Computer Science 211 Lab: Optimization of Assembly Language Code

Optimization of Assembly Language Code

Goals: This laboratory investigates the assembly code generated by the pc and gcc compilers. The lab asks you to compare the code generated when the compiler tries to optimize code.

Resource: As with the previous lab, you will want to refer to some or all of the following references.

Intel's Pentium III Manuals:
Note: The order of instructions operands for Intel assembly is reversed from the order used for AT&T Pentium Assembly -- as used by the gcc compiler and as assembler!
- IA-32 Intel® Architecture Software Developer's Manual, Volume 1: Basic Architecture (pdf format: 4,005K bytes, 422 pages)
- IA-32 Intel® Architecture Software Developer's Manual, Volume 2: Instruction Set Reference (pdf format: 6,593 Kbytes, 964 pages)
- IA-32 Intel® Architecture Software Developer's Manual, Volume 3: System Programming Guide (pdf format: 6,269 Kbytes, 696 pages)
- Intel® Pentium® 4 and Intel® Xeon^TM Processor Optimization (pdf format: 1,505 Kbytes, 333 pages)
Randall Hyde, The Art of Assembly Language Programming.
Available from Dominique Thiébaut at Smitt College or from a somewhat more general site maintained by Randall Hyde.
Dean Elsner, Jay Fenlason, and friends, Using as : The GNU Assembler, http://www.gnu.org/manual/gas-2.9.1/html_node/as_toc.html

Preparation: Before the laboratory session, read the C program FahrToCelsius which is found as file table.c in directory ~walker/assembly/pentium.

Steps for this Lab:

Focusing on this C program, compile the program to assembly to determine what code is produced using the gcc compiler.
Compile table.c to assembly language using the gcc compiler, and discuss how double precision floating point computations are handled.
- What registers are used for the addresses of the floating point numbers?
- What registers are used when performing floating point operations?
- What floating point instructions are performed, and how are those operations specified?
The gcc compiler allows you to request one of several levels of optimization of the code, using the -O option. E.g., for level 2 optimization, type:
```
     gcc -S -O2 table.c
```
Compare the non-optimized and optimized code generated by the compiler you selected. For example, you should observe the nature of any changes between the two assembly language listings. Are registers used differently? Are some instructions omitted? Are different types of instructions used? Is one listing significantly shorter?
Also, in addition to the above discussion, for both your non-optimized and optimized code:
- draw a picture of the registers and show which registers are used for what purposes,
- draw a picture of the run-time stack to show what values are stored where (relative to esp and/or ebp), and
- discuss how the floating point arithmetic is handled.

Note: In answering the above questions, you need not annotate the assembly language programs. In making comparisons or in discussing approaches, you may find it helpful to comment upon specific instructions or code segments, but there is no need to annotate the entire assembly language code.

Work to be turned in:

Steps 2 and 3.

This document is available on the World Wide Web as

     http://www.cs.grinnell.edu/~walker/courses/211.fa01/labs/lab.code-optimization.html

created October 1, 1997 last revised October 15, 2002


For more information, please contact Henry M. Walker at walker@cs.grinnell.edu.