core/asm: Fix encoding of pushed labels

EVM uses big-endian byte-order, so to pad a label value to 4 bytes,
zeros must be added to the front, not the end.

core/asm/compiler.go

@@ -201,8 +201,8 @@ func (c *Compiler) compileElement(element token) error {
 		case stringValue:
 			value = []byte(rvalue.text[1 : len(rvalue.text)-1])
 		case label:
-			value = make([]byte, 4)
-			copy(value, big.NewInt(int64(c.labels[rvalue.text])).Bytes())
+			value = big.NewInt(int64(c.labels[rvalue.text])).Bytes()
+			value = append(make([]byte, 4-len(value)), value...)
 		default:
 			return compileErr(rvalue, rvalue.text, "number, string or label")
 		}

core/asm/compiler_test.go

@@ -0,0 +1,49 @@
+// Copyright 2019 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package asm
+
+import (
+	"testing"
+)
+
+func TestCompiler(t *testing.T) {
+	tests := []struct {
+		input, output string
+	}{
+		{
+			input: `
+	GAS
+	label:
+	PUSH @label
+`,
+			output: "5a5b6300000001",
+		},
+	}
+	for _, test := range tests {
+		ch := Lex([]byte(test.input), false)
+		c := NewCompiler(false)
+		c.Feed(ch)
+		output, err := c.Compile()
+		if len(err) != 0 {
+			t.Errorf("compile error: %v\ninput: %s", err, test.input)
+			continue
+		}
+		if output != test.output {
+			t.Errorf("incorrect output\ninput: %sgot:  %s\nwant: %s\n", test.input, output, test.output)
+		}
+	}
+}