The purpose of this assignment is to help you learn about computer architecture, assembly language programming, and testing strategies. It also will give you the opportunity to learn more about the GNU/Unix programming tools, especially bash, emacs, gcc, gdb, and gprof.
The assignment consists of two parts, each of which has subparts. We encourage you to complete Part 1 by the end of the first week of the assignment period.
Part 2f, as defined below, is the "extra challenge" part of this assignment. While doing the "extra challenge" part of the assignment, you are bound to observe the course policies regarding assignment conduct as given in the course Policies web page, plus one additional policy: you may not use any "human" sources of information. That is, you may not consult with the course's staff members, the lab teaching assistants, other current students via Piazza, or any other people while working on the "extra challenge" part of an assignment, except for clarification of requirements.
The extra challenge part is worth 12 percent of this assignment. So if you don't do any of the "extra challenge" part and all other parts of your assignment solution are perfect and submitted on time, then your grade for the assignment will be 88 percent.
The Unix operating system has a command named wc (word count). In its simplest form, wc reads characters from stdin until end-of-file, and writes to stdout a count of how many lines, words, and characters it has read. A word is a sequence of characters that is delimited by one or more white space characters.
Consider some examples. In the following, a space is shown as s and a newline character as n.
If the file named proverb contains these characters:
Learningsissan treasureswhichn accompaniessitsn ownerseverywhere.n --sChinesesproverbn
then the command:
$ wc < proverb
writes this line to stdout:
5 12 82
If the file proverb2 contains these characters:
Learningsissan treasureswhichn accompaniessitsn ownerseverywhere.n --sssChinesesproverb
(note that the last "line" does not end with a newline character) then the command:
$ wc < proverb2
writes this line to stdout:
4 12 83
The file mywc.c in the /u/cos217/Assignment4 directory contains a C program that implements the subset of the wc command described above. Translate that program into assembly language, thus creating a file named mywc.s. It is acceptable to use global (i.e. bss section or data section) variables in mywc.s, but we encourage you to use local (i.e. stack) variables instead. Your assembly language program should have exactly the same behavior (i.e. should write exactly the same characters to stdout) as the given C program.
Design a test plan for your mywc program. Your test plan should include tests in three categories: (1) boundary testing, (2) statement testing, and (3) stress testing.
Create text files to test your programs. Name each such file such that its prefix is mywc and its suffix is .txt. The command ls mywc*.txt should display the names of all mywc test files, and only those files.
Describe your mywc test plan in your readme file. Your description should have this structure:
mywc boundary tests:
mywcXXX.txt: Description of the characteristics of that file, and how it tests boundary conditions of your mywc program.
mywcYYY.txt: Description of the characteristics of that file, and how it tests boundary conditions of your mywc program.
...
mywc statement tests:
mywcXXX.txt: Description of the characteristics of that file, and which statements of your mywc program it tests. Refer to the statements using the line numbers of the given mywc.c program.
mywcYYY.txt: Description of the characteristics of that file, and which statements of your mywc program it tests. Refer to the statements using the line numbers of the given mywc.c program.
...
Your descriptions of the test files should be of the form "This file contains such-and-such characteristics, and so tests lines such-and-such of the program." Please identify the lines of code tested by line numbers. The line numbers should refer to the given C code.
mywc stress tests:
mywcXXX.txt: Description of the characteristics of that file, and how it stress tests your mywc program.
mywcYYY.txt: Description of the characteristics of that file, and how it stress tests your mywc program.
...
Submit no more than three large test files. Make sure that each large test file contains no more than 50000 characters. Submitting files containing more than 50000 characters could exhaust the course's allotted disk space on nobel, and so could prohibit other students from submitting any files at all. Also make sure that each large test file contains no more than 1000 lines. It would be difficult for your grader to scroll through a file that contains more than 1000 lines.
In a more realistic context, your test files should contain non-ASCII characters. However, in the context of this assignment, submit test files that contain only printable ASCII characters. Specifically, make sure your computer-generated test files contain only characters having ASCII codes (in hexadecimal) 09, 0A, and 20 through 7E. It would be difficult for your grader to examine files that contain other characters.
The nobel /u/cos217/Assignment4 directory contains bash shell scripts named testmywc and testmywcdiff that automate your testing. Comments at the beginning of those files describe how to use them. After copying the scripts to your project directory, you may need to execute the commands chmod 700 testmywc and chmod 700 testmywcdiff to give them "executable" permissions.
Many programming environments contain modules to handle high-precision integer arithmetic. For example, the Java Development Kit (JDK) contains the BigDecimal and BigInteger classes.
The Fibonacci numbers are used often in computer science. See http://en.wikipedia.org/wiki/Fibonacci_numbers for some background information. Note in particular that Fibonacci numbers can be very large integers.
This part of the assignment asks you to use assembly language to write a minimal but fast high-precision integer arithmetic module, and use it to compute large Fibonacci numbers.
BigInt Objects Using C CodeSuppose you must compute Fibonacci number 500000, that is, fib(500000)...
The /u/cos217/Assignment4 directory contains a C program that computes Fibonacci numbers. It consists of two modules: a client module and a BigInt ADT.
The client consists of the file fib.c. The client accepts an integer x as a command-line argument, where x must be a non-negative integer. The client performs some boundary condition and stress tests, writing the results to stdout. Then it computes and writes fib(x) to stdout as a hexadecimal number. It writes to stderr the amount of CPU time consumed while performing the computation. The client module delegates most of the work to BigInt objects.
The BigInt ADT performs high precision integer arithmetic. It is a minimal ADT; essentially it implements only an "add" operation. The BigInt ADT consists of four files:
bigint.h is the interface. Note that the ADT makes eight functions available to clients: BigInt_new, BigInt_free, BigInt_assignFromHexString, BigInt_largest, BigInt_random, BigInt_writeHex, BigInt_writeHexAbbrev, and BigInt_add.
bigint.c contains implementations of the BigInt_new, BigInt_free, BigInt_assignFromHexString, BigInt_largest, BigInt_random, BigInt_writeHex, and BigInt_writeHexAbbrev functions.
bigintadd.c contains an implementation of the BigInt_add function.
bigintprivate.h is a "private header file" -- private in the sense that clients never use it. It allows code sharing between the two implementation files, bigint.c and bigintadd.c.
Study the given code. Then build a fib program consisting of the files fib.c, bigint.c, and bigintadd.c, with no optimizations. Run the program to compute fib(500000). In your readme file note the amount of CPU time consumed.
BigInt Objects Using C Code Built with Compiler OptimizationSuppose you decide that the amount of CPU time consumed is unacceptably large. You decide to command the compiler to optimize the code that it produces...
Build the fib program using optimization. Specifically, specify the -D NDEBUG option so the preprocessor disables the assert macro, and the -O3 option so the compiler generates optimized code. Run the resulting program to compute fib(500000). In your readme file note the amount of CPU time consumed.
Suppose you decide that the amount of CPU time consumed still is too large. You decide to investigate by doing a gprof analysis to determine which functions are consuming the most time...
Perform a gprof analysis of the code from Part 2b. Store the textual report in a file named gprofreport. Save the file; as described later in this document, you should submit that file.
BigInt Objects Using Assembly Language CodeSuppose, not surprisingly, your gprof analysis shows that most CPU time is spent executing the BigInt_add function. In an attempt to gain speed, you decide to code the BigInt_add function manually in assembly language..
Manually translate the C code in the bigintadd.c file into assembly language, thus creating the file bigintadd.s. You should not translate the code in other files into assembly language.
Your assembly language code should store all local variables defined in the BigInt_larger and BigInt_add functions in memory, on the stack.
Note that assert is a parameterized macro, not a function. (See Section 14.3 of the King book for a description of parameterized macros.) So assembly language code cannot call assert. When translating bigintadd.c to assembly language, simply pretend that the calls of assert are not in the C code.
Build a fib program consisting of the files fib.c, bigint.c, and bigintadd.s using the -D NDEBUG and -O3 options. Run the program to compute fib(x) for various values of x, and make sure it writes the same output to stdout as the program built from C code does. Finally, run the program to compute fib(500000). In your readme file note the amount of CPU time consumed.
BigInt Objects Using Optimized Assembly Language CodeSuppose, to your horror, you discover that you have taken a step backward: the CPU time consumed by your assembly language code is approximately the same as that of the non-optimized compiler-generated code! So you decide to optimize your assembly language code...
Manually optimize your assembly language code in bigintadd.s, thus creating the file bigintaddopt.s. Specifically, perform these optimizations:
Store local variables (but not parameters) in registers instead of in memory. Remember that EBX, ESI, and EDI are callee-save registers, and that EAX, ECX, and EDX are caller-save registers.
"Inline" the call of the BigInt_larger function. That is, eliminate the BigInt_larger function, placing its code within the BigInt_add function.
Build a fib program consisting of the files fib.c, bigint.c, and bigintaddopt.s using the -D NDEBUG and -O3 options. Run the program to compute fib(x) for various values of x, and make sure it writes the same output to stdout as the program built from C code does. Finally, run the program to compute fib(500000). In your readme file note the amount of CPU time consumed.
Can you write assembly language code that is approximately as fast as the optimized code that the compiler generates? That is, can you approximately tie the compiler?
BigInt Objects Using Highly Optimized Assembly Language CodeFinally, suppose you decide to optimize your assembly language code even further, moving away from a statement-by-statement translation of C code into assembly language...
Further optimize your assembly language code in bigintaddopt.s, thus creating the file bigintaddoptopt.s. Specifically, perform these optimizations:
Factor as much code as possible out of the loop.
Use the assembly language loop patterns described in Section 3.6.5 of the Bryant & O'Hallaron book instead of the simpler but less efficient patterns described in precepts.
Instead of using a uiCarry variable (stored in memory or in a register) to keep track of carries during addition, use the "carry" bit in the EFLAGS register. The carry bit is examined and set by the adcl ("add with carry long") instruction.
Feel free to implement any additional optimizations you want. However, your BigInt_add function must be a general-purpose function for adding two given BigInt objects; the function cannot be specific to the task of adding two Fibonacci numbers to generate a third Fibonacci number. In other words, your function must work with the given fib.c client.
This part is difficult; we will not think unkindly of you if you decide not to do it. To do it properly you will need to learn about the adcl instruction, and about which instructions affect and do not affect the C ("carry") flag in the EFLAGS register.
Hint: When writing bigintaddoptopt.s, the problem is this: How can you preserve the value of the C flag between executions of the adcl instruction? One solution is to save and then restore the value of the EFLAGS register. Another solution is to express the logic such that only instructions that do not affect the C flag are executed between each execution of the adcl instruction.
Build a fib program consisting of the files fib.c, bigint.c, and bigintaddoptopt.s using the -D NDEBUG and -O3 options. Run the program to compute fib(x) for various values of x, and make sure it writes the same output to stdout as the program built from C code does. Finally, run the program to compute fib(500000). In your readme file note the amount of CPU time consumed.
Can you beat the compiler?
Develop on nobel. Use emacs to create source code. Use gdb to debug. Use gprof to generate an execution profile.
Do not use a C compiler to produce any of your assembly language code. Doing so would be considered academically dishonest. Instead produce your assembly language code manually.
We encourage you to develop "flattened" C code (as described in precepts) to bridge the gap between the given "normal" C code and your assembly language code. Using flattened C code as a bridge can eliminate logic errors from your assembly language code, leaving only the possibility of translation errors.
We also encourage you to use your flattened C code as comments in your assembly language code. Such comments can clarify your assembly language code substantially.
Create a readme file by copying the file /u/cos217/Assignment4/readme to your project directory, and editing the copy by replacing each area marked "<Complete this section.>" as appropriate.
Submit your work electronically on nobel using these commands:
submit 4 mywc.s submit 4 mywc*.txt submit 4 gprofreport submit 4 bigintadd.s submit 4 bigintaddopt.s submit 4 bigintaddoptopt.s submit 4 readme
We will grade your code on quality from the user's and programmer's points of view. From the user's point of view, your code has quality if it behaves as it should. The correct behavior of your code is defined by the previous sections of this assignment specification.
From the programmer's point of view, your code has quality if it is well-styled and thereby easy to maintain. Comments in your assembly language code are especially important. Each assembly language function -- especially the main function -- should have a comment that describes what the function does. Local comments within your assembly language functions are equally important. Comments copied from corresponding flattened C code are particularly helpful.
Your assembly language code should use .equ directives to avoid "magic numbers." In particular, you should use .equ directives to give meaningful names to:
Enumerated constants, for example: .equ TRUE, 1.
Parameter stack offsets, for example: .equ OADDEND1, 8.
Local variable stack offsets, for example: .equ UICARRY, -4.
Structure field offsets, for example: .equ ILENGTH, 0.
Stack offsets at which callee-save registers are stored, for example: .equ EBXOFFSET, -4.
Also, your bigintaddopt.s should use .equ directives to give meaningful names to registers that store local variables, for example, .equ REG_UICARRY, %edx. Those names should begin with REG_. (Otherwise they appear to be ordinary labels.)
To encourage good coding practices, we will deduct points if gcc217 generates warning messages.
Testing is a substantial aspect of the assignment. Approximately 10% of the grade will be based upon your mywc test plan as described in your readme file and as implemented by your data files.
Your grade for Part 2f will be based upon:
Raw performance (6 percent). That is, how quickly does your code compute fib(500000)? We can't award any raw performance points unless your code passes all tests in the fib.c client.
Style (6 percent). That is, does your code contain optimizations that logically should improve performance?
This assignment was written by Robert M. Dondero, Jr.,
based in part upon suggestions from Andrew Appel,
and with contributions by other faculty members.