Optimisations and API changes for smart contracts (#67)

* support VM to pass method and arguments to a script.

* added support for type assertions in smartcontracts.

* added native vm support for print.

* moved VM API packages to vm -> API

* reverted the native Print opcode in favor of runtime.Log

* added support for registering custom interop hooks in the VM.

* Updated README

* Updated compiler with @OPTIMIZE tags

* Moved more tests to VM package.

* optimized and refactored compiler and vm API

* updated README with new smartcontract apis

* bumped version

pkg/vm/compiler/codegen.go

@@ -118,19 +118,6 @@ func (c *codegen) emitStoreStructField(i int) {
 	emitOpcode(c.prog, vm.Osetitem)
 }
 
-func (c *codegen) emitSyscallReturn() {
-	emitOpcode(c.prog, vm.Ojmp)
-	emitOpcode(c.prog, vm.Opcode(0x03))
-	emitOpcode(c.prog, vm.Opush0)
-
-	emitInt(c.prog, int64(0))
-
-	emitOpcode(c.prog, vm.Onop)
-	emitOpcode(c.prog, vm.Ofromaltstack)
-	emitOpcode(c.prog, vm.Odrop)
-	emitOpcode(c.prog, vm.Oret)
-}
-
 // convertGlobals will traverse the AST and only convert global declarations.
 // If we call this in convertFuncDecl then it will load all global variables
 // into the scope of the function.
@@ -160,7 +147,7 @@ func (c *codegen) convertFuncDecl(file *ast.File, decl *ast.FuncDecl) {
 	}
 
 	c.scope = f
-	ast.Inspect(decl, c.scope.analyzeVoidCalls)
+	ast.Inspect(decl, c.scope.analyzeVoidCalls) // @OPTIMIZE
 
 	// All globals copied into the scope of the function need to be added
 	// to the stack size of the function.
@@ -193,17 +180,19 @@ func (c *codegen) convertFuncDecl(file *ast.File, decl *ast.FuncDecl) {
 		l := c.scope.newLocal(name)
 		c.emitStoreLocal(l)
 	}
-
-	// If this function is a syscall we will manipulate the return value to 0.
-	// All the syscalls are just signatures functions and bring no real return value.
-	// The return values you will find in the smartcontract package is just for
-	// satisfying the typechecker and the user experience.
-	if isSyscall(f.name) {
-		c.emitSyscallReturn()
-	} else {
-		// After loading the arguments we can convert the globals into the scope of the function.
+	// Load in all the global variables in to the scope of the function.
+	// This is not necessary for syscalls.
+	if !isSyscall(f.name) {
 		c.convertGlobals(file)
-		ast.Walk(c, decl.Body)
+	}
+
+	ast.Walk(c, decl.Body)
+
+	// If this function returs the void (no return stmt) we will cleanup its junk on the stack.
+	if !hasReturnStmt(decl) {
+		emitOpcode(c.prog, vm.Ofromaltstack)
+		emitOpcode(c.prog, vm.Odrop)
+		emitOpcode(c.prog, vm.Oret)
 	}
 }
 
@@ -268,6 +257,7 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 
 		// @OPTIMIZE: We could skip these 3 instructions for each return statement.
 		// To be backwards compatible we will put them them in.
+		// See issue #65 (https://github.com/CityOfZion/neo-go/issues/65)
 		l := c.newLabel()
 		emitJmp(c.prog, vm.Ojmp, int16(l))
 		c.setLabel(l)
@@ -276,9 +266,9 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 			ast.Walk(c, n.Results[0])
 		}
 
-		emitOpcode(c.prog, vm.Onop)
+		emitOpcode(c.prog, vm.Onop) // @OPTIMIZE
 		emitOpcode(c.prog, vm.Ofromaltstack)
-		emitOpcode(c.prog, vm.Odrop)
+		emitOpcode(c.prog, vm.Odrop) // Cleanup the stack.
 		emitOpcode(c.prog, vm.Oret)
 		return nil
 
@@ -439,8 +429,8 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 		}
 
 		// If we are not assigning this function to a variable we need to drop
-		// the top stack item. It's not a void but you get the point \o/.
-		if _, ok := c.scope.voidCalls[n]; ok && !isNoRetSyscall(f.name) {
+		// (cleanup) the top stack item. It's not a void but you get the point \o/.
+		if _, ok := c.scope.voidCalls[n]; ok {
 			emitOpcode(c.prog, vm.Odrop)
 		}
 		return nil
@@ -518,6 +508,14 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 		c.setLabel(fend)
 
 		return nil
+
+	// We dont really care about assertions for the core logic.
+	// The only thing we need is to please the compiler type checking.
+	// For this to work properly, we only need to walk the expression
+	// not the assertion type.
+	case *ast.TypeAssertExpr:
+		ast.Walk(c, n.X)
+		return nil
 	}
 	return c
 }
@@ -528,7 +526,7 @@ func (c *codegen) convertSyscall(name string) {
 		log.Fatalf("unknown VM syscall api: %s", name)
 	}
 	emitSyscall(c.prog, api)
-	emitOpcode(c.prog, vm.Onop)
+	emitOpcode(c.prog, vm.Onop) // @OPTIMIZE
 }
 
 func (c *codegen) convertBuiltin(name string, expr *ast.CallExpr) {