Expression Simplification

The expression simplification optimization attempts to replace arithmetic expressions with simpler expressions. Additionally, the compiler also looks for operations in expressions that can be avoided completely without affecting the final outcome of the expression. This optimization reduces size and increases speed.

Controlling expression simplification explains how to control the optimization for expression simplification.

Table 1. Controlling expression simplification
Turn control this option from here...	use this setting
CodeWarrior IDE	Choose an Optimization Level value `(1,2,3,4)` from the Properties > C/C++ Build > Settings > Tool Settings > PowerPC Compiler > Optimization panel.
source code	There is no pragma to control this optimization.
command line	`-opt level=1` , `-opt level=2` , `-opt level=3` , `-opt level=4`

For example, Before expression simplification contains a few assignments to some arithmetic expressions:

addition to zero
multiplication by a power of 2
subtraction of a value from itself
arithmetic expression with two or more literal values
Figure 1. Before expression simplification

void func_from(int* result1, int* result2, int* result3, int* result4, 
int x)
{
    *result1 = x + 0;
    *result2 = x * 2;
    *result3 = x - x;
    *result4 = 1 + x + 4;
}

After expression simplification shows source code that is equivalent to expression simplification. The compiler has modified these assignments to:

remove the addition to zero
replace the multiplication of a power of 2 with bit-shift operation
replace a subtraction of x from itself with 0
consolidate the additions of 1 and 4 into 5
Figure 2. After expression simplification

void func_to(int* result1, int* result2, int* result3, int* result4, 
int x)
{
    *result1 = x;
    *result2 = x << 1;
    *result3 = 0;
    *result4 = 5 + x;
}