(R.TSC);
     StackTrieNode *&StackCursor = StackCursorByThreadId[R.TId];
     switch (R.Type) {
     case RecordTypes::CUSTOM_EVENT:
     case RecordTypes::TYPED_EVENT:
       // TODO: Support typed and custom event rendering on Chrome Trace Viewer.
       break;
     case RecordTypes::ENTER:
     case RecordTypes::ENTER_ARG:
       StackCursor = findOrCreateStackNode(StackCursor, R.FuncId, R.TId,
                                           StackRootsByThreadId, StacksByStackId,
                                           &id_counter, NodeStore);
       // Each record is represented as a json dictionary with function name,
       // type of B for begin or E for end, thread id, process id,
       // timestamp in microseconds, and a stack frame id. The ids are logged
       // in an id dictionary after the events.
+      if (NumOutputRecords++ > 0) {
+        OS << ",\n";
+      }
       writeTraceViewerRecord(Version, OS, R.FuncId, R.TId, R.PId, Symbolize,
                              FuncIdHelper, EventTimestampUs, *StackCursor, "B");
       break;
     case RecordTypes::EXIT:
     case RecordTypes::TAIL_EXIT:
       // No entries to record end for.
       if (StackCursor == nullptr)
         break;
       // Should we emit an END record anyway or account this condition?
       // (And/Or in loop termination below)
       StackTrieNode *PreviousCursor = nullptr;
       do {
-        if (PreviousCursor != nullptr) {
+        if (NumOutputRecords++ > 0) {
           OS << ",\n";
         }
         writeTraceViewerRecord(Version, OS, StackCursor->FuncId, R.TId, R.PId,
                                Symbolize, FuncIdHelper, EventTimestampUs,
                                *StackCursor, "E");
         PreviousCursor = StackCursor;
         StackCursor = StackCursor->Parent;
       } while (PreviousCursor->FuncId != R.FuncId && StackCursor != nullptr);