AN3350 datasheet(104/185 Pages) STMICROELECTRONICS

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Intermediate optimizations

AN3350

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Doc ID 018512 Rev 2

func1();

}

func2();

}

Example 50 After dead code elimination

void func_to(void)

{

func2();

}

13.1.2

Expression simplification

The expression simplification optimization attempts to replace arithmetic expressions with

simpler expressions. Additionally, the compiler also looks for operations in expressions that

can be avoided completely without affecting the final outcome of the expression. This

optimization reduces size and increases speed.

Table 18 explains how to control the optimization for expression simplification.

For example, Example 51 contains a few assignments to some arithmetic expressions:

●

Addition to zero

●

Multiplication by a power of 2

●

Subtraction of a value from itself

●

Arithmetic expression with two or more literal values

Example 51 Before expression simplification

void func_from(int* result1, int* result2, int* result3, int*

result4, int x)

{

*result1 = x + 0;

*result2 = x * 2;

*result3 = x - x;

*result4 = 1 + x + 4;

}

Example 52 shows source code that is equivalent to expression simplification. The compiler

has modified these assignments to:

●

Remove the addition to zero

●

Replace the multiplication of a power of 2 with bit-shift operation

●

Replace a subtraction of x from itself with 0

●

Consolidate the additions of 1 and 4 into 5

Table 18.

Controlling expression simplification

Turn control this option from here...

use this setting

Source code

There is no pragma to control this optimization.

Command line

-opt level=1

, -opt level=2, -opt level=3, -opt level=4

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