Project 6: Compilation

In this project, you will complete a compiler for SwiJay (called JaySea, inspired by javac) which produces Java VM assembly (which can then be used to generate classfiles).

You should mainly think of this as a compiler for Jay, except that one extra tricky part--switch statements---is added at the end.

1. Set-up.

Many other pieces are provided for you, and can be found in a tar file.

cp ~tvandrun/Public/cs365/proj6.tar .

The system works like this. The program swijay/jaysea.py reads in a swijay file and parses it. Then the CompilerVisitor (which your task is to finish implementing) transforms that into Java assembly as defined for the Jasmin tool (the output here is a .jj file). Then JaySea feeds the Java assembly into Jasmin (whose output is a .class file). Finally JaySea executes the classfile using the Java VM.

Assembling and running the swijay program are optional. The --no-run flag makes JaySea produce a .class file supresses execution of the resulting file. The --no-assem flag supresses Jasmin (and hence also supresses execution, since we wouldn't have a .class file to execute)

2. Your task

Your task is finishing the visit methods in compilerVisitor.py. For all Jay statements and expressions, what series of instructions needs to be emitted, and how does that affect the stack?

Some specifics:

• The only changes to the code you'll need to do are to finish the visit methods for assignments, conditionals, switch statements, variables, and literal expressions.
• To serve as examples, Skips, blocks, loops, prints, unary expressions, and binary expressions are done for you.
• I've provided a lot of "machinery". The code I've provided keeps track of the stack depth (and maximum stack depth). You can generate new labels using gen_lab(). Each label needs to have a consistent stack depth in the final version, and the instance variable lab_tab associates labels with stack depths. You can interact with this table using get_depth() and set_depth(), but I recommend you mainly use mark_lab(), which associates a given lable with the current stack depth. Specifically, any time you emit an instruction that jumps to a label, you should mark that label. The variables of the program are stored as a list in sym_tab. What you really want is the number for each variable, which you can get from the index of that variable in sym_tab
• The emit() function takes a string and an integer; the integer is the effect that the instruction has on the stack. This keeps track of stack depth consistency and the maximum stack depth. If you emit a goto, you should adjust the stack by its current height so that the resulting height is zero, since the instruction after a goto must be the target of a jump or branch.
• Use emit_lab() to emit labels.
• In theory, we could print out booleans as booleans, not as ints. However, that would require type checking, which I never did for SwiJay. So, for simplicity's sake, you may print out all values--integer or boolean--as ints.

3. Suggestions

Make use of these resources:

• The Jasmin Home Page, especially the description of instructions.
• The Java VM Spec, especially the VM instruction set (Notice the alphabetical indices at the top of the page.)

Make your changes to JaySea incrementally. Then gradually add statements and expressions. Do switch statements last. Switch statements can be compiled to the instruction lookupswitch (or, if you're really ambitious, tableswitch). Remember that the statements inside a switch are called InSwitchStatements, and there are "regular" statements, cases, breaks, and defaults. You will need to do some processing of the whole switch statement at the beginning to generate the switch instruction, and then visit the InSwitchStatements.

As you work, I recommend you have both Jay and Java versions of your test files. Compile the Java versions and disassemble them with javap to see what the result might look like. Your output does not have to be identical to what javac produces--often there is an equivalent series of instructions that you'll find are easier to produce---but javac can guide you through some of the knottier statements.

4. Extra credit

All you need is a compiler that produces programs that work to get full credit. But there are some optimizations you could perform for which I will award extra credit. For example, if (x > 0)... would naively be complied by loading x, pushing 0, generating the sequence for >, and then ifne, but a more efficient version would load x and use if_icmpgt. Also, x = x + 1; could be done using the iinc instruction. Some switch statements can be compiled to a tableswitch instead of a lookupswitch.

5. To turn in

To turn in this project, please copy your compile.sml and, if you did any optimizations for extra credit, a text file describing what you did to

/cslab.all/ubuntu/cs365/turnin/xxxxxx/proj6

where "xxxxxx" is [elliot|payton|davidemmanuel|nathan|gill|brandon|sean|simon]. I will grade your project by running it against a collection of test files.

Due: Wed, April 2, 5:00 pm.