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) {

[Vyacheslav Egorov (Google), 2014/07/12 00:19:40]

I don't think you actually need tmp. When I was trying this sequence out I just
changed all usages to ensure that register that was used as tmp would contain
object already, because it is often used as 

movl(R1, address(...))
LoadTaggedClassIdMayBeSmi(R1, R1, R2)

this can obviously become 

movl(R2, address(...))
LoadTaggedClassIdMayBeSmi(R1, R2)

instead.

   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);

[Vyacheslav Egorov (Google), 2014/07/12 00:19:40]

I think this move is really redundant in all cases in stubs (see the pattern in
the comment above)

+  } else {
+    tmp = object;
+  }
+
+  // Make a dummy "Object" whose cid is kSmiCid.
+  movl(result, Immediate(reinterpret_cast<int32_t>(&kSmiCidSource) + 1));

[Vyacheslav Egorov (Google), 2014/07/12 00:19:40]

+ kHeapObjectTag instead of +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);
 }