[stubs] Fix allocation memento detection.

src/mips/macro-assembler-mips.cc

Index: src/mips/macro-assembler-mips.cc
diff --git a/src/mips/macro-assembler-mips.cc b/src/mips/macro-assembler-mips.cc
index 5c0b901c1f5f3740ceddacadd75147f4b8ce5a61..c3abe4fa6fe2f5405d00441331d075aedf770077 100644
--- a/src/mips/macro-assembler-mips.cc
+++ b/src/mips/macro-assembler-mips.cc
@@ -6560,13 +6560,14 @@ void MacroAssembler::TestJSArrayForAllocationMemento(Register receiver_reg,
   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.
-  Addu(scratch_reg, receiver_reg, Operand(kMementoEndOffset));
+  Addu(scratch_reg, receiver_reg, Operand(kMementoLastWordOffset));
   li(at, Operand(new_space_allocation_top_adr));
   lw(at, MemOperand(at));
   Xor(scratch_reg, scratch_reg, Operand(at));
@@ -6575,7 +6576,7 @@ void MacroAssembler::TestJSArrayForAllocationMemento(Register receiver_reg,
   // 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.
-  Addu(scratch_reg, receiver_reg, Operand(kMementoEndOffset));
+  Addu(scratch_reg, receiver_reg, Operand(kMementoLastWordOffset));
   Xor(scratch_reg, scratch_reg, Operand(receiver_reg));
   And(scratch_reg, scratch_reg, Operand(~Page::kPageAlignmentMask));
   Branch(no_memento_found, ne, scratch_reg, Operand(zero_reg));
@@ -6584,10 +6585,10 @@ void MacroAssembler::TestJSArrayForAllocationMemento(Register receiver_reg,
   // If top is on the same page as the current object, we need to check whether
   // we are below top.
   bind(&top_check);
-  Addu(scratch_reg, receiver_reg, Operand(kMementoEndOffset));
+  Addu(scratch_reg, receiver_reg, Operand(kMementoLastWordOffset));
   li(at, Operand(new_space_allocation_top_adr));
   lw(at, MemOperand(at));
-  Branch(no_memento_found, gt, scratch_reg, Operand(at));
+  Branch(no_memento_found, ge, scratch_reg, Operand(at));
   // Memento map check.
   bind(&map_check);
   lw(scratch_reg, MemOperand(receiver_reg, kMementoMapOffset));