Reviewers: Mads Ager,
Description:
Trim fast elements tail on significant length decreases.
Runtime_RegExpExecMultiple had to be updated because it assumed
setting an array's length to zero still keeps some capacity in the
backing store.
[email protected]
Please review this at http://codereview.chromium.org/7237004/
SVN Base: https://v8.googlecode.com/svn/branches/bleeding_edge
Affected files:
M src/objects.cc
M src/runtime.cc
Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index
6c9f52b2df89839306e30b8f925f87412fc111f4..cff87bdb471395cf498e84246334fa7dda44c0cb
100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -7507,12 +7507,24 @@ MaybeObject* JSObject::SetElementsLength(Object*
len) {
{ MaybeObject* maybe_obj = EnsureWritableFastElements();
if (!maybe_obj->ToObject(&obj)) return maybe_obj;
}
- int old_length =
FastD2I(JSArray::cast(this)->length()->Number());
- // NOTE: We may be able to optimize this by removing the
- // last part of the elements backing storage array and
- // setting the capacity to the new size.
- for (int i = value; i < old_length; i++) {
- FixedArray::cast(elements())->set_the_hole(i);
+ FixedArray* fast_elements = FixedArray::cast(elements());
+ if (2 * value <= old_capacity) {
+ // If more than half the elements won't be used, trim the
array.
+ if (value == 0) {
+ initialize_elements();
+ } else {
+ fast_elements->set_length(value);
+ Address filler_start = fast_elements->address() +
+
FixedArray::OffsetOfElementAt(value);
+ int filler_size = (old_capacity - value) * kPointerSize;
+ GetHeap()->CreateFillerObjectAt(filler_start, filler_size);
+ }
+ } else {
+ // Otherwise, fill the unused tail with holes.
+ int old_length =
FastD2I(JSArray::cast(this)->length()->Number());
+ for (int i = value; i < old_length; i++) {
+ fast_elements->set_the_hole(i);
+ }
}
JSArray::cast(this)->set_length(Smi::cast(smi_length));
}
Index: src/runtime.cc
diff --git a/src/runtime.cc b/src/runtime.cc
index
32d25179233275c62ff07d5592d1dfabd2d75e77..f11d745d8f8fc258076653877dd57022b48d414d
100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -3532,7 +3532,8 @@ RUNTIME_FUNCTION(MaybeObject*,
Runtime_RegExpExecMultiple) {
if (result_array->HasFastElements()) {
result_elements =
Handle<FixedArray>(FixedArray::cast(result_array->elements()));
- } else {
+ }
+ if (result_elements.is_null() || result_elements->length() < 16) {
result_elements = isolate->factory()->NewFixedArrayWithHoles(16);
}
FixedArrayBuilder builder(result_elements);
--
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