[runtime/heap] Introduce CopyFixedArrayUpTo to match CopyFixedArrayAndGrow, copying to a smaller ar…

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index 713cda82f57b786da7ac63195b81969cd715cbc9..b373bb5ef01f7453f575dc4c91805abd1c5dce9c 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -3781,18 +3781,41 @@ AllocationResult Heap::CopyFixedArrayAndGrow(FixedArray* src, int grow_by,
     AllocationResult allocation = AllocateRawFixedArray(new_len, pretenure);
     if (!allocation.To(&obj)) return allocation;
   }
+
   obj->set_map_no_write_barrier(fixed_array_map());
   FixedArray* result = FixedArray::cast(obj);
   result->set_length(new_len);
 
   // Copy the content.
   DisallowHeapAllocation no_gc;
-  WriteBarrierMode mode = result->GetWriteBarrierMode(no_gc);
+  WriteBarrierMode mode = obj->GetWriteBarrierMode(no_gc);
   for (int i = 0; i < old_len; i++) result->set(i, src->get(i), mode);
   MemsetPointer(result->data_start() + old_len, undefined_value(), grow_by);
   return result;
 }
 
+AllocationResult Heap::CopyFixedArrayUpTo(FixedArray* src, int new_len,
+                                          PretenureFlag pretenure) {
+  if (new_len == 0) return empty_fixed_array();
+
+  DCHECK_LE(new_len, src->length());
+
+  HeapObject* obj = nullptr;
+  {
+    AllocationResult allocation = AllocateRawFixedArray(new_len, pretenure);
+    if (!allocation.To(&obj)) return allocation;
+  }
+  obj->set_map_no_write_barrier(fixed_array_map());
+
+  FixedArray* result = FixedArray::cast(obj);
+  result->set_length(new_len);
+
+  // Copy the content.
+  DisallowHeapAllocation no_gc;
+  WriteBarrierMode mode = result->GetWriteBarrierMode(no_gc);
+  for (int i = 0; i < new_len; i++) result->set(i, src->get(i), mode);
+  return result;
+}
 
 AllocationResult Heap::CopyFixedArrayWithMap(FixedArray* src, Map* map) {
   int len = src->length();
@@ -3801,13 +3824,12 @@ AllocationResult Heap::CopyFixedArrayWithMap(FixedArray* src, Map* map) {
     AllocationResult allocation = AllocateRawFixedArray(len, NOT_TENURED);
     if (!allocation.To(&obj)) return allocation;
   }
+  obj->set_map_no_write_barrier(map);
   if (InNewSpace(obj)) {
-    obj->set_map_no_write_barrier(map);
     CopyBlock(obj->address() + kPointerSize, src->address() + kPointerSize,
               FixedArray::SizeFor(len) - kPointerSize);
     return obj;
   }
-  obj->set_map_no_write_barrier(map);
   FixedArray* result = FixedArray::cast(obj);
   result->set_length(len);