Exercise 3 - Winter 2012 (Answers)

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1. A function f has the following overall structure:

   
   int f(unsigned x)
   {
     int n = 0;
     while( _______ /* A.*/) {
       __________;  /* B.*/
      
       __________;  /* C. */
   
     }
     return _____;  /* D. */
   }
   	    

   The gcc C compiler generates the following assembly code:

   
   f:
           pushl   %ebp
           xorl    %eax, %eax
           movl    %esp, %ebp
           movl    8(%ebp), %edx
           testl   %edx, %edx
           je      .L4
   .L5:
           addl    $1, %eax
           shrl    %edx
           jne     .L5
   .L4:
           popl    %ebp
           ret
   

   Determine the register usage.

   That is, determine which register is used for variable n and which register is used for variable x.

   Using this register to variable correspondence, fill in the 4 missing parts of the C code.

   Answers:

   1. A.
   2. B.
   3. C.
   4. D.

2. What value would be returned by each function call below if the IA32 assembly code for f is:

   f:
           pushl   %ebp
           movl    %esp, %ebp
           movl    8(%ebp), %edx
           cmpl    $10, %edx
           jle     .L2	   
           leal    -2(%edx), %eax
           jmp     .L4
   .L2:
           leal    1(%edx,%edx,2), %eax
   .L4:
           popl    %ebp
           ret
   	  

   Function Call	Return value
   f(12)
   f(2)

3. Consider the following struct declarations on an IA32 machine.

   Assume the alignment rules for Windows. That is, each primitive data type (char, short, int, float, double) of size X must begin at an address that is a multiple of X.

   Assume

   • sizeof(char) = 1
   • sizeof(short) = 2
   • sizeof(int) = 4
   • sizeof(double) = 8
   • sizeof(any pointer type) = 4

   For each struct declaration, fill in

   • the offset in bytes from the beginning of the struct for each member field
   • number of padding bytes (0 or more) after each member field
   • the total number of bytes allocated for the struct
   • the alignment for the struct; that is, what its beginning address must be a multiple of: 1, 2, 4, or 8

   Example

    struct p
    {
      char ch1;
      char *str;
      char ch2;
    };
   	

   member	offset	member size	padding (0 if none)
   ch1	0	1	3
   str	4	4	0
   ch2	8	1	3

   total struct size	12
   struct alignment (1,2,4,or 8)	4

   1.  struct p1
       {
         char ch;
         short s;
         int n;
       };
      	

      member	offset	member size	padding (0 if none)
      ch
      s
      n

      total struct size
      struct alignment (1,2,4,or 8)

   2. struct p2
      {
        char ch;
        int n;
        short s;
      };
      	

      member	offset	member size	padding (0 if none)
      ch
      n
      s

      total struct size
      struct alignment (1,2,4,or 8)

   3. 
      struct p3
      {
        int *ip;
        double d;
        char ch;
      };
      	  

      member	offset	member size	padding (0 if none)
      ip
      d
      ch

      total struct size
      struct alignment (1,2,4,or 8)