Simplify tagged event handling.

Don't make copies of Events on replay. The same events may be replayed a lot,
so it's better to copy before storing the events. Also avoid a copy if the
tag doesn't actually change.

The intent of this change is to reduce gc churn on incremental builds. When
I wrote this change (~a year ago), this was a noticable source of gc churn in
some benchmarks I ran at the time.

PiperOrigin-RevId: 177450696

3 files changed, 25 insertions, 22 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/events/Event.java b/src/main/java/com/google/devtools/build/lib/events/Event.java
index 61db7dcad9..5bf5b1dd05 100644
--- a/src/main/java/com/google/devtools/build/lib/events/Event.java
+++ b/src/main/java/com/google/devtools/build/lib/events/Event.java
@@ -66,6 +66,9 @@ public final class Event implements Serializable {
   }
 
   public Event withTag(String tag) {
+    if (Objects.equals(tag, this.tag)) {
+      return this;
+    }
     if (this.message != null) {
       return new Event(this.kind, this.location, this.message, tag);
     } else {
diff --git a/src/main/java/com/google/devtools/build/skyframe/ParallelEvaluatorContext.java b/src/main/java/com/google/devtools/build/skyframe/ParallelEvaluatorContext.java
index 840b5801e4..6860433c01 100644
--- a/src/main/java/com/google/devtools/build/skyframe/ParallelEvaluatorContext.java
+++ b/src/main/java/com/google/devtools/build/skyframe/ParallelEvaluatorContext.java
@@ -231,9 +231,8 @@ class ParallelEvaluatorContext {
 
     @Override
     public void accept(TaggedEvents events) {
-      String tag = events.getTag();
       for (Event e : events.getEvents()) {
-        reporter.handle(e.withTag(tag));
+        reporter.handle(e);
       }
     }
   }
diff --git a/src/main/java/com/google/devtools/build/skyframe/TaggedEvents.java b/src/main/java/com/google/devtools/build/skyframe/TaggedEvents.java
index bf5fc69798..0d3e422b65 100644
--- a/src/main/java/com/google/devtools/build/skyframe/TaggedEvents.java
+++ b/src/main/java/com/google/devtools/build/skyframe/TaggedEvents.java
@@ -13,10 +13,10 @@
 // limitations under the License.
 package com.google.devtools.build.skyframe;
 
+import com.google.common.collect.Collections2;
 import com.google.common.collect.ImmutableCollection;
-import com.google.common.collect.Iterables;
+import com.google.common.collect.ImmutableList;
 import com.google.devtools.build.lib.events.Event;
-
 import java.io.Serializable;
 import java.util.Objects;
 import javax.annotation.Nullable;
@@ -33,23 +33,21 @@ import javax.annotation.concurrent.Immutable;
  */
 @Immutable
 public final class TaggedEvents implements Serializable {
+  private final ImmutableList<Event> events;
+  private final int hashCode;
 
-  @Nullable
-  private final String tag;
-  private final ImmutableCollection<Event> events;
-
-  TaggedEvents(@Nullable String tag, ImmutableCollection<Event> events) {
-
-    this.tag = tag;
-    this.events = events;
+  TaggedEvents(final @Nullable String tag, ImmutableCollection<Event> events) {
+    this.events =
+        events.isEmpty()
+            ? ImmutableList.of()
+            : ImmutableList.copyOf(
+                Collections2.transform(
+                    events,
+                    (Event e) -> e.withTag(tag)));
+    this.hashCode = events.hashCode();
   }
 
-  @Nullable
-  String getTag() {
-    return tag;
-  }
-
-  ImmutableCollection<Event> getEvents() {
+  ImmutableList<Event> getEvents() {
     return events;
   }
 
@@ -59,20 +57,23 @@ public final class TaggedEvents implements Serializable {
    */
   @Override
   public String toString() {
-    return tag == null ? "<unknown>" : tag + ": " + Iterables.toString(events);
+    return events.toString();
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(tag, events);
+    return hashCode;
   }
 
   @Override
   public boolean equals(Object other) {
-    if (other == null || !other.getClass().equals(getClass())) {
+    if (this == other) {
+      return true;
+    }
+    if (!(other instanceof TaggedEvents)) {
       return false;
     }
     TaggedEvents that = (TaggedEvents) other;
-    return Objects.equals(this.tag, that.tag) && Objects.equals(this.events, that.events);
+    return (hashCode == that.hashCode) && Objects.equals(this.events, that.events);
   }
 }