CS 301 - Homework 6

1. Speed this program up by eliminating the divide. Keep the loop in place.

   int divby=128;
   int foo(void) {
   	unsigned int i, sum=0, val=1024;
   	for (i=0;i<val;i++) sum+=i>>7; /* FIX: shift is same as dividing by 128, but way cheaper */
   	return sum;
   }
   

2. Figure out why this is slow, and speed it up. You must leave the loop in place; but anything else is fair game.

   int get_thingy(void); /* prototype for routine below */
   inline int get_thingy(void) { /* FIX: declare subroutine *above* use, and make it "inline" */
   	return 2;
   }
   
   int foo(void) {
   	int i, sum=0, val=1024;
   	for (i=0;i<val;i++) sum+=get_thingy();
   	return sum;
   }
   
   

3. Here's a C++ performance pitfall I've seen too many times. Don't mess with my_class.
   

   class my_class {
   public:
   	int val;
   	my_class() {val=1;}
   private:
   	char scratch_data[0x54321];
   };
   
   int some_function(my_class &c /* FIX: pass by reference! */,int d) {
   	return c.val+d;
   }
   
   int foo(void) {
   	my_class c;
   	return some_function(c,0x1234+sizeof(my_class));
   }

O. Lawlor, ffosl@uaf.edu
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