[array] Properly handle COW arrays in Array.p.reverse fast-path

Instead of using the slow-path for COW arrays, we now properly copy
them and use the fast-path.

R=jgruber@chromium.org

Change-Id: Iebbad5f761d97c5400c457877571c7930269d52f
Reviewed-on: https://chromium-review.googlesource.com/1188130
Commit-Queue: Simon Zünd <szuend@google.com>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55384}

src/builtins/array-reverse.tq

@@ -143,14 +143,25 @@ module array {
     return object;
   }
 
+  macro EnsureWriteableFastElements(array: JSArray) {
+    const elements: FixedArrayBase = array.elements;
+    if (elements.map != kCOWMap) return;
+
+    // There are no COW *_DOUBLE_ELEMENTS arrays, so we are allowed to always
+    // extract FixedArrays and don't have to worry about FixedDoubleArrays.
+    assert(IsFastSmiOrTaggedElementsKind(array.map.elements_kind));
+
+    const length: Smi = array.length_fast;
+    array.elements = ExtractFixedArray(
+        unsafe_cast<FixedArray>(elements), 0, length, length, kFixedArrays);
+  }
+
   macro TryFastPackedArrayReverse(receiver: Object) labels Slow {
     const array: JSArray = cast<JSArray>(receiver) otherwise Slow;
-    const map: Map = array.map;
-
-    if (!IsExtensibleMap(map)) goto Slow;
-    if (array.elements.map == kCOWMap) goto Slow;
+    EnsureWriteableFastElements(array);
+    assert(array.elements.map != kCOWMap);
 
-    const kind: ElementsKind = map.elements_kind;
+    const kind: ElementsKind = array.map.elements_kind;
     if (kind == PACKED_SMI_ELEMENTS) {
       FastPackedArrayReverse<FastPackedSmiElements, Smi>(
           array.elements, array.length_fast);

src/builtins/base.tq

@@ -109,6 +109,8 @@ type FixedBigInt64Array extends FixedTypedArray;
 
 const kAllFixedArrays: constexpr ExtractFixedArrayFlags generates
 'ExtractFixedArrayFlag::kAllFixedArrays';
+const kFixedArrays: constexpr ExtractFixedArrayFlags generates
+'ExtractFixedArrayFlag::kFixedArrays';
 
 const kFixedCOWArrayMapRootIndex: constexpr RootListIndex generates
 'Heap::kFixedCOWArrayMapRootIndex';