Справочник Пользователя для AMD 250
154
Scheduling Optimizations
Chapter 7
25112
Rev. 3.06
September 2005
Software Optimization Guide for AMD64 Processors
7.6
Pointer Arithmetic in Loops
Optimization
Minimize pointer arithmetic in loops, especially if the loop bodies are small. Take advantage of
scaled-index addressing modes to utilize the loop counter as an index into memory arrays.
scaled-index addressing modes to utilize the loop counter as an index into memory arrays.
Application
This optimization applies to:
•
32-bit software
•
64-bit software
Rationale
In small loops, pointer arithmetic causes significant overhead. Using scaled-index addressing modes
has no negative impact on execution speed, but the reduced number of instructions preserves decode
bandwidth.
has no negative impact on execution speed, but the reduced number of instructions preserves decode
bandwidth.
Example
Consider the following C code, which adds the elements of two arrays and stores them in a third
array:
array:
int a[MAXSIZE], b[MAXSIZE], c[MAXSIZE], i;
for (i = 0; i < MAXSIZE; i++) {
c[i] = a[i] + b[i];
}