Avoiding Loop Rotation By Hand - Analog Devices VISUALDSP++ 3.5 Manual

Loop Guidelines
xc = xa + xb; yc = ya + yb;
c[i] = xc; c[i+1] = yc;
}
}
Bad: harder for the compiler to optimize.

Avoiding Loop Rotation by Hand

Tip: Do not rotate loops by hand.
Programmers are often tempted to "rotate" loops in DSP code by "hand"
attempting to execute loads and stores from earlier or future iterations at
the same time as computation from the current iteration. This technique
introduces loop-carried dependencies that prevent the compiler from rear-
ranging the code effectively. However, it is better to give the compiler a
"normalized" version, and leave the rotation to the compiler.
int ss(short *a, short *b, int n) {
int sum = 0;
int i;
for (i = 0; i < n; i++) {
sum += a[i] + b[i];
}
return sum;
}
Good: will be rotated by the compiler.
int ss(short *a, short *b, int n) {
short ta, tb;
int sum = 0;
int i = 0;
ta = a[i]; tb = b[i];
for (i = 1; i < n; i++) {
sum += ta + tb;
ta = a[i]; tb = b[i];
}
sum += ta + tb;
2-18 VisualDSP++ 3.5 C/C++ Compiler and Library Manual
for ADSP-219x DSPs