From 76925fe3e0f2a7d425e3b33b2546b681a6417c54 Mon Sep 17 00:00:00 2001
From: Roman Khimov <roman@nspcc.ru>
Date: Thu, 9 Jul 2020 13:06:49 +0300
Subject: [PATCH] compiler: slices must default to nil

---
 pkg/compiler/codegen.go    | 22 ++++++++++++++--------
 pkg/compiler/slice_test.go | 37 +++++++++++++++++++++++++++++++++++++
 2 files changed, 51 insertions(+), 8 deletions(-)

diff --git a/pkg/compiler/codegen.go b/pkg/compiler/codegen.go
index c1014cfd7..c7d888a42 100644
--- a/pkg/compiler/codegen.go
+++ b/pkg/compiler/codegen.go
@@ -231,13 +231,6 @@ func (c *codegen) emitDefault(t types.Type) {
 		default:
 			emit.Opcode(c.prog.BinWriter, opcode.PUSHNULL)
 		}
-	case *types.Slice:
-		if isCompoundSlice(t) {
-			emit.Opcode(c.prog.BinWriter, opcode.NEWARRAY0)
-		} else {
-			emit.Int(c.prog.BinWriter, 0)
-			emit.Opcode(c.prog.BinWriter, opcode.NEWBUFFER)
-		}
 	case *types.Struct:
 		num := t.NumFields()
 		emit.Int(c.prog.BinWriter, int64(num))
@@ -1183,9 +1176,22 @@ func (c *codegen) convertBuiltin(expr *ast.CallExpr) {
 	case "append":
 		arg := expr.Args[0]
 		typ := c.typeInfo.Types[arg].Type
+		emit.Opcode(c.prog.BinWriter, opcode.OVER)
+		emit.Opcode(c.prog.BinWriter, opcode.ISNULL)
+		emit.Instruction(c.prog.BinWriter, opcode.JMPIFNOT, []byte{2 + 4})
 		if isByteSlice(typ) {
+			emit.Opcode(c.prog.BinWriter, opcode.NIP)
+			emit.Opcode(c.prog.BinWriter, opcode.PUSH0)
+			emit.Opcode(c.prog.BinWriter, opcode.NEWBUFFER)
+			emit.Opcode(c.prog.BinWriter, opcode.SWAP)
+			// Jump target.
 			emit.Opcode(c.prog.BinWriter, opcode.CAT)
 		} else {
+			emit.Opcode(c.prog.BinWriter, opcode.NIP)
+			emit.Opcode(c.prog.BinWriter, opcode.NEWARRAY0)
+			emit.Opcode(c.prog.BinWriter, opcode.SWAP)
+			emit.Opcode(c.prog.BinWriter, opcode.NOP)
+			// Jump target.
 			emit.Opcode(c.prog.BinWriter, opcode.OVER)
 			emit.Opcode(c.prog.BinWriter, opcode.SWAP)
 			emit.Opcode(c.prog.BinWriter, opcode.APPEND)
@@ -1513,7 +1519,7 @@ func (c *codegen) writeJumps(b []byte) error {
 			opcode.JMPEQ, opcode.JMPNE,
 			opcode.JMPGT, opcode.JMPGE, opcode.JMPLE, opcode.JMPLT:
 			// Noop, assumed to be correct already. If you're fixing #905,
-			// make sure not to break "len" handling above.
+			// make sure not to break "len" and "append" handling above.
 		case opcode.JMPL, opcode.JMPIFL, opcode.JMPIFNOTL,
 			opcode.JMPEQL, opcode.JMPNEL,
 			opcode.JMPGTL, opcode.JMPGEL, opcode.JMPLEL, opcode.JMPLTL,
diff --git a/pkg/compiler/slice_test.go b/pkg/compiler/slice_test.go
index 2dfe9adbc..f386beddf 100644
--- a/pkg/compiler/slice_test.go
+++ b/pkg/compiler/slice_test.go
@@ -243,6 +243,43 @@ var sliceTestCases = []testCase{
 		}`,
 		big.NewInt(42),
 	},
+	{
+		"defaults to nil for byte slice",
+		`
+		package foo
+		func Main() int {
+			var a []byte
+			if a != nil { return 1}
+			return 2
+		}
+		`,
+		big.NewInt(2),
+	},
+	{
+		"defaults to nil for int slice",
+		`
+		package foo
+		func Main() int {
+			var a []int
+			if a != nil { return 1}
+			return 2
+		}
+		`,
+		big.NewInt(2),
+	},
+	{
+		"defaults to nil for struct slice",
+		`
+		package foo
+		type pair struct { a, b int }
+		func Main() int {
+			var a []pair
+			if a != nil { return 1}
+			return 2
+		}
+		`,
+		big.NewInt(2),
+	},
 }
 
 func TestSliceOperations(t *testing.T) {