[stubs] Fix allocation memento detection.

src/arm/macro-assembler-arm.cc

Index: src/arm/macro-assembler-arm.cc
diff --git a/src/arm/macro-assembler-arm.cc b/src/arm/macro-assembler-arm.cc
index 8976d9fde1af877654e9fa21452c1da934702dff..9afb7604af3743d79c3dca24647074ee6b4c13e0 100644
--- a/src/arm/macro-assembler-arm.cc
+++ b/src/arm/macro-assembler-arm.cc
@@ -3804,13 +3804,14 @@ void MacroAssembler::TestJSArrayForAllocationMemento(
   ExternalReference new_space_allocation_top_adr =
       ExternalReference::new_space_allocation_top_address(isolate());
   const int kMementoMapOffset = JSArray::kSize - kHeapObjectTag;
-  const int kMementoEndOffset = kMementoMapOffset + AllocationMemento::kSize;
+  const int kMementoLastWordOffset =
+      kMementoMapOffset + AllocationMemento::kSize - kPointerSize;
 
   // Bail out if the object is not in new space.
   JumpIfNotInNewSpace(receiver_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.
-  add(scratch_reg, receiver_reg, Operand(kMementoEndOffset));
+  add(scratch_reg, receiver_reg, Operand(kMementoLastWordOffset));
   mov(ip, Operand(new_space_allocation_top_adr));
   ldr(ip, MemOperand(ip));
   eor(scratch_reg, scratch_reg, Operand(ip));
@@ -3819,7 +3820,7 @@ void MacroAssembler::TestJSArrayForAllocationMemento(
   // The object is on a different page than allocation top. Bail out if the
   // object sits on the page boundary as no memento can follow and we cannot
   // touch the memory following it.
-  add(scratch_reg, receiver_reg, Operand(kMementoEndOffset));
+  add(scratch_reg, receiver_reg, Operand(kMementoLastWordOffset));
   eor(scratch_reg, scratch_reg, Operand(receiver_reg));
   tst(scratch_reg, Operand(~Page::kPageAlignmentMask));
   b(ne, no_memento_found);
@@ -3828,11 +3829,11 @@ void MacroAssembler::TestJSArrayForAllocationMemento(
   // If top is on the same page as the current object, we need to check whether
   // we are below top.
   bind(&top_check);
-  add(scratch_reg, receiver_reg, Operand(kMementoEndOffset));
+  add(scratch_reg, receiver_reg, Operand(kMementoLastWordOffset));
   mov(ip, Operand(new_space_allocation_top_adr));
   ldr(ip, MemOperand(ip));
   cmp(scratch_reg, ip);
-  b(gt, no_memento_found);
+  b(ge, no_memento_found);
   // Memento map check.
   bind(&map_check);
   ldr(scratch_reg, MemOperand(receiver_reg, kMementoMapOffset));