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44 C and C++ Source-Level Optimizations Chapter 2
25112 Rev. 3.06 September 2005
Software Optimization Guide for AMD64 Processors
2.21 Frequently Dereferenced Pointer Arguments
Optimization
Avoid dereferenced pointer arguments inside a function.
Application
This optimization applies to:
• 32-bit software
• 64-bit software
Rationale
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.
Examples
Listing 16. Avoid
// 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;
}