Re: [PATCH] Optimization of AbstractStringBuilder.ensureCapacityInternal()

[Claes Redestad]

Hi,

interesting - do you have any numbers showing a benefit from doing this
(on various sets of input)?

My concerns are that count might typically be close enough to value.length
to make the difference small in practice, and that there are some (fragile)
JIT optimizations to try and avoid zeroing out the newly allocated array
that we have to take care not to break.

/Claes

On 2018-02-22 17:51, Roman Leventov wrote:

AbstractStringBuilder.ensureCapacityInternal() doesn't need to copy the
whole underlying array, only the part until the current count.

diff --git
a/src/java.base/share/classes/java/lang/AbstractStringBuilder.java
b/src/java.base/share/classes/java/lang/AbstractStringBuilder.java
--- a/src/java.base/share/classes/java/lang/AbstractStringBuilder.java
+++ b/src/java.base/share/classes/java/lang/AbstractStringBuilder.java
@@ -140,11 +140,12 @@
       * overflow, this method throws {@code OutOfMemoryError}.
       */
      private void ensureCapacityInternal(int minimumCapacity) {
+        byte[] oldValue = value;
          // overflow-conscious code
-        int oldCapacity = value.length >> coder;
+        int oldCapacity = oldValue.length >> coder;
          if (minimumCapacity - oldCapacity > 0) {
-            value = Arrays.copyOf(value,
-                    newCapacity(minimumCapacity) << coder);
+            value = new byte[newCapacity(minimumCapacity) << coder];
+            System.arraycopy(oldValue, 0, value, 0, count << coder);
          }
      }