Use shorter write-barrier filtering sequence when value is known to be non-smi.

runtime/vm/assembler_x64.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/globals.h"
 #if defined(TARGET_ARCH_X64)
 
 #include "vm/assembler.h"
 #include "vm/heap.h"
 #include "vm/memory_region.h"
@@ skipping 1672 matching lines @@
 
 
 void Assembler::negq(Register reg) {
   AssemblerBuffer::EnsureCapacity ensured(&buffer_);
   EmitRegisterREX(reg, REX_W);
   EmitUint8(0xF7);
   EmitOperand(3, Operand(reg));
 }
 
 
+void Assembler::notl(Register reg) {
+  AssemblerBuffer::EnsureCapacity ensured(&buffer_);
+  EmitRegisterREX(reg, REX_NONE);
+  EmitUint8(0xF7);
+  EmitUint8(0xD0 | (reg & 7));
+}
+
+
 void Assembler::notq(Register reg) {
   AssemblerBuffer::EnsureCapacity ensured(&buffer_);
   EmitRegisterREX(reg, REX_W);
   EmitUint8(0xF7);
   EmitUint8(0xD0 | (reg & 7));
 }
 
 
 void Assembler::enter(const Immediate& imm) {
   AssemblerBuffer::EnsureCapacity ensured(&buffer_);
@@ skipping 298 matching lines @@
     cmpq(reg, Immediate(reinterpret_cast<int64_t>(object.raw())));
   } else {
     ASSERT(reg != TMP);
     LoadObject(TMP, object);
     cmpq(reg, TMP);
   }
 }
 
 
 // Destroys the value register.
+void Assembler::StoreIntoObjectFilterNoSmi(Register object,
+                                           Register value,
+                                           Label* no_update) {
+  COMPILE_ASSERT((kNewObjectAlignmentOffset == kWordSize) &&
+                 (kOldObjectAlignmentOffset == 0), young_alignment);
+
+  // Write-barrier triggers if the value is in the new space (has bit set) and
+  // the object is in the old space (has bit cleared).
+  // To check that we could compute value & ~object and skip the write barrier
+  // if the bit is not set. However we can't destroy the object.
+  // However to preserve the object we compute negated expression
+  // ~value | object instead and skip the write barrier if the bit is set.
+  notl(value);
+  orl(value, object);
+  testl(value, Immediate(kNewObjectAlignmentOffset));
+  j(NOT_ZERO, no_update, Assembler::kNearJump);
+}
+
+
+// Destroys the value register.
 void Assembler::StoreIntoObjectFilter(Register object,
                                       Register value,
                                       Label* no_update) {
   // For the value we are only interested in the new/old bit and the tag bit.
   andl(value, Immediate(kNewObjectAlignmentOffset | kHeapObjectTag));
   // Shift the tag bit into the carry.
   shrl(value, Immediate(1));
   // Add the tag bits together, if the value is not a Smi the addition will
   // overflow into the next bit, leaving us with a zero low bit.
   adcl(value, object);
   // Mask out higher, uninteresting bits which were polluted by dest.
   andl(value, Immediate(kObjectAlignment - 1));
   // Compare with the expected bit pattern.
   cmpl(value, Immediate(
       (kNewObjectAlignmentOffset >> 1) + kHeapObjectTag +
       kOldObjectAlignmentOffset + kHeapObjectTag));
   j(NOT_ZERO, no_update, Assembler::kNearJump);
 }
 
 
 void Assembler::StoreIntoObject(Register object,
                                 const Address& dest,
-                                Register value) {
+                                Register value,
+                                bool can_value_be_smi) {
   ASSERT(object != value);
   movq(dest, value);
   Label done;
-  StoreIntoObjectFilter(object, value, &done);
+  if (can_value_be_smi) {
+    StoreIntoObjectFilter(object, value, &done);
+  } else {
+    StoreIntoObjectFilterNoSmi(object, value, &done);
+  }
   // A store buffer update is required.
   if (value != RAX) pushq(RAX);
   leaq(RAX, dest);
   call(&StubCode::UpdateStoreBufferLabel());
   if (value != RAX) popq(RAX);
   Bind(&done);
 }
 
 
 void Assembler::StoreIntoObjectNoBarrier(Register object,
@@ skipping 413 matching lines @@
 
 const char* Assembler::FpuRegisterName(FpuRegister reg) {
   ASSERT((0 <= reg) && (reg < kNumberOfXmmRegisters));
   return xmm_reg_names[reg];
 }
 
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_X64