AN3350 datasheet(107/185 Pages) STMICROELECTRONICS

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AN3350

Intermediate optimizations

Doc ID 018512 Rev 2

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13.1.5

Dead store elimination

Dead store elimination removes unused assignment statements. This optimization reduces

size and improves speed.

Table 21 explains how to control the optimization for dead store elimination.

For example, in Example 57 the variable x is first assigned the value of y * y. However,

this result is not used before x is assigned the result returned by a call to getresult().

In Example 58 the compiler can safely remove the first assignment to x since the result of

this assignment is never used.

Example 57 Before dead store elimination

void func_from(int x, int y)

{

x = y * y;

otherfunc1(y);

x = getresult();

otherfunc2(y);

}

Example 58 After dead store elimination

void func_to(int x, int y)

{

otherfunc1(y);

x = getresult();

otherfunc2(y);

}

13.1.6

Live range splitting

Live range splitting attempts to reduce the number of variables used in a function. This

optimization reduces a function’s runtime stack size, requiring fewer instructions to invoke

the function. This optimization potentially improves execution speed.

Table 22 explains how to control the optimization for live range splitting.

Table 21.

Controlling dead store elimination

Turn control this option from here...

use this setting

source code

#pragma opt_dead_assignments on | off | reset

command line

-opt [no]deadstore

Table 22.

Controlling live range splitting

Turn control this option from here...

use this setting

Source code

There is no pragma to control this optimization.

Command line

-opt level=3, -opt level=4

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