AN3350 datasheet(108/185 Pages) STMICROELECTRONICS

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Intermediate optimizations

AN3350

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Doc ID 018512 Rev 2

For example, in Example 59 three variables, a, b, and c, are defined. Although each

variable is eventually used, each of their uses is exclusive to the others. In other words, a is

not referred to in the same expressions as b or c, b is not referred to with a or c, and c is

not used with a or b.

In Example 60, the compiler has replaced a, b, and c, with a single variable. This

optimization reduces the number of registers that the object code uses to store variables,

allowing more variables to be stored in registers instead of slower memory. This optimization

also reduces a function’s stack memory.

Example 59 Before live range splitting

void func_from(int x, int y)

{

int a;

int b;

int c;

a = x * y;

otherfunc(a);

b = x + y;

otherfunc(b);

c = x - y;

otherfunc(c);

}

Example 60 After live range splitting

void func_to(int x, int y)

{

int a_b_or_c;

a_b_or_c = x * y;

otherfunc(temp);

a_b_or_c = x + y;

otherfunc(temp);

a_b_or_c = x - y;

otherfunc(temp);

}

13.1.7

Loop-invariant code motion

Loop-invariant code motion moves expressions out of a loop if the expressions are not

affected by the loop or the loop does not affect the expression. This optimization improves

execution speed.

Table 23 explains how to control the optimization for loop-invariant code motion.

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