Roll back middle atttempt at micro-optimizing singletonIterator.

The overall impact of my optimization attempts is unclear, so I'm going to put us back in the state we were in 6 months ago: That old implementation used slightly more memory than the later ones, and it can't be any faster now than it was then, but at least we know that it won't be any slower. PiperOrigin-RevId: 630373651
google · May 3, 2024 · addb4a8 · addb4a8
1 parent 2f4154d
commit addb4a8
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Showing 2 changed files with 24 additions and 26 deletions.
diff --git a/android/guava/src/com/google/common/collect/Iterators.java b/android/guava/src/com/google/common/collect/Iterators.java
@@ -1105,31 +1105,30 @@ protected T get(int index) {
 
   private static final class SingletonIterator<T extends @Nullable Object>
       extends UnmodifiableIterator<T> {
-    private @Nullable Object valueOrThis;
+    private static final Object SENTINEL = new Object();
+
+    private @Nullable Object valueOrSentinel;
 
     SingletonIterator(T value) {
-      this.valueOrThis = value;
+      this.valueOrSentinel = value;
     }
 
     @Override
     public boolean hasNext() {
-      return valueOrThis != this;
+      return valueOrSentinel != SENTINEL;
     }
 
     @Override
     @ParametricNullness
     public T next() {
-      Object result = valueOrThis;
-      valueOrThis = this;
-      // We put the common case first, even though it's unlikely to matter if the code is run much:
-      // https://shipilev.net/jvm/anatomy-quarks/28-frequency-based-code-layout/
-      if (result != this) {
-        // The field held either a `T` or `this`, and it turned out not to be `this`.
-        @SuppressWarnings("unchecked")
-        T t = (T) result;
-        return t;
+      if (valueOrSentinel == SENTINEL) {
+        throw new NoSuchElementException();
       }
-      throw new NoSuchElementException();
+      // The field held either a T or SENTINEL, and it turned out not to be SENTINEL.
+      @SuppressWarnings("unchecked")
+      T t = (T) valueOrSentinel;
+      valueOrSentinel = SENTINEL;
+      return t;
     }
   }
 

diff --git a/guava/src/com/google/common/collect/Iterators.java b/guava/src/com/google/common/collect/Iterators.java
@@ -1105,31 +1105,30 @@ protected T get(int index) {
 
   private static final class SingletonIterator<T extends @Nullable Object>
       extends UnmodifiableIterator<T> {
-    private @Nullable Object valueOrThis;
+    private static final Object SENTINEL = new Object();
+
+    private @Nullable Object valueOrSentinel;
 
     SingletonIterator(T value) {
-      this.valueOrThis = value;
+      this.valueOrSentinel = value;
     }
 
     @Override
     public boolean hasNext() {
-      return valueOrThis != this;
+      return valueOrSentinel != SENTINEL;
     }
 
     @Override
     @ParametricNullness
     public T next() {
-      Object result = valueOrThis;
-      valueOrThis = this;
-      // We put the common case first, even though it's unlikely to matter if the code is run much:
-      // https://shipilev.net/jvm/anatomy-quarks/28-frequency-based-code-layout/
-      if (result != this) {
-        // The field held either a `T` or `this`, and it turned out not to be `this`.
-        @SuppressWarnings("unchecked")
-        T t = (T) result;
-        return t;
+      if (valueOrSentinel == SENTINEL) {
+        throw new NoSuchElementException();
       }
-      throw new NoSuchElementException();
+      // The field held either a T or SENTINEL, and it turned out not to be SENTINEL.
+      @SuppressWarnings("unchecked")
+      T t = (T) valueOrSentinel;
+      valueOrSentinel = SENTINEL;
+      return t;
     }
   }