Remove usages of Heap::NewSpaceStart and its external reference

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 02f7bee24811f9280148137a2df53782e9508855..a188cab3021a953a9b80870d4a9120a3c45dd504 100644
--- a/src/mips/macro-assembler-mips.cc
+++ b/src/mips/macro-assembler-mips.cc
@@ -6057,28 +6057,39 @@ void MacroAssembler::ClampDoubleToUint8(Register result_reg,
   bind(&done);
 }
 
-
-void MacroAssembler::TestJSArrayForAllocationMemento(
-    Register receiver_reg,
-    Register scratch_reg,
-    Label* no_memento_found,
-    Condition cond,
-    Label* allocation_memento_present) {
-  ExternalReference new_space_start =
-      ExternalReference::new_space_start(isolate());
+void MacroAssembler::TestJSArrayForAllocationMemento(Register receiver_reg,
+                                                     Register scratch_reg,
+                                                     Label* no_memento_found) {
+  Label map_check;
   ExternalReference new_space_allocation_top =
       ExternalReference::new_space_allocation_top_address(isolate());
-  Addu(scratch_reg, receiver_reg,
-       Operand(JSArray::kSize + AllocationMemento::kSize - kHeapObjectTag));
-  Branch(no_memento_found, lt, scratch_reg, Operand(new_space_start));
+  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.
+  Addu(scratch_reg, receiver_reg, Operand(kMementoEndOffset));
+  Xor(scratch_reg, scratch_reg, Operand(receiver_reg));
+  Branch(no_memento_found, gt, scratch_reg, Operand(Page::kPageSize));
+  // Bail out if the object is not in new space.
+  Addu(scratch_reg, receiver_reg, Operand(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.
+  Addu(scratch_reg, receiver_reg, Operand(kMementoEndOffset));
+  Xor(scratch_reg, scratch_reg, Operand(new_space_allocation_top));
+  And(scratch_reg, scratch_reg, Operand(~Page::kPageAlignmentMask));
+  Branch(&map_check, ne, scratch_reg, Operand(zero_reg));
+  // Otherwise, we have to check whether we are still below top, to ensure that
+  // we are not checking against a stale memento.
+  Addu(scratch_reg, receiver_reg, Operand(kMementoEndOffset));
   li(at, Operand(new_space_allocation_top));
   lw(at, MemOperand(at));
   Branch(no_memento_found, gt, scratch_reg, Operand(at));
-  lw(scratch_reg, MemOperand(scratch_reg, -AllocationMemento::kSize));
-  if (allocation_memento_present) {

[Michael Lippautz, 2016/03/30 15:22:22]

The way this was implemented was slightly better (only 1 instead of 2 branches),
but I found it quite unreadable (cond + presence of label). See
lithium-codegen-mips.cc for the caller...

The new implementation is now the same on all platforms.

-    Branch(allocation_memento_present, cond, scratch_reg,
-           Operand(isolate()->factory()->allocation_memento_map()));
-  }
+  bind(&map_check);
+  lw(scratch_reg, MemOperand(receiver_reg, kMementoMapOffset));
+  Branch(no_memento_found, ne, scratch_reg,
+         Operand(isolate()->factory()->allocation_memento_map()));
 }