core/asm: Fix PC calculations when a label is pushed

Incrementing PC by 5 is only correct if the label appears after a jump,
in which case there is an implicit push. When it appears after an explicit
push, PC should only be incremented by 4.

core/asm/compiler.go

@@ -57,6 +57,7 @@ func NewCompiler(debug bool) *Compiler {
 // second stage to push labels and determine the right
 // position.
 func (c *Compiler) Feed(ch <-chan token) {
+	var prev token
 	for i := range ch {
 		switch i.typ {
 		case number:
@@ -73,10 +74,14 @@ func (c *Compiler) Feed(ch <-chan token) {
 			c.labels[i.text] = c.pc
 			c.pc++
 		case label:
-			c.pc += 5
+			c.pc += 4
+			if prev.typ == element && isJump(prev.text) {
+				c.pc++
+			}
 		}
 
 		c.tokens = append(c.tokens, i)
+		prev = i
 	}
 	if c.debug {
 		fmt.Fprintln(os.Stderr, "found", len(c.labels), "labels")

core/asm/compiler_test.go

@@ -32,6 +32,13 @@ func TestCompiler(t *testing.T) {
 `,
 			output: "5a5b6300000001",
 		},
+		{
+			input: `
+	PUSH @label
+	label:
+`,
+			output: "63000000055b",
+		},
 	}
 	for _, test := range tests {
 		ch := Lex([]byte(test.input), false)