Expressions

If components of an expression are duplicate expressions, code them either at the left end of the expression or within parentheses. For example:

a = b*(x*y*z);       /* Duplicates recognized */
c = x*y*z*d;
e = f + (x + y);
g = x + y + h;
a = b*x*y*z;         /* No duplicates recognized */
c = x*y*z*d;
e = f + x + y;
g = x + y + h;

When components of an expression in a loop are constant, code the expressions either at the left end of the expression, or within parentheses. If c, d, and e are constant and v, w, and x are variable, the following examples show the difference in evaluation:

v*w*x*(c*d*e);         /* Loop invariant expressions recognized    */
c + d + e + v + w + x;
v*w*x*c*d*e;           /* Optimization required for constant */
v + w + x + c + d + e; /* expressions to be recognized       */

For integer expressions, the loop invariant expression will be recognized if -O is specified. For floating-point expressions, the loop invariant expression will be recognized if -O3 is specified.

Program Optimization with IBM C and C++ Compilers

Writing Optimized Code...
    Overview
    Variables
    Pointers
    Functions
    Function Arguments
    Critical Loops
    Conversions
    Arithmetic Conversions
    Inlined Components