Common Subexpression Elimination

Common subexpression elimination replaces multiple instances of the same expression with a single instance. This optimization reduces size and increases execution speed.

Controlling common subexpression elimination explains how to control the optimization for common subexpression elimination.

Table 1. Controlling common subexpression elimination
Turn control this option from here...	use this setting
CodeWarrior IDE	Choose an Optimization Level value `(2,3,4)` from the Properties > C/C++ Build > Settings > Tool Settings > PowerPC Compiler > Optimization panel.
source code	#pragma opt_common_subs on \| off \| reset
command line	`-opt [no]cse`

For example, in Before common subexepression elimination , the subexpression x * y occurs twice.

Figure 1. Before common subexepression elimination

void func_from(int* vec, int size, int x, int y, int value)
{
    if (x * y < size)
    {
        vec[x * y - 1] = value;
    }
}

After common subexpression elimination shows equivalent source code after the compiler applies common subexpression elimination. The compiler generates instructions to compute x * y and store it in a hidden, temporary variable. The compiler then replaces each instance of the subexpression with this variable.

Figure 2. After common subexpression elimination

void func_to(int* vec, int size, int x, int y, int value)
{
    int temp = x * y;
    if (temp < size)
    {
        vec[temp - 1] = value;
    }
}