CMSC 430, Spring 2011

Theory of Language Translation

Project 4 - C-- : Byte Code Analyzer

Due 11:59pm Mon, May 9th, 2011


In this project you will add some global data-flow analysis to your C-- compiler.

Getting Started

Project 4 will be implemented in Java in Eclipse. Download the following file:
  • Eclipse project archive file
You can import the project into Eclipse as an existing project in an archive file.

Along with files used to make direct submissions to the submit server (submit.jar, .submit), you will find the following project files:


See project 1 description for description of project files and syntax/semantics of C--. Compared to project 3, there are some changes:

  • now contains skeleton code to perform global data-flow analysis
  • contains code to represent and manipulate basic blocks
  • Public/release tests are now applied directly to sequences of Java byte codes. Tests only rely on optimizeCode() in, and do not use your mycc compiler or code generator. The byte code sequences used for public tests specified in are generated from test*.c. Your optimizer output for public tests are put in opt*.log and should be compared to opt*.out.
  • You may still use your mycc compiler to generate additional test cases for your data-flow analysis code. Student tests show examples of how to test your compiler on C-- code. Examples of expected outputs are shown in test*.out. Your output may differ from example outputs if your code generator generates different Java byte code sequences.
You should not need to make any changes to your scanner, parser, or type checker to perform code generation.


You are given a skeleton data flow analyzer for calculating live variables. The existing code builds a simplified control flow graph (CFG) assuming all statements are in a single basic block. It then performs iterative data-flow analysis on the (reverse) CFG, but lacks code to compute local effects and propagate data-flow information between basic blocks.

Data-flow framework

Your must extend the compiler to implement a global data-flow analysis framework. To begin with, you must build the control flow graph correctly in the presence of control flow (i.e., handle IF, GOTO, and RETURN statements) For simplicity, you can put every statement in its own basic block. Existing code in the compiler will display the control-flow graph as a list of basic blocks, with lists of predecessor & successors.

Live Variables

Next you must compute data flow information to solve the live variable problem for all local scalar variables (you can ignore arrays & global vars)
  • insert code to compute local information (i.e., GEN and KILL) by looking for ILOAD and ISTORE instructions
  • merge information at joins (combine data to compute OUT for each basic block)
  • propagate information through the basic block (compute IN from OUT and local information)
  • ensure the iterative framework is properly initialized and halts
Existing code in the compiler will display the data-flow information for each basic blocks, and will also use the IN set for the first basic block to warn of possible references to uninitialized variables.

Dead code elimination

Finally, you must use live variable information to eliminate dead stores for local scalar variables (ISTOREs). These instructions should be replaced with POP instructions. A more sophisticated implementation would actually eliminate the computations, but is not necessary.

Useful Classes

BitSet (from java.util.BitSet)

BitSet is used to represent data-flow information in each basic block. A bit at position X is set to represent information about local variable at index X. BitSet has the following methods:
 void and(BitSet set) 
    Performs a logical AND of this target bit set with the argument bit set. 

 void andNot(BitSet set) 
    Clears all of the bits in this BitSet whose corresponding bit is set in the specified BitSet. 

 void clear(int bitIndex) 
    Sets the bit specified by the index to false. 

 Object clone() 
    Cloning this BitSet produces a new BitSet that is equal to it. 

 boolean equals(Object obj) 
    Compares this object against the specified object. 

 boolean get(int bitIndex) 
    Returns the value of the bit with the specified index. 

 void or(BitSet set) 
    Performs a logical OR of this bit set with the bit set argument. 

 void set(int bitIndex) 
    Sets the bit specified by the index to true. 

 void xor(BitSet set) 
    Performs a logical XOR of this bit set with the bit set argument. 


bBlock is used to represent an individual basic block. It maintains its connections to other bBlocks, and uses BitSets to store data-flow information. bBlock has the following fields and methods:
   int               num;     // number for basic block
   int               len;     // number of instructions in basic block

   InstructionHandle first;   // first instruction in basic block
   InstructionHandle last;    // last instruction in basic block

   bBlock []         pred;    // predecessors
   bBlock []         succ;    // successors

   BitSet          in;        //
   BitSet         out;        //
   BitSet         gen;        //
   BitSet        kill;        //

   int               currPred;     // iterator: number of current predecessor
   int               currSucc;     // iterator: number of current successor

   public void setSucc(bBlock bb)  // add bb to successors, updates bb's predecessors
   public void setPred(bBlock bb)  // add bb to predecessors, updates bb's successors

   public bBlock getSucc()         // used together to iterate over all successors
   public bBlock getSuccNext() 
control flow graph ( bBlock[] cfg ) The CFG is simply represented by an array of bBlocks. By convention, blocks are kept in the order of the original code for 0..X, with an extra bBlock at X+1 (with 0 instructions) to represent the end of the procedure.


The only file you need to submit is You can submit your project in two ways:
  • Submit using the submit server by clicking on the submit link in the column "web submission".

    Select using the "Browse" button, then press the "Submit project!" button.

  • Submit directly by executing a Java program on a computer with Java and network access. Use the submit.jar file from the archive, To submit, place and submit.jar in the same directory, then execute goSubmit or type the following command directly:

    java -jar submit.jar

    You will be asked to enter your class account and password, then all files in the directory (and its subdirectories) will be put in a jar file and submitted to the submit server. If your submission is successful you will see the message:

    Successful submission # received for project 4

Academic Integrity

Please carefully read the academic honesty section of the course syllabus. In particular, remember that you are not allowed to show or copy any code for your project, either from the web or other students. Any evidence of impermissible cooperation on projects, use of disallowed materials or resources, or unauthorized use of computer accounts, will be submitted to the Student Honor Council, which could result in an XF for the course, or suspension or expulsion from the University. Be sure you understand what you are and what you are not permitted to do in regards to academic integrity when it comes to project assignments. These policies apply to all students, and the Student Honor Council does not consider lack of knowledge of the policies to be a defense for violating them. Full information is found in the course syllabus---please review it at this time.