Add faster tensor rename if dimension after rename are in the same order

1 files changed, 17 insertions, 0 deletions

diff --git a/vespajlib/src/main/java/com/yahoo/tensor/functions/Rename.java b/vespajlib/src/main/java/com/yahoo/tensor/functions/Rename.java
index 53d774de329..e18af235d59 100644
--- a/vespajlib/src/main/java/com/yahoo/tensor/functions/Rename.java
+++ b/vespajlib/src/main/java/com/yahoo/tensor/functions/Rename.java
@@ -95,6 +95,11 @@ public class Rename extends PrimitiveTensorFunction {
             toIndexes[i] = renamedType.indexOfDimension(newDimensionName).get();
         }
 
+        // avoid building a new tensor if dimensions can simply be renamed
+        if (simpleRenameIsPossible(toIndexes)) {
+            return tensor.withType(renamedType);
+        }
+
         Tensor.Builder builder = Tensor.Builder.of(renamedType);
         for (Iterator<Tensor.Cell> i = tensor.cellIterator(); i.hasNext(); ) {
             Map.Entry<TensorAddress, Double> cell = i.next();
@@ -104,6 +109,18 @@ public class Rename extends PrimitiveTensorFunction {
         return builder.build();
     }
 
+    /**
+     * If none of the dimensions change order after rename we can do a simple rename.
+     */
+    private boolean simpleRenameIsPossible(int[] toIndexes) {
+        for (int i = 0; i < toIndexes.length; ++i) {
+            if (toIndexes[i] != i) {
+                return false;
+            }
+        }
+        return true;
+    }
+
     private TensorAddress rename(TensorAddress address, int[] toIndexes) {
         String[] reorderedLabels = new String[toIndexes.length];
         for (int i = 0; i < toIndexes.length; i++)