AMD 250 Manuale Utente

25112

Rev. 3.06

September 2005

Avoid dereferenced pointer arguments inside a function. 

This optimization applies to:

•

32-bit software

•

64-bit software

Because the compiler has no knowledge of whether aliasing exists between the pointers, such 
dereferencing cannot be “optimized away” by the compiler. Since data may not be maintained in 
registers, memory traffic can significantly increase.

Many compilers have an “assume no aliasing” optimization switch. This allows the compiler to 
assume that two different pointers always have disjoint contents and does not require copying of 
pointer arguments to local variables. If your compiler does not have this type of optimization, then 
copy the data pointed to by the pointer arguments to local variables at the start of the function and if 
necessary copy them back at the end of the function.

// Assumes pointers are different and q != r.

void isqrt(unsigned long a, unsigned long *q, unsigned long *r) {

   *q = a;

   if (a > 0) {

      while (*q > (*r = a / *q)) {

         *q = (*q + *r) >> 1;

      }

   }

   *r = a - *q * *q;

}