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AN3350

Intermediate optimizations

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Example 52 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;

}

13.1.3

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.

Table 19 explains how to control the optimization for common subexpression elimination.

For example, in Example 53, the subexpression x * y occurs twice.

Example 53 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;

}

Example 54 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.

Example 54 After common subexpression elimination

void func_to(int* vec, int size, int x, int y, int value)

{

int temp = x * y;

if (temp < size)

{

Table 19.

Controlling common subexpression elimination

Turn control this option from here...

use this setting

Source code

#pragma opt_common_subs on | off | reset

Command line

-opt [no]cse

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