Intern empty Depsets

Empty `Depset`s are interned per order, which decreases allocations as well as retained heap when a `Depset` is stored in a `struct` in a provider (providers with schema already unwrap most `Depset`s).

Fixes #19380

Closes #19387.

PiperOrigin-RevId: 563104923
Change-Id: If66fb4a108ef7569a4f95e7af3a74ac84d1c4636

src/main/java/com/google/devtools/build/lib/collect/nestedset/Depset.java

@@ -102,7 +102,7 @@ public final class Depset implements StarlarkValue, Debug.ValueWithDebugAttribut
   @Nullable private final Class<?> elemClass;
   private final NestedSet<?> set;
 
-  private Depset(@Nullable Class<?> elemClass, NestedSet<?> set) {
+  Depset(@Nullable Class<?> elemClass, NestedSet<?> set) {
     this.elemClass = elemClass;
     this.set = set;
   }
@@ -145,9 +145,10 @@ private static void checkElement(Object x, boolean strict) throws EvalException
   // Object.class means "any Starlark value" but in fact allows any Java value.
   public static <T> Depset of(Class<T> elemClass, NestedSet<T> set) {
     Preconditions.checkNotNull(elemClass, "elemClass cannot be null");
-    // Having a shared EMPTY_DEPSET instance, could reduce working heap. Empty depsets are not
-    // retained though, because of optimization in StarlarkProvider.optimizeField.
-    return new Depset(set.isEmpty() ? null : ElementType.getTypeClass(elemClass), set);
+    if (set.isEmpty()) {
+      return set.getOrder().emptyDepset();
+    }
+    return new Depset(ElementType.getTypeClass(elemClass), set);
   }
 
   /**
@@ -259,15 +260,11 @@ public static <T> NestedSet<T> cast(Object x, Class<T> type, String what) throws
   public static <T> NestedSet<T> noneableCast(Object x, Class<T> type, String what)
       throws EvalException {
     if (x == Starlark.NONE) {
-      @SuppressWarnings("unchecked")
-      NestedSet<T> empty = (NestedSet<T>) EMPTY;
-      return empty;
+      return NestedSetBuilder.emptySet(Order.STABLE_ORDER);
     }
     return cast(x, type, what);
   }
 
-  private static final NestedSet<?> EMPTY = NestedSetBuilder.<Object>emptySet(Order.STABLE_ORDER);
-
   public boolean isEmpty() {
     return set.isEmpty();
   }
@@ -377,6 +374,9 @@ public static Depset fromDirectAndTransitive(
       }
     }
 
+    if (builder.isEmpty()) {
+      return builder.getOrder().emptyDepset();
+    }
     return new Depset(type, builder.build());
   }
 

src/main/java/com/google/devtools/build/lib/collect/nestedset/Order.java

@@ -112,10 +112,12 @@ public enum Order {
 
   private final String starlarkName;
   private final NestedSet<?> emptySet;
+  private final Depset emptyDepset;
 
-  private Order(String starlarkName) {
+  Order(String starlarkName) {
     this.starlarkName = starlarkName;
     this.emptySet = new NestedSet<>(this);
+    this.emptyDepset = new Depset(null, this.emptySet);
   }
 
   @SerializationConstant @AutoCodec.VisibleForSerialization
@@ -150,6 +152,11 @@ <E> NestedSet<E> emptySet() {
     return (NestedSet<E>) emptySet;
   }
 
+  /** Returns an empty depset of the given ordering. */
+  Depset emptyDepset() {
+    return emptyDepset;
+  }
+
   public String getStarlarkName() {
     return starlarkName;
   }

src/test/java/com/google/devtools/build/lib/collect/nestedset/DepsetTest.java

@@ -406,4 +406,17 @@ ev.new Scenario()
                 + " NoneType'",
             "depset(direct='hello')");
   }
+
+  @Test
+  public void testEmptyDepsetInternedPerOrder() throws Exception {
+    ev.exec(
+        "stable1 = depset()",
+        "stable2 = depset()",
+        "preorder1 = depset(order = 'preorder')",
+        "preorder2 = depset(order = 'preorder')");
+    assertThat(ev.lookup("stable1")).isSameInstanceAs(ev.lookup("stable2"));
+    assertThat(ev.lookup("preorder1")).isSameInstanceAs(ev.lookup("preorder2"));
+    assertThat(ev.lookup("stable1")).isNotSameInstanceAs(ev.lookup("preorder1"));
+    assertThat(ev.lookup("stable2")).isNotSameInstanceAs(ev.lookup("preorder2"));
+  }
 }