AN3350 datasheet(109/185 Pages) STMICROELECTRONICS

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AN3350

Intermediate optimizations

Doc ID 018512 Rev 2

109/185

For example, in Example 61, the assignment to the variable circ does not refer to the

counter variable of the for loop, i. But the assignment to circ will be executed at each

loop iteration.

Example 62 shows source code that is equivalent to how the compiler would rearrange

instructions after applying this optimization. The compiler has moved the assignment to

circ

outside the for loop so that it is only executed once instead of each time the for loop

iterates.

Example 61 Before loop-invariant code motion

void func_from(float* vec, int max, float val)

{

float circ;

int i;

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

{

circ = val * 2 * PI;

vec[i] = circ;

}

Example 62 After loop-invariant code motion

void func_to(float* vec, int max, float val)

{

float circ;

int i;

circ = val * 2 * PI;

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

{

vec[i] = circ;

}

13.1.8

Strength reduction

Strength reduction attempts to replace slower multiplication operations with faster addition

operations. This optimization improves execution speed but increases code size.

Table 24 explains how to control the optimization for strength reduction.

Table 23.

Controlling loop-invariant code motion

Turn control this option from here...

use this setting

Source code

#pragma opt_loop_invariants on | off | reset

Command line

-opt [no]loop[invariants]

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