AN3350 datasheet(111/185 Pages) STMICROELECTRONICS

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AN3350

Intermediate optimizations

Doc ID 018512 Rev 2

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For example, in Example 65, the for loop’s body is a single call to a function,

otherfunc()

. For each time the loop’s completion test executes

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

the function executes the loop body only once.

In Example 66, the compiler has inserted another copy of the loop body and rearranged the

loop to ensure that variable i is incremented properly. With this arrangement, the loop’s

completion test executes once for every 2 times that the loop body executes.

Example 65 Before loop unrolling

const int MAX = 100;

void func_from(int* vec)

{

int i;

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

{

otherfunc(vec[i]);

}

Example 66 After loop unrolling

const int MAX = 100;

void func_to(int* vec)

{

int i;

for (i = 0; i < MAX;)

{

otherfunc(vec[i]);

++i;

otherfunc(vec[i]);

++i;

}

13.2

Inlining

Inlining replaces instructions that call a function and return from it with the actual

instructions of the function being called. Inlining functions makes your program faster

because it executes the function code immediately without the overhead of a function call

and return. However, inlining can also make your program larger because the compiler may

insert the function’s instructions many times throughout your program.

Table 25.

Controlling loop unrolling

Turn control this option from here...

use this setting

Source code

#pragma opt_unroll_loops on | off | reset

Command line

-opt level=3, -opt level=4

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