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

runtime/vm/assembler_ia32.cc

Index: runtime/vm/assembler_ia32.cc
diff --git a/runtime/vm/assembler_ia32.cc b/runtime/vm/assembler_ia32.cc
index 2acf8ce65f0caefe8152567b374bda5c7b033994..9d0160bb09deca67e74592fec5cb74eacc5ad012 100644
--- a/runtime/vm/assembler_ia32.cc
+++ b/runtime/vm/assembler_ia32.cc
@@ -1850,6 +1850,26 @@ void Assembler::CompareObject(Register reg, const Object& object) {
 
 
 // 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.

[Ivan Posva, 2013/03/22 04:11:35]

Neat!

+  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) {
@@ -1872,12 +1892,17 @@ void Assembler::StoreIntoObjectFilter(Register object,
 
 void Assembler::StoreIntoObject(Register object,
                                 const Address& dest,
-                                Register value) {
+                                Register value,
+                                ValueSminess value_sminess) {
   ASSERT(object != value);
   TraceStoreIntoObject(object, dest, value);
   movl(dest, value);
   Label done;
-  StoreIntoObjectFilter(object, value, &done);
+  if (value_sminess == kValueIsNotSmi) {
+    StoreIntoObjectFilterNoSmi(object, value, &done);
+  } else {
+    StoreIntoObjectFilter(object, value, &done);

[Ivan Posva, 2013/03/22 04:11:35]

Please assert for kValueCanBeSmi in case we add more values to the enum. Or just
make the parameter a boolean.

[Vyacheslav Egorov (Google), 2013/03/22 11:22:50]

Done.

+  }
   // A store buffer update is required.
   if (value != EAX) pushl(EAX);  // Preserve EAX.
   leal(EAX, dest);