Add support for unary plus in hydrogen compiler.

src/hydrogen.cc

Index: src/hydrogen.cc
diff --git a/src/hydrogen.cc b/src/hydrogen.cc
index 6cb2b240a95b61103609b49d4f2532282985591d..4a99aa90b76e98d13daada80238b42f68d001bff 100644
--- a/src/hydrogen.cc
+++ b/src/hydrogen.cc
@@ -485,6 +485,11 @@ HConstant* HGraph::GetConstant(SetOncePointer<HConstant>* pointer,
 }
 
 
+HConstant* HGraph::GetConstant0() {
+  return GetConstant(&constant_0_, Smi::FromInt(0));
+}
+
+
 HConstant* HGraph::GetConstant1() {
   return GetConstant(&constant_1_, Smi::FromInt(1));
 }
@@ -4519,7 +4524,7 @@ void HGraphBuilder::VisitUnaryOperation(UnaryOperation* expr) {
       VisitForEffect(expr->expression());
     }
 
-  } else if (op == Token::BIT_NOT || op == Token::SUB) {
+  } else if (op == Token::BIT_NOT || op == Token::SUB || op == Token::ADD) {
     VISIT_FOR_VALUE(expr->expression());
     HValue* value = Pop();
     HInstruction* instr = NULL;
@@ -4528,8 +4533,10 @@ void HGraphBuilder::VisitUnaryOperation(UnaryOperation* expr) {
         instr = new HBitNot(value);
         break;
       case Token::SUB:
-        instr = new HMul(graph_->GetConstantMinus1(), value);
+        instr = new HSub(graph_->GetConstant0(), value);

[Kevin Millikin (Chromium), 2011/03/14 14:18:46]

I'm suspicious of this case.  Isn't it true that:

-(0) = 0-0 = 0+(-0) = +0

?  We should probably have a test that checks the corner cases for unary minus.

There is type feedback for the stub in unoptimized code, so it seems better to
call HGraphBuilder::BuildBinaryOperation and get our hands on that feedback.

[Lasse Reichstein, 2011/03/15 12:18:33]

Argh, foiled again by -0.
Yes, you are correct.
Incidentally, it would work if we used a ConstantMinus0 instead, but then we
would lose smi-ness.
I guess multiplication is safer (and the slight speedup I saw was probably due
to suddently being able to ignore -0)

         break;
+      case Token::ADD:
+        instr = new HSub(value, graph_->GetConstant0());

[Kevin Millikin (Chromium), 2011/03/14 14:18:46]

I'm also suspicious of this:

+(-0) = 0-(-0) = 0+0 = +0

[Lasse Reichstein, 2011/03/15 12:18:33]

Agree, that's bad too.

       default:
         UNREACHABLE();
         break;