AN3350 datasheet(106/185 Pages) STMICROELECTRONICS

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Intermediate optimizations

AN3350

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Doc ID 018512 Rev 2

vec[temp - 1] = value;

}

13.1.4

Copy propagation

Copy propagation replaces variables with their original values if the variables do not change.

This optimization reduces runtime stack size and improves execution speed.

Table 20 explains how to control the optimization for copy propagation.

For example, in Example 55 , the variable j is assigned the value of x. But j’s value is

never changed, so the compiler replaces later instances of j with x, as shown in Example

56.

By propagating x, the compiler is able to reduce the number of registers it uses to hold

variable values, allowing more variables to be stored in registers instead of slower memory.

Also, this optimization reduces the amount of stack memory used during function calls.

Example 55 Before copy propagation

void func_from(int* a, int x)

{

int i;

int j;

j = x;

for (i = 0; i < j; i++)

{

a[i] = j;

}

Example 56 After copy propagation

void func_to(int* a, int x)

{

int i;

int j;

j = x;

for (i = 0; i < x; i++)

{

a[i] = x;

}

Table 20.

Controlling copy propagation

Turn control this option from here...

use this setting

Source code

#pragma opt_propagation on | off | reset

Command line

-opt [no]prop[agation]

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