Minimize ArrayList resizing in MutableList construction

Specialize the creation of internal contents if we have a collection and can
properly presize the ArrayList (this is almost always the case), and provide a
special case constructor for concat.

--
MOS_MIGRATED_REVID=129919134

1 files changed, 14 insertions, 4 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/syntax/SkylarkList.java b/src/main/java/com/google/devtools/build/lib/syntax/SkylarkList.java
index 51be3e9b2d..7098a1d790 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/SkylarkList.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/SkylarkList.java
@@ -20,14 +20,12 @@ import com.google.devtools.build.lib.events.Location;
 import com.google.devtools.build.lib.skylarkinterface.SkylarkModule;
 import com.google.devtools.build.lib.skylarkinterface.SkylarkModuleCategory;
 import com.google.devtools.build.lib.syntax.SkylarkMutable.MutableCollection;
-
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.List;
 import java.util.ListIterator;
 import java.util.RandomAccess;
-
 import javax.annotation.Nullable;
 
 /** A class to handle lists and tuples in Skylark. */
@@ -231,7 +229,7 @@ public abstract class SkylarkList<E> extends MutableCollection<E> implements Lis
      * @return a MutableList containing the elements
      */
     @SuppressWarnings("unchecked")
-    MutableList(Iterable<? extends E> contents, Mutability mutability) {
+    private MutableList(Iterable<? extends E> contents, Mutability mutability) {
       super();
       addAllUnsafe(contents);
       if (contents instanceof GlobList) {
@@ -240,6 +238,18 @@ public abstract class SkylarkList<E> extends MutableCollection<E> implements Lis
       this.mutability = mutability;
     }
 
+    /** Specialized constructor for concat. */
+    private MutableList(
+        MutableList<? extends E> lhs,
+        MutableList<? extends E> rhs,
+        @Nullable Environment env) {
+      super();
+      this.contents.ensureCapacity(lhs.size() + rhs.size());
+      this.contents.addAll(lhs);
+      this.contents.addAll(rhs);
+      this.mutability = env == null ? Mutability.IMMUTABLE : env.mutability();
+    }
+
     /**
      * Creates a MutableList from contents and an Environment.
      * @param contents the contents of the list
@@ -334,7 +344,7 @@ public abstract class SkylarkList<E> extends MutableCollection<E> implements Lis
         MutableList<? extends E> right,
         Environment env) {
       if (left.getGlobList() == null && right.getGlobList() == null) {
-        return new MutableList(Iterables.concat(left, right), env);
+        return new MutableList<>(left, right, env);
       }
       return new MutableList(GlobList.concat(
           left.getGlobListOrContentsUnsafe(), right.getGlobListOrContentsUnsafe()), env);