Better sequences for class id loading on ia32.

runtime/vm/assembler_ia32.cc

Index: runtime/vm/assembler_ia32.cc
===================================================================
--- runtime/vm/assembler_ia32.cc	(revision 38158)
+++ runtime/vm/assembler_ia32.cc	(working copy)
@@ -290,6 +290,14 @@
 }
 
 
+void Assembler::cmovne(Register dst, Register src) {
+  AssemblerBuffer::EnsureCapacity ensured(&buffer_);
+  EmitUint8(0x0F);
+  EmitUint8(0x45);
+  EmitRegisterOperand(dst, src);
+}
+
+
 void Assembler::cmovs(Register dst, Register src) {
   AssemblerBuffer::EnsureCapacity ensured(&buffer_);
   EmitUint8(0x0F);
@@ -2708,24 +2716,26 @@
     Register result, Register object, Register tmp) {
   ASSERT(object != tmp);
   ASSERT(result != tmp);
+  static const intptr_t kSmiCidSource = kSmiCid << RawObject::kClassIdTagPos;
 
-  // Make a copy of object since result and object can be the same register.
-  movl(tmp, object);
-  // Load up a null object. We only need it so we can use LoadClassId on it in
-  // the case that object is a Smi.
-  movl(result, Immediate(reinterpret_cast<intptr_t>(Object::null())));
-  // Check if the object is a Smi.
+  if (result == object) {
+    movl(tmp, object);
+  } else {
+    tmp = object;
+  }
+
+  // Make a dummy "Object" whose cid is kSmiCid.
+  movl(result, Immediate(reinterpret_cast<int32_t>(&kSmiCidSource) + 1));
+
+  // Check if object (in tmp) is a Smi.
   testl(tmp, Immediate(kSmiTagMask));
-  // If the object *is* a Smi, load the null object into tmp. o/w leave alone.
-  cmove(tmp, result);
-  // Loads either the cid of the object if it isn't a Smi, or the cid of null
-  // if it is a Smi, which will be ignored.
-  LoadClassId(result, tmp);
 
-  movl(tmp, Immediate(kSmiCid));
-  // If object is a Smi, move the Smi cid into result. o/w leave alone.
-  cmove(result, tmp);
-  // Finally, tag the result.
+  // If the object is not a Smi, use the original object to load the cid.
+  // Otherwise, the dummy object is used, and the result is kSmiCid.
+  cmovne(result, tmp);
+  LoadClassId(result, result);
+
+  // Tag the result.
   SmiTag(result);
 }