Improve performance of absurdly large range()s

For instance, for a simple .bzl program that just iterates over a range
of size 200M, the total running time drops from about two minutes to half
a minute.

--
MOS_MIGRATED_REVID=127942625

1 files changed, 5 insertions, 1 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/syntax/MethodLibrary.java b/src/main/java/com/google/devtools/build/lib/syntax/MethodLibrary.java
index f02b58546e..a7d168dc87 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/MethodLibrary.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/MethodLibrary.java
@@ -2032,13 +2032,17 @@ public class MethodLibrary {
       if (step == 0) {
         throw new EvalException(loc, "step cannot be 0");
       }
-      List<Integer> result = Lists.newArrayList();
+      ArrayList<Integer> result = Lists.newArrayList();
       if (step > 0) {
+        int size = (stop - start) / step;
+        result.ensureCapacity(size);
         while (start < stop) {
           result.add(start);
           start += step;
         }
       } else {
+        int size = (start - stop) / step;
+        result.ensureCapacity(size);
         while (start > stop) {
           result.add(start);
           start += step;