Programming Assignment 6 — The Code Generator

Project Overview

Programming assignments 2 through 4 involved the constructed of the front-end (lexer, parser) and gatekeeping (semantic analyzer) stages of a compiler. In this assignment you will write a program that generates assembly instructions.

You may do this assignment in OCaml, Python, JavaScript, Haskell or Ruby.

You may work in a team of two people for this assignment. You may work in a team for any or all subsequent programming assignments. You do not need to keep the same teammate. The course staff are not responsible for finding you a willing teammate.

Goal

For this assignment you will write a code generator. Among other things, this involves implementing the operational semantics specification of Cool. You will track enough information to generate legitimate run-time errors (e.g., dispatch on void). You do not have to worry about "malformed input" because the semantic analyzer (from PA4) has already ruled out bad programs.

You will also write additional code to unserialize the class map, implementation map, parent map, and annotated AST produced by the semantic analyzer.

The Specification

You must create three artifacts:
  1. A program that takes a single command-line argument (e.g., file.cl-type). That argument will be an ASCII text Cool class map, implementation map, parent map and annotated AST file (as described in PA4). Your program must emit either Cool Assembly Language (file.cl-asm) or x86-64 Assembly Language (file.s). Your program will consist of a number of OCaml files, a number of Python files, or a number of Ruby files.
  2. A plain ASCII text file called readme.txt describing your design decisions and choice of test cases. See the grading rubric. A few paragraphs should suffice.
  3. Testcases test1.cl, test2.cl, test3.cl and test4.cl. The testcases should exercise compiler and run-time error corner cases.

Assembly Language

You have your choice of either Cool Assembly Language or x86-64 Assembly Language for this project. You may pick either one, or even switch if you don't like your initial choice (but the deadlines remain unchanged).

Cool Assembly Language is a simplified RISC-style assembly language that is similar to Java Bytecode.

x86-64 Assembly Language is the real deal — it's what most desktop machines use.

To find out if your development machine supports our flavor of x86-64 assembly language, try this:
./cool --x86 hello-world.cl
gcc hello-world.s
./a.out
If there were no assembler or linker errors and you saw "Hello, world.", your system can handle our x86-64 assembly. If not, you must use Cool Assembly Language.

Although we're invoking gcc, that's just shorthand: it's calling as and collect2 and whatnot to assemble the .s file into an object file and link that object file into an executable. You can see the individual steps with gcc -v.

Error Reporting

You are guaranteed that the file.cl-type input file will be correctly formed and will correspond to a well-typed Cool program. Thus, there will not be any direct static errors in the input. Those were all caught by the semantic analyzer earlier.

However, you must generate file.cl-asm (or file.s) so that it checks for and reports run-time errors. When your file.{cl-asm,s} program detects an error, it should use the Syscall IO.out_string and Syscall exit assembly instructions to cause an error string to be printed to the screen.

To report an error, write the string ERROR: line_number: Exception: message (for example, using Syscall IO.out_string) and terminate the program with Syscall exit. You may generate your file.{cl-asm,s} so that it writes whatever you want in the message, but it should be fairly indicative. Example erroneous input:

class Main inherits IO {
  my_void_io : IO ; -- no initializer => void value
  main() : Object {
    my_void_io.out_string("Hello, world.\n")
  } ;
} ;

For such an input, you must generate a well-formed file.{cl-asm,s} assmebly language file. However, when that file is executed (either in a Cool CPU Simulator or on an x86-64 machine), it will produce output such as:

ERROR: 4: Exception: dispatch on void
To put this another way, rather than actually checking for errors directly, you must generate assembly code that will later check for and report errors.

Line Number Error Reporting

The typing rules do not directly specify the line numbers on which errors are to be reported. As of v1.25, the Cool reference compiler uses these guidelines: Note that the reference interpreter uses different line numbers in some cases; you must match the reference compiler.

Commentary

You will have to handle all of the internal functions (e.g., IO.out_string) that are defined in PA4.

You can do basic testing as follows:

Whitespace and newlines do not matter in your file.{cl-asm,s} assembly code. However, whitespace and newlines do matter for your simulated Cool CPU output. This is because you are specifically being asked to implement IO and substring functions.

You should implement all of the operational semantics rules in the Reference Manual. You will also have to implement all of the built-in functions on the five Basic Classes.

What To Turn In For PA6c

PA6c is a checkpoint for PA6. A compiler is a large and complicated program; you must start it early to have any hope of success.

For the checkpoint, you will only be tested on hello-world.cl. If you can generate code for that, you pass the checkpoint. (Yes, you can "trick" the checkpoint by hard-coding output for that one test. Ultimately, however, you're only hurting yourself because you'll have more work to do later. This is not an honor violation.) You must turn in a zip file containing these files:

  1. source_files -- your implementation, including
Your zip file may also contain: Submit the file as you did for PA1.

What To Turn In For PA6

You must turn in a zip file containing these files:
  1. readme.txt -- your README file
  2. test1.cl -- a testcase
  3. test2.cl -- a testcase
  4. test3.cl -- a testcase
  5. test4.cl -- a testcase
  6. source_files -- your implementation, including
Your zip file may also contain: Submit the file as you did for PA1.

Working In Pairs

You may complete this project in a team of two. Teamwork imposes burdens of communication and coordination, but has the benefits of more thoughtful designs and cleaner programs. Team programming is also the norm in the professional world.

Students on a team are expected to participate equally in the effort and to be thoroughly familiar with all aspects of the joint work. Both members bear full responsibility for the completion of assignments. Partners turn in one solution for each programming assignment; each member receives the same grade for the assignment. If a partnership is not going well, the teaching assistants will help to negotiate new partnerships. Teams may not be dissolved in the middle of an assignment.

If you are working in a team, exactly one team member should submit a PA6 zipfile. That submission should include the file team.txt, a one-line flat ASCII text file that contains exactly and only the email address of your teammate. Don't include the @virgnia.edu bit. Example: If ph4u and wrw6y are working together, ph4u would submit ph4u-pa5.zip with a team.txt file that contains the word wrw6y. Then ph4u and wrw6y will both receive the same grade for that submission.

Autograding

We will use scripts to run your program on various testcases. The testcases will come from the good.cl and bad.cl files you and your classsmates submit as well as held-out testcases used only for grading. Your programs cannot use any special libraries (aside from the OCaml unix and str libraries, which are not necessary for this assignment). We will use (loosely) the following commands to execute them: You may thus have as many source files as you like (although two or three should suffice) -- they will be passed to your language compiler in alphabetical order (if it matters).

In each case we will then compare your output to the correct answer:

Note that this time we do not ignore newlines and whitespace since we are explicitly testing your implementation of a string IO subsystem. You must get every character correct in non-error instances. If your answer is not the same as the reference answer you get 0 points for that testcase. Otherwise you get 1 point for that testcase.

For error messages and negative testcases we will compare your output but not the particular error message. Basically, your generated code need only correctly identify that there is an error on line X. You do not have to faithfully duplicate our English error messages. Many people choose to (because it makes testing easier) — but it's not required.

We will perform the autograding on a 64-bit Linux system. However, your submissions must officialy be platform-independent (not that hard with a scripting language). You cannot depend on your compiler running on any particular platform (although you can depend on the resulting assembly code running on its associated platform).

There is more to your grade than autograder results. See the Programming Assignment page for a point breakdown.

Your submission may not create any temporary files. Your submission may not read or write any files beyond its input and output. We may test your submission in a special "jail" or "sandbox".