Optimize write barrier of map-only elements transitions

src/ia32/macro-assembler-ia32.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 219 matching lines @@
 
   // Clobber clobbered input registers when running with the debug-code flag
   // turned on to provoke errors.
   if (emit_debug_code()) {
     mov(value, Immediate(BitCast<int32_t>(kZapValue)));
     mov(dst, Immediate(BitCast<int32_t>(kZapValue)));
   }
 }
 
 
+void MacroAssembler::RecordWriteForMap(
+    Register object,
+    Handle<Map> map,
+    Register scratch1,
+    Register scratch2,
+    SaveFPRegsMode save_fp) {
+  // First, check if a write barrier is even needed. The tests below
+  // catch stores of Smis.

[Erik Corry, 2012/06/06 11:04:26]

Comment seems to be out of date.

+  Label done;
+  
+  Register address = scratch1;
+  Register value = scratch2;
+  if (emit_debug_code()) {
+    Label ok;
+    lea(address, FieldOperand(object, HeapObject::kMapOffset));
+    test_b(address, (1 << kPointerSizeLog2) - 1);
+    j(zero, &ok, Label::kNear);
+    int3();
+    bind(&ok);
+  }
+
+  ASSERT(!object.is(value));
+  ASSERT(!object.is(address));
+  ASSERT(!value.is(address));
+  if (emit_debug_code()) {
+    AbortIfSmi(object);
+  }
+
+  if (!FLAG_incremental_marking) {
+    return;
+  }
+
+  // A single check of the map's pages interesting flag suffices, since it is
+  // only set during incremental collection, and it's guaranteed that the from
+  // object's interesting flag is also set.

[Michael Starzinger, 2012/06/06 09:41:17]

Can we add to the comment that this relies on "maps not being compacted" and
"maps never being in new-space". We can even assert that ...

ASSERT(!isolate()->heap()->InNewSpace(*map));
ASSERT(!isolate()->heap()->mark_compact_collector()->IsOnEvacuationCandidate(*map));

The second assert is not as useful as the first, because it only checks whether
the map is on an evacuation candidate during compile-time, but not during
runtime. But I would still include it.

[danno, 2012/06/06 10:58:28]

Done.

+  CheckPageFlag(map,
+                MemoryChunk::kPointersToHereAreInterestingMask,
+                zero,
+                &done,
+                Label::kNear);
+
+  // Delay the initialization of |address| and |value| for the stub until it's
+  // known that the will be needed. Up until this point their value are not
+  // needed since they are embedded in the operands of instructions that need
+  // them.
+  lea(address, FieldOperand(object, HeapObject::kMapOffset));
+  mov(value, Immediate(map));
+  RecordWriteStub stub(object, value, address, OMIT_REMEMBERED_SET, save_fp);
+  CallStub(&stub);
+
+  bind(&done);
+
+  // Clobber clobbered input registers when running with the debug-code flag
+  // turned on to provoke errors.
+  if (emit_debug_code()) {
+    mov(value, Immediate(BitCast<int32_t>(kZapValue)));
+    mov(scratch1, Immediate(BitCast<int32_t>(kZapValue)));
+    mov(scratch2, Immediate(BitCast<int32_t>(kZapValue)));
+  }
+}
+
+
 void MacroAssembler::RecordWrite(Register object,
                                  Register address,
                                  Register value,
                                  SaveFPRegsMode fp_mode,
                                  RememberedSetAction remembered_set_action,
                                  SmiCheck smi_check) {
   ASSERT(!object.is(value));
   ASSERT(!object.is(address));
   ASSERT(!value.is(address));
   if (emit_debug_code()) {
@@ skipping 2361 matching lines @@
   if (mask < (1 << kBitsPerByte)) {
     test_b(Operand(scratch, MemoryChunk::kFlagsOffset),
            static_cast<uint8_t>(mask));
   } else {
     test(Operand(scratch, MemoryChunk::kFlagsOffset), Immediate(mask));
   }
   j(cc, condition_met, condition_met_distance);
 }
 
 
+void MacroAssembler::CheckPageFlag(
+    Handle<Map> map,
+    int mask,
+    Condition cc,
+    Label* condition_met,
+    Label::Distance condition_met_distance) {
+  ASSERT(cc == zero || cc == not_zero);
+  uint32_t masked_address = reinterpret_cast<uint32_t>(*map) &
+      ~Page::kPageAlignmentMask;
+  Page* page = reinterpret_cast<Page*>(masked_address);

[Michael Starzinger, 2012/06/06 09:41:17]

Better use the following "Page* page = Page::FromAddress(map->address());" here.

[danno, 2012/06/06 10:58:28]

Done.

+  ExternalReference reference(ExternalReference::page_flags(page));
+  if (mask < (1 << kBitsPerByte)) {
+    test_b(Operand::StaticVariable(reference), static_cast<uint8_t>(mask));
+  } else {
+    test(Operand::StaticVariable(reference), Immediate(mask));
+  }
+  j(cc, condition_met, condition_met_distance);
+}
+
+
 void MacroAssembler::JumpIfBlack(Register object,
                                  Register scratch0,
                                  Register scratch1,
                                  Label* on_black,
                                  Label::Distance on_black_near) {
   HasColor(object, scratch0, scratch1,
            on_black, on_black_near,
            1, 0);  // kBlackBitPattern.
   ASSERT(strcmp(Marking::kBlackBitPattern, "10") == 0);
 }
@@ skipping 191 matching lines @@
   // Load the prototype from the map and loop if non-null.
   bind(&check_prototype);
   mov(ecx, FieldOperand(ebx, Map::kPrototypeOffset));
   cmp(ecx, isolate()->factory()->null_value());
   j(not_equal, &next);
 }
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_IA32