AN3350 datasheet(113/185 Pages) STMICROELECTRONICS

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AN3350

Intermediate optimizations

Doc ID 018512 Rev 2

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#pragma dont_inline off

/* Will be inlined, if possible. */

inline int also_attempt_to_inline(void)

{

return 10;

}

Some kinds of functions are never inlined:

●

Functions with variable argument lists

●

Functions defined with __attribute__((never_inline))

●

Functions compiled with #pragma optimize_for_size on or the Optimize For

Size setting in the IDE’s Global Optimizations panel

●

Functions which have their addresses stored in variables

The compiler will not inline these functions, even if they are defined with the inline,

__inline__

, or __inline keywords.

13.2.2

Inlining techniques

The depth of inlining explains how many levels of function calls the compiler will inline. The

Inline Depth setting in the IDE’s C/C++ Language settings panel and the inline_depth

pragma control inlining depth.

Normally, the compiler only inlines an eligible function if it has already translated the

function’s definition. In other words, if an eligible function has not yet been compiled, the

compiler has no object code to insert. To overcome this limitation, the compiler can perform

interprocedural analysis (IPA) either in file or program mode. This lets the compiler evaluate

all the functions in a file or even the entire program before inlining the code. The IPA setting

in the IDE’s C/C++ Language settings panel and the ipa pragma control this capability.

The compiler normally inlines functions from the first function in a chain of function calls to

the last function called. Alternately, the compiler may inline functions from the last function

called to the first function in a chain of function calls. The Bottom-up Inlining option in the

IDE’s C/C++ Language settings panel and the inline_bottom_up and

inline_bottom_up_once

pragmas control this reverse method of inlining.

Some functions that have not been defined with the inline, __inline__, or __inline

keywords may still be good candidates to be inlined. Automatic inlining allows the compiler

to inline these functions in addition to the functions that you explicitly specify as eligible for

inlining. The Auto-Inline option in the IDE’s C/C++ Language panel and the auto_inline

pragma control this capability.

When inlining, the compiler calculates the complexity of a function by counting the number

of statements, operands, and operations in a function to determine whether or not to inline

an eligible function. The compiler does not inline functions that exceed a maximum

complexity. The compiler uses three settings to control the extent of inlined functions:

●

Maximum auto-inlining complexity: the threshold for which a function may be auto-

inlined

●

Maximum complexity: the threshold for which any eligible function may be inlined

●

Maximum total complexity: the threshold for all inlining in a function

The inline_max_auto_size, inline_max_size, and inline_max_total_size

pragmas control these thresholds, respectively.

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