Bug 1517409 - (part 4) Add a postCapacity measurement r=jonco

Alternative version of part 4 which does not add new telemetry.

--HG--
extra : rebase_source : fa733a8d1394e1eca4c9f43e0c8ee1c250e736a3
extra : histedit_source : fa838e0cdbaf9c287a9997a2d85f2aa773920987%2Ccbc0208c43057dc5f8424281d90a08d9265d9fbe

js/src/gc/Statistics.cpp

@@ -606,7 +606,7 @@ UniqueChars Statistics::renderJsonMessage(uint64_t timestamp, Statistics::JSONUs
 
   json.beginObject();
   json.property("status", "completed");    // JSON Key #1
-  formatJsonDescription(timestamp, json);  // #2-22
+  formatJsonDescription(timestamp, json, use);  // #2-22
 
   if (use == Statistics::JSONUse::TELEMETRY) {
     json.beginListProperty("slices_list");  // #23
@@ -626,7 +626,8 @@ UniqueChars Statistics::renderJsonMessage(uint64_t timestamp, Statistics::JSONUs
 }
 
 void Statistics::formatJsonDescription(uint64_t timestamp,
-                                       JSONPrinter& json) const {
+                                       JSONPrinter& json,
+                                       JSONUse use) const {
   // If you change JSON properties here, please update:
   // Telemetry ping code:
   //   toolkit/components/telemetry/other/GCTelemetry.jsm
@@ -676,6 +677,10 @@ void Statistics::formatJsonDescription(uint64_t timestamp,
                   ExplainAbortReason(nonincrementalReason_));  // #16
   }
   json.property("allocated_bytes", preHeapSize);  // #17
+  if (use == Statistics::JSONUse::PROFILER) {
+    json.property("post_heap_size", postHeapSize);
+  }
+
   uint32_t addedChunks = getCount(COUNT_NEW_CHUNK);
   if (addedChunks) {
     json.property("added_chunks", addedChunks);  // #18
@@ -742,6 +747,7 @@ Statistics::Statistics(JSRuntime* rt)
       nonincrementalReason_(gc::AbortReason::None),
       allocsSinceMinorGC({0, 0}),
       preHeapSize(0),
+      postHeapSize(0),
       thresholdTriggered(false),
       triggerAmount(0.0),
       triggerThreshold(0.0),
@@ -978,6 +984,7 @@ void Statistics::beginGC(JSGCInvocationKind kind) {
 void Statistics::endGC() {
   TimeDuration sccTotal, sccLongest;
   sccDurations(&sccTotal, &sccLongest);
+  postHeapSize = runtime->gc.heapSize.gcBytes();
 
   runtime->addTelemetry(JS_TELEMETRY_GC_IS_ZONE_GC,
                         !zoneStats.isFullCollection());

js/src/gc/Statistics.h

@@ -292,7 +292,7 @@ struct Statistics {
   };
 
   // Return JSON for a whole major GC.  If use == PROFILER then
-  // detailed per-slice data will be included.
+  // detailed per-slice data and some other fields will be included.
   UniqueChars renderJsonMessage(uint64_t timestamp, JSONUse use) const;
 
   // Return JSON for the timings of just the given slice.
@@ -360,8 +360,9 @@ struct Statistics {
     uint32_t tenured;
   } allocsSinceMinorGC;
 
-  /* Heap size before the GC started. */
+  /* Heap size before and after the GC ran. */
   size_t preHeapSize;
+  size_t postHeapSize;
 
   /* If the GC was triggered by exceeding some threshold, record the
    * threshold and the value that exceeded it. */
@@ -443,7 +444,7 @@ struct Statistics {
   UniqueChars formatDetailedPhaseTimes(const PhaseTimeTable& phaseTimes) const;
   UniqueChars formatDetailedTotals() const;
 
-  void formatJsonDescription(uint64_t timestamp, JSONPrinter&) const;
+  void formatJsonDescription(uint64_t timestamp, JSONPrinter&, JSONUse) const;
   void formatJsonSliceDescription(unsigned i, const SliceData& slice,
                                   JSONPrinter&) const;
   void formatJsonPhaseTimes(const PhaseTimeTable& phaseTimes,