Remove usages of Heap::NewSpaceStart and its external reference

src/x64/macro-assembler-x64.cc

Index: src/x64/macro-assembler-x64.cc
diff --git a/src/x64/macro-assembler-x64.cc b/src/x64/macro-assembler-x64.cc
index 0c31809588befb995187f4fc66431a936e36dfa2..68c4ecabdc349f36dc0b4d75978e0cca16cb1646 100644
--- a/src/x64/macro-assembler-x64.cc
+++ b/src/x64/macro-assembler-x64.cc
@@ -5615,19 +5615,36 @@ void MacroAssembler::TestJSArrayForAllocationMemento(
     Register receiver_reg,
     Register scratch_reg,
     Label* no_memento_found) {
-  ExternalReference new_space_start =
-      ExternalReference::new_space_start(isolate());
+  Label map_check;
   ExternalReference new_space_allocation_top =
       ExternalReference::new_space_allocation_top_address(isolate());
-
-  leap(scratch_reg, Operand(receiver_reg,
-      JSArray::kSize + AllocationMemento::kSize - kHeapObjectTag));
-  Move(kScratchRegister, new_space_start);
-  cmpp(scratch_reg, kScratchRegister);
-  j(less, no_memento_found);
+  const int kMementoMapOffset = JSArray::kSize - kHeapObjectTag;
+  const int kMementoEndOffset = kMementoMapOffset + AllocationMemento::kSize;
+
+  // Bail out if the object sits on the page boundary as no memento can follow
+  // and we cannot touch the memory following it.
+  leap(scratch_reg, Operand(receiver_reg, kMementoEndOffset));
+  xorp(scratch_reg, receiver_reg);
+  cmpp(scratch_reg, Immediate(Page::kPageSize));
+  j(greater, no_memento_found);
+  // Bail out if the object is not in new space.
+  leap(scratch_reg, Operand(receiver_reg, kMementoEndOffset));
+  JumpIfNotInNewSpace(scratch_reg, scratch_reg, no_memento_found);
+  // If the object is in new space, we need to check whether it is on the same
+  // page as the current top.
+  leap(scratch_reg, Operand(receiver_reg, kMementoEndOffset));
+  xorp(scratch_reg, ExternalOperand(new_space_allocation_top));
+  testp(scratch_reg, Immediate(~Page::kPageAlignmentMask));
+  // If top is not on the same page as the current object, we can just continue
+  // with the map check.
+  j(not_zero, &map_check);
+  // Otherwise, we have to check whether we are still below top, to ensure that
+  // we are not checking against a stale memento.
+  leap(scratch_reg, Operand(receiver_reg, kMementoEndOffset));
   cmpp(scratch_reg, ExternalOperand(new_space_allocation_top));
   j(greater, no_memento_found);
-  CompareRoot(MemOperand(scratch_reg, -AllocationMemento::kSize),
+  bind(&map_check);
+  CompareRoot(MemOperand(receiver_reg, kMementoMapOffset),
               Heap::kAllocationMementoMapRootIndex);
 }