Move empty elements canonicalization from call sites of

AllocateUninitializedJSArrayWithElements into the method.

Prior to the change, if the caller forgets to handle empty case on
their side, AllocateUninitializedJSArrayWithElements would allocate a
new empty FixedArray rather than return the canonical one. This refactor
shifts the burden of canonicalization from the callers to
AllocateUninitializedJSArrayWithElements.


Bug: v8:6777
Change-Id: I1246cb288861b65b51938414a454f21af78f8399
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1480330
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Irina Yatsenko <irinayat@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#60015}

src/code-stub-assembler.cc

@@ -3822,6 +3822,30 @@ CodeStubAssembler::AllocateUninitializedJSArrayWithElements(
   CHECK_EQ(allocation_flags & ~kAllowLargeObjectAllocation, 0);
   CSA_SLOW_ASSERT(this, TaggedIsPositiveSmi(length));
 
+  TVARIABLE(JSArray, array);
+  TVARIABLE(FixedArrayBase, elements);
+
+  if (IsIntPtrOrSmiConstantZero(capacity, capacity_mode)) {
+    TNode<FixedArrayBase> empty_array = EmptyFixedArrayConstant();
+    array = AllocateJSArray(array_map, empty_array, length, allocation_site);
+    return {array.value(), empty_array};
+  }
+
+  Label out(this), empty(this), nonempty(this);
+
+  Branch(SmiEqual(ParameterToTagged(capacity, capacity_mode), SmiConstant(0)),
+         &empty, &nonempty);
+
+  BIND(&empty);
+  {
+    TNode<FixedArrayBase> empty_array = EmptyFixedArrayConstant();
+    array = AllocateJSArray(array_map, empty_array, length, allocation_site);
+    elements = empty_array;
+    Goto(&out);
+  }
+
+  BIND(&nonempty);
+  {
     int base_size = JSArray::kSize;
     if (allocation_site != nullptr) base_size += AllocationMemento::kSize;
 
@@ -3832,24 +3856,19 @@ CodeStubAssembler::AllocateUninitializedJSArrayWithElements(
     TNode<IntPtrT> size =
         ElementOffsetFromIndex(capacity, kind, capacity_mode, base_size);
 
-  TVARIABLE(JSArray, array);
-  TVARIABLE(FixedArrayBase, elements);
-
-  Label out(this);
-
     // For very large arrays in which the requested allocation exceeds the
     // maximal size of a regular heap object, we cannot use the allocation
     // folding trick. Instead, we first allocate the elements in large object
-  // space, and then allocate the JSArray (and possibly the allocation memento)
-  // in new space.
+    // space, and then allocate the JSArray (and possibly the allocation
+    // memento) in new space.
     if (allocation_flags & kAllowLargeObjectAllocation) {
       Label next(this);
       GotoIf(IsRegularHeapObjectSize(size), &next);
 
       CSA_CHECK(this, IsValidFastJSArrayCapacity(capacity, capacity_mode));
 
-    // Allocate and initialize the elements first. Full initialization is needed
-    // because the upcoming JSArray allocation could trigger GC.
+      // Allocate and initialize the elements first. Full initialization is
+      // needed because the upcoming JSArray allocation could trigger GC.
       elements =
           AllocateFixedArray(kind, capacity, capacity_mode, allocation_flags);
 
@@ -3862,8 +3881,8 @@ CodeStubAssembler::AllocateUninitializedJSArrayWithElements(
       }
 
       // The JSArray and possibly allocation memento next. Note that
-    // allocation_flags are *not* passed on here and the resulting JSArray will
-    // always be in new space.
+      // allocation_flags are *not* passed on here and the resulting JSArray
+      // will always be in new space.
       array =
           AllocateJSArray(array_map, elements.value(), length, allocation_site);
 
@@ -3894,6 +3913,7 @@ CodeStubAssembler::AllocateUninitializedJSArrayWithElements(
     StoreObjectFieldNoWriteBarrier(elements.value(), FixedArray::kLengthOffset,
                                   capacity_smi);
     Goto(&out);
+  }
 
   BIND(&out);
   return {array.value(), elements.value()};
@@ -3929,53 +3949,18 @@ TNode<JSArray> CodeStubAssembler::AllocateJSArray(
 
   TNode<JSArray> array;
   TNode<FixedArrayBase> elements;
-  int capacity_as_constant;
 
-  if (IsIntPtrOrSmiConstantZero(capacity, capacity_mode)) {
-    // Array is empty. Use the shared empty fixed array instead of allocating a
-    // new one.
-    TNode<FixedArrayBase> empty_fixed_array =
-        CAST(LoadRoot(RootIndex::kEmptyFixedArray));
-    array =
-        AllocateJSArray(array_map, empty_fixed_array, length, allocation_site);
-  } else if (TryGetIntPtrOrSmiConstantValue(capacity, &capacity_as_constant,
-                                            capacity_mode)) {
-    CHECK_GT(capacity_as_constant, 0);
-    // Allocate both array and elements object, and initialize the JSArray.
   std::tie(array, elements) = AllocateUninitializedJSArrayWithElements(
       kind, array_map, length, allocation_site, capacity, capacity_mode,
       allocation_flags);
-    // Fill in the elements with holes.
-    FillFixedArrayWithValue(kind, elements,
-                            IntPtrOrSmiConstant(0, capacity_mode), capacity,
-                            RootIndex::kTheHoleValue, capacity_mode);
-  } else {
-    Label out(this), empty(this), nonempty(this);
-    TVARIABLE(JSArray, var_array);
+
+  Label out(this), nonempty(this);
 
     Branch(SmiEqual(ParameterToTagged(capacity, capacity_mode), SmiConstant(0)),
-           &empty, &nonempty);
-
-    BIND(&empty);
-    {
-      // Array is empty. Use the shared empty fixed array instead of allocating
-      // a new one.
-      TNode<FixedArrayBase> empty_fixed_array =
-          CAST(LoadRoot(RootIndex::kEmptyFixedArray));
-      var_array = AllocateJSArray(array_map, empty_fixed_array, length,
-                                  allocation_site);
-      Goto(&out);
-    }
+          &out, &nonempty);
 
   BIND(&nonempty);
   {
-      // Allocate both array and elements object, and initialize the JSArray.
-      TNode<JSArray> array;
-      std::tie(array, elements) = AllocateUninitializedJSArrayWithElements(
-          kind, array_map, length, allocation_site, capacity, capacity_mode,
-          allocation_flags);
-      var_array = array;
-      // Fill in the elements with holes.
     FillFixedArrayWithValue(kind, elements,
                             IntPtrOrSmiConstant(0, capacity_mode), capacity,
                             RootIndex::kTheHoleValue, capacity_mode);
@@ -3983,9 +3968,6 @@ TNode<JSArray> CodeStubAssembler::AllocateJSArray(
   }
 
   BIND(&out);
-    array = var_array.value();
-  }
-
   return array;
 }
 

test/mjsunit/object-get-own-property-names.js

@@ -28,6 +28,9 @@
 // Test ES5 section 15.2.3.4 Object.getOwnPropertyNames.
 
 // Check simple cases.
+var obj = {};
+assertEquals(0, Object.getOwnPropertyNames(obj).length);
+
 var obj = { a: 1, b: 2};
 var propertyNames = Object.getOwnPropertyNames(obj);
 propertyNames.sort();
@@ -52,6 +55,13 @@ assertEquals("a", propertyNames[0]);
 assertEquals("c", propertyNames[1]);
 
 // Check that non-enumerable properties are being returned.
+var obj = {};
+Object.defineProperty(obj, 'x', {
+  value: 1,
+  enumerable: false
+});
+assertEquals(1, Object.getOwnPropertyNames(obj).length);
+
 var propertyNames = Object.getOwnPropertyNames([1, 2]);
 propertyNames.sort();
 assertEquals(3, propertyNames.length);

test/mjsunit/object-keys.js

@@ -4,6 +4,31 @@
 
 // Flags: --allow-natives-syntax
 
+// Ensure empty keys are handled properly
+(function() {
+  const a = {};
+  let k = Object.keys(a);
+  %HeapObjectVerify(k);
+  assertEquals(0, k.length);
+})();
+
+// Ensure non-enumerable keys are handled properly
+(function() {
+  const a = {};
+  Object.defineProperty(a, 'x', {
+    value: 1,
+    enumerable: false
+  });
+  let k = Object.keys(a);
+  %HeapObjectVerify(k);
+  assertEquals(0, k.length);
+
+  a.y = 2;
+  k = Object.keys(a);
+  %HeapObjectVerify(k);
+  assertEquals(1, k.length);
+})();
+
 // Ensure that mutation of the Object.keys result doesn't affect the
 // enumeration cache for fast-mode objects.
 (function() {