Conditional Execution

Tom Kelliher, CS 220

Sept. 7, 2007

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Assignment

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From Last Time

Logical and branch instructions.

Outline

1. Compiling HLL control structures.

2. Class assignment.

Coming Up

Intro to Linux.

Compiling HLL Control Structures

Write MIPS code fragments corresponding to the following:

1. Compiling an if:
   

   if (i < 12)
      ++i;
   else
      --j;
   

2. Compiling a loop:
   

   i = 1;
   j = 0;
   while (i < 200)
   {
      j += i;
      i *= i;
   }
   

Class Assignment

Write MIPS code corresponding/solving each of the following:

1. j = 0;
   for (i = 0; i < 10; ++i)
      j += i;
   

2. j = 0;
   for (i = 0; i < 10; ++i)
      if (i > 5)
         j += i;
   

3. while (i > 0 && i < 10)
      ++i;
   

4. if (i < 12 && j > 3 || k != 0)
      ++i;
   else if (i == 33)
      --j;
   else
      k += 2;
   

5. (3.9 from the text) The naive way of compiling

   while (save[i] == k)
      i += k;
   

   requires execution of both a conditional branch and an unconditional jump each time through the loop. Produce the naive code.

   Optimize the naive code so that only a conditional branch is executed each time through the loop.

6. (3.24 from the text, a variation) Write a code segment which takes two ``parameters:''
   1. An ASCII character in $a0.

   2. A pointer to a NULL-terminated string in $a1.

   and ``returns'' a count of the number of occurrences of the character in the string in $v0.

Tom Kelliher