AN3350 datasheet(110/185 Pages) STMICROELECTRONICS

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Intermediate optimizations

AN3350

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Doc ID 018512 Rev 2

For example, in Example 63, the assignment to elements of the vec array use a

multiplication operation that refers to the for loop’s counter variable, i.

In Example 64, the compiler has replaced the multiplication operation with a hidden variable

that is increased by an equivalent addition operation. Processors execute addition

operations faster than multiplication operations.

Example 63 Before strength reduction

void func_from(int* vec, int max, int fac)

{

int i;

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

{

vec[i] = fac * i;

}

Example 64 After strength reduction

void func_to(int* vec, int max, int fac)

{

int i;

int strength_red;

hidden_strength_red = 0;

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

{

vec[i] = hidden_strength_red;

hidden_strength_red = hidden_strength_red + i;

}

13.1.9

Loop unrolling

Loop unrolling inserts extra copies of a loop’s body in a loop to reduce processor time

executing a loop’s overhead instructions for each iteration of the loop body. In other words,

this optimization attempts to reduce the ratio of time that the processor executes a loop’s

completion test and branching instructions compared to the time the processor executes the

loop’s body. This optimization improves execution speed but increases code size.

Table 25 explains how to control the optimization for loop unrolling.

Table 24.

Controlling strength reduction

Turn control this option from here...

use this setting

Source code

#pragma opt_strength_reduction on | off | reset

Command line

-opt [no]strength

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