([Ljava/lang/String;)V) Unable to pop operand off an empty stack

Question

For school I have to make my own programming language. One part of the language is that the user is able to make and use arrays. I am using antlr 4.1 to generate the grammar, and the bytecode is in Jasmin since the compiler and code generator are make using java. The compiler is able to compile my code, but whenever i use a unit test to test the program, I get the following error:

java.lang.VerifyError: (class: HelloWorld, method: main signature: ([Ljava/lang/String;)V) Unable to pop operand off an empty stack

This is the code i am trying to compile (which is able to compile like i said).

NMBR ARRAY nummers[20]
nummers.ADD(15)
PRINT(nummers.GET[0])

And these are all the methods that the array uses

@Override
public ArrayList<String> visitArrayStatement(CAPSParser.ArrayStatementContext ctx) {
    Symbol s = symbols.get(ctx);

    if (s.getName().equals(ctx.IDENTIFIER().getText())) {

        jasminCode.add("ldc " + ctx.NMBR_VALUE().getText());
        if (s.getType().equals(DataType.NMBR)) {
            jasminCode.add("newarray int");
        } else if (s.getType().equals(DataType.STR)) {
            jasminCode.add("newarray java/lang/String");
        }
        jasminCode.add("astore " + s.getLocalslot());
    }
    return null;
}

@Override
public ArrayList<String> visitArrayAssignment(CAPSParser.ArrayAssignmentContext ctx) {
    Symbol s = symbols.get(ctx);
    arrayCounter++;
    if (s.getName().equals(ctx.IDENTIFIER().getText())) {
        jasminCode.add("aload " + s.getLocalslot());
        jasminCode.add("ldc " + arrayCounter);
        visit(ctx.expression());
        if (s.getType().equals(DataType.NMBR)) {
            jasminCode.add("iastore");
            jasminCode.add("astore " + s.getLocalslot());
        } else if (s.getType().equals(DataType.STR)) {
            jasminCode.add("aastore");
            jasminCode.add("astore " + s.getLocalslot());
        }
    }
    return null;
}

@Override
public ArrayList<String> visitArrayGetValue(CAPSParser.ArrayGetValueContext ctx) {
    Symbol s = symbols.get(ctx);
    array = s;

    if (s.getName().equals(ctx.IDENTIFIER().getText())) {
        jasminCode.add("aload " + s.getLocalslot());
        jasminCode.add("ldc " + ctx.NMBR_VALUE().getText());

        if (s.getType().equals(DataType.NMBR)) {
            jasminCode.add("iaload");
        } else if (s.getType().equals(DataType.STR)) {
            jasminCode.add("aaload");
        }
    }
    return null;
}

And finaly this is the test I'm trying to run:

@Test
void checkOutputFile() throws Exception {
    // Compile and assemble testFiles/hello.exlang
    Compiler c = new Compiler();
    JasminBytecode code = c.compileFile("testFiles/hello.exlang", "HelloWorld");
    assertNotNull(code);

    // Check that output matches what we expect
    List<String> output = runCode(code);
    assertArrayEquals(new String[] {
            "15"
    }, output.toArray());
}

This is the jasmin code that is generated:

.bytecode 49.0
.class public hello
.super java/lang/Object

.method public static main([Ljava/lang/String;)V
.limit stack 99
.limit locals 99

ldc 20
newarray int
astore 1
aload 1
ldc 1
ldc 15
iastore
astore 1
aload 1
ldc 2
ldc 5
iastore
astore 1
aload 1
ldc 3
ldc 10
iastore
astore 1
aload 1
ldc 4
ldc 60
iastore
astore 1
getstatic java/lang/System/out Ljava/io/PrintStream;
aload 1
ldc 3
iaload
invokevirtual java/io/PrintStream/println(I)V
astore 1
return
.end method

Does anyone know why this gives me the following error? Any help would be appriciated.

Answer 1

The stack transition for iastore is:

..., arrayref, index, value →

...

After iastore, the array, the index, and the value will all be popped from the stack, so it doesn't make sense to have another astore 1 instruction after it, as it would be storing whatever is below the arrayref. If the operand stack happens to be empty at this point, astore would be trying to pop an empty stack, which is what the error message is referring to.

Are you trying to store the modified array back into the variable? You don't need to do that. iastore modifies the array that the variable has a reference to, and does not create a copy. You can just delete the two occurrences of this line in visitArrayAssignment:

jasminCode.add("astore " + s.getLocalslot());

I also noticed that you are doing astore after invoking System.out.println. But println returns void, so there won't be any return value on the stack to store. You should remove that astore too.