compiler: support nil checks

pkg/compiler/codegen.go

@@ -659,8 +659,26 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 				return nil
 			}
 
+			var checkForNull bool
+
+			if isExprNil(n.X) {
+				checkForNull = true
+			} else {
 				ast.Walk(c, n.X)
+			}
+			if isExprNil(n.Y) {
+				checkForNull = true
+			} else {
 				ast.Walk(c, n.Y)
+			}
+			if checkForNull {
+				emit.Opcode(c.prog.BinWriter, opcode.ISNULL)
+				if n.Op == token.NEQ {
+					emit.Opcode(c.prog.BinWriter, opcode.NOT)
+				}
+
+				return nil
+			}
 
 			switch {
 			case n.Op == token.ADD:

pkg/compiler/nilcheck_test.go

@@ -0,0 +1,55 @@
+package compiler_test
+
+import (
+	"math/big"
+	"testing"
+)
+
+var nilTestCases = []testCase{
+	{
+		"nil check positive right",
+		`
+		package foo
+		func Main() int {
+			var t interface{}
+			if t == nil {
+				return 1
+			}
+			return 2
+		}
+		`,
+		big.NewInt(1),
+	},
+	{
+		"nil check negative right",
+		`
+		package foo
+		func Main() int {
+			t := []byte{}
+			if t == nil {
+				return 1
+			}
+			return 2
+		}
+		`,
+		big.NewInt(2),
+	},
+	{
+		"nil check positive left",
+		`
+		package foo
+		func Main() int {
+			var t interface{}
+			if nil == t {
+				return 1
+			}
+			return 2
+		}
+		`,
+		big.NewInt(1),
+	},
+}
+
+func TestNil(t *testing.T) {
+	runTestCases(t, nilTestCases)
+}