Computer Science - C language

[joe111]

Can anyone help me with my assignment please. thank you

 

 

 

Consider the following function that forms part of a C program that has

been written to measure the execution time of a simple arithmetic

statement like sum = sum + increment;

long sumLoop(long increment, long iterations)

{

long i, sum=0;

for (i=0 ; i < iterations ; i++) {

sum = sum + increment;

}

return (sum);

}

The following (numbered) lines of IA-32 (in GAS format) assembly

language corresponding to the above function have been generated

when the C program was compiled without optimisation:

10 sumLoop:

11 pushl %ebp

12 movl %esp, %ebp

13 subl $8, %esp

14 movl $0, -8(%ebp)

15 movl $0, -4(%ebp)

16 .L7:

17 movl -4(%ebp), %eax

18 cmpl 12(%ebp), %eax

19 jl .L10

20 jmp .L8

21 .L10:

22 movl 8(%ebp), %eax

23 leal -8(%ebp), %edx

24 addl %eax, (%edx)

25 leal -4(%ebp), %eax

26 incl (%eax)

27 jmp .L7

28 .L8:

29 movl -8(%ebp), %eax

30 leave

31 ret

Where appropriate please refer to these corresponding line number(s)

in your answers to the following questions:

(a) Explain the purpose and/or operation of these assembly language

statements in the context of this C function. Little credit will be given to

answers that merely repeat what is obvious from each statement. For

example, don't answer "pushes register ebp onto the stack" for line 11.

(10 marks)

(b) Outline a technique that a C programmer might use in an attempt to

measure the execution time of a single sum = sum + increment;

statement with a high degree of accuracy. Identify the issues in the

technique you propose that may lead to inaccuracies in timing and

discuss briefly how you would overcome these.

(5 marks)

© When the program is recompiled with optimisation (the –O3 compiler

switch) the programmer discovers that the sumLoop () function runs

faster and that the execution time of a sum = sum + increment;

statement drops to 1 nanosecond in the optimised code from the

original measurement of 5 nanoseconds in the non-optimised version

of the program. Give an explanation of the optimisations that are likely

to have been made by the compiler in order to achieve this fivefold

improvement in performance and compare and contrast these

optimisations with the original assembly code in the above listing.

(5 marks)

Edited March 27, 2013 by joe111

[Phi for All]

Please give some indications that you understand the questions, and show us what work you've done so far on each question. Teachers don't want sites like ours giving out answers without trying to help you understand the questions.

[Ben Banana]

Hey, the first thing you need to start working on is understanding how the registers correspond to the C program. Here are some resources:

 

http://www.swansontec.com/sregisters.html

http://cs.smith.edu/~thiebaut/ArtOfAssembly/CH03/CH03-3.html#HEADING3-4

 

Good luck.