Remove over-zealous hole checking in Array.slice()

R=jkummerow@chromium.org
BUG=chromium:165637
TEST=regress-165637.js

Review URL: https://codereview.chromium.org/11442054

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13211 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/builtins.cc

@@ -858,23 +858,6 @@ BUILTIN(ArraySlice) {
   }
 
   JSObject* object = JSObject::cast(receiver);
-  ElementsKind kind = object->GetElementsKind();
-
-  if (IsHoleyElementsKind(kind)) {
-    bool packed = true;
-    ElementsAccessor* accessor = ElementsAccessor::ForKind(kind);
-    for (int i = 0; i < len; i++) {
-      if (!accessor->HasElement(object, object, i, elms)) {
-        packed = false;
-        break;
-      }
-    }
-    if (packed) {
-      kind = GetPackedElementsKind(kind);
-    } else if (!receiver->IsJSArray()) {
-      return CallJsBuiltin(isolate, "ArraySlice", args);
-    }
-  }
 
   ASSERT(len >= 0);
   int n_arguments = args.length() - 1;
@@ -924,6 +907,23 @@ BUILTIN(ArraySlice) {
   // Calculate the length of result array.
   int result_len = Max(final - k, 0);
 
+  ElementsKind kind = object->GetElementsKind();
+  if (IsHoleyElementsKind(kind)) {
+    bool packed = true;
+    ElementsAccessor* accessor = ElementsAccessor::ForKind(kind);
+    for (int i = k; i < final; i++) {
+      if (!accessor->HasElement(object, object, i, elms)) {
+        packed = false;
+        break;
+      }
+    }
+    if (packed) {
+      kind = GetPackedElementsKind(kind);
+    } else if (!receiver->IsJSArray()) {
+      return CallJsBuiltin(isolate, "ArraySlice", args);
+    }
+  }
+
   JSArray* result_array;
   MaybeObject* maybe_array = heap->AllocateJSArrayAndStorage(kind,
                                                              result_len,

test/mjsunit/regress/regress-165637.js

+// Copyright 2012 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
+//       disclaimer in the documentation and/or other materials provided
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+// Flags: --allow-natives-syntax
+
+// Should not take a very long time (n^2 algorithms are bad)
+
+function do_slices() {
+  var data = new Array(1024 * 12); // 12kB
+
+  for (var i = 0; i < data.length; i++) {
+    data[i] = 255;
+  }
+
+  var start = Date.now();
+
+  for (i = 0; i < 20000; i++) {
+    data.slice(4, 1);
+  }
+
+  return Date.now() - start;
+}
+
+// Should never take more than 3 seconds (if the bug is fixed, the test takes
+// considerably less time than 3 seconds).
+assertTrue(do_slices() < (3 * 1000));
+
+// Make sure that packed and unpacked array slices are still properly handled
+var holey_array = [1, 2, 3, 4, 5,,,,,,];
+assertFalse(%HasFastHoleyElements(holey_array.slice(6, 1)));
+assertEquals(undefined, holey_array.slice(6, 7)[0])
+assertFalse(%HasFastHoleyElements(holey_array.slice(2, 1)));
+assertEquals(3, holey_array.slice(2, 3)[0])