VM(CoreLib): Work around JIT optimization issue revealed by cec963f028198ec5871840491ba78b096ad0b819

runtime/lib/growable_array.dart

Index: runtime/lib/growable_array.dart
diff --git a/runtime/lib/growable_array.dart b/runtime/lib/growable_array.dart
index 463214700e356ba0810b9d7ba4294226272cf31c..917950aac726a8987eab184e38e4506ee6e857a5 100644
--- a/runtime/lib/growable_array.dart
+++ b/runtime/lib/growable_array.dart
@@ -234,16 +234,27 @@ class _GrowableList<T> extends ListBase<T> {
 
   void _grow(int new_capacity) {
     var newData = _allocateData(new_capacity);
-    for (int i = 0; i < length; i++) {
-      newData[i] = this[i];
+    // This is a work-around for dartbug.com/30090: array-bound-check
+    // generalization causes excessive deoptimizations because it
+    // hoists CheckArrayBound(i, ...) out of the loop below and turns it
+    // into CheckArrayBound(length - 1, ...). Which deoptimizes
+    // if length == 0. However the loop itself does not execute
+    // if length == 0.
+    if (length > 0) {
+      for (int i = 0; i < length; i++) {
+        newData[i] = this[i];
+      }
     }
     _setData(newData);
   }
 
   void _shrink(int new_capacity, int new_length) {
     var newData = _allocateData(new_capacity);
-    for (int i = 0; i < new_length; i++) {
-      newData[i] = this[i];
+    // This is a work-around for dartbug.com/30090. See the comment in _grow.
+    if (new_length > 0) {
+      for (int i = 0; i < new_length; i++) {
+        newData[i] = this[i];
+      }
     }
     _setData(newData);
   }