Difference between revisions of "Inline"

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{{inline}}
 
{{inline}}
  
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The [[modifier]] <syntaxhighlight lang="pascal" inline>inline</syntaxhighlight> requests the [[FPC]] to consider copying the definition of a [[Routine|routine]] to the call site (“inlining”).
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The modifier <syntaxhighlight lang="pascal" inline>noinline</syntaxhighlight> prevents the FPC from ever inlining a routine, even automatically (since {{gitlab|commit|FPC|503ea604f33b5a7dd72d7a6417f9a38774f19263|SVN revision 41198}}, as of 2022 only available in Trunk).
  
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Return to [[Reserved words]].
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== use ==
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The use of <syntaxhighlight lang="pascal" inline>inline</syntaxhighlight> routines is switched off by default.
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You can enable it with the <syntaxhighlight lang="bash" inline>‑Si</syntaxhighlight> compiler switch or the <syntaxhighlight lang="delphi" inline>{$inline on}</syntaxhighlight> [[local compiler directives|local compiler directive]].
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The reserved word '''inline''' allows the compiler to copy a function or procedure in place of its call.
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The <syntaxhighlight lang="pascal" inline>inline</syntaxhighlight> directive is placed after a routine’s signature at its defining point.
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Example:
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<syntaxhighlight lang="pascal" highlight="1">
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function cube(const x: ALUSInt): ALUSInt; inline;
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begin
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cube := sqr(x) * x;
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end;
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</syntaxhighlight>
  
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If the inline function or procedure is used frequently, this increases the speed of the program since the program does not have to branch to the subroutine.
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== implications ==
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Inlining means that a routine’s implementation exists at ''multiple'' places in the final [[Executable program|executable file]].
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There is no single address the program jumps to, but every time you invoke that routine there is dedicated copy in the source code.
  
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The use of inline functions and inline procedures is switched off as standard. The compiler switch '''-Si''' or '''{$inline on}''' enables the use of inline functions and procedures.
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=== advantages ===
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* Avoid call overhead for frequently invoked routines. This ''could'' increase the speed of the program.
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* Elimination of an extra level of indirection in the case of [[Variable parameter|parameters passed by reference]].
  
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{{Warning| inline is a compiler hint and the compiler can ignore it. If the compiler warns you it has not inlined a certain code part marked as inline, you should remove the inline directive. This is not a bug. It is about code complexity}}
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=== disadvantages ===
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* More difficult to debug: There is no extra frame on the stack indicating the subroutine.
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* Inlining requires space. You will necessarily have numerous copies of the same code at many places.
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* It is not possible to “fine tune” the use of <syntaxhighlight lang="pascal" inline>inline</syntaxhighlight>: You cannot ask for inlining ''just'' at specific places (e.&#8239;g. in a loop).
  
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== caveats ==
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* <syntaxhighlight lang="pascal" inline>inline</syntaxhighlight> is a compiler ''hint''. The compiler can ignore it. If the compiler warns you it has not inlined a certain code part marked as <syntaxhighlight lang="pascal" inline>inline</syntaxhighlight>, you should remove the <syntaxhighlight lang="pascal" inline>inline</syntaxhighlight> directive. This is not a bug; it is about code complexity.
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* Recursive routines cannot be inlined.
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* Regardless of the <syntaxhighlight lang="pascal" inline>inline</syntaxhighlight> request ''there is'' one instance the routine exists in memory like usual.
  
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Example:
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== application ==
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If you want to optimize for speed, consider trying [[Optimization|<syntaxhighlight lang="delphi" inline>{$optimization autoInline}</syntaxhighlight>]] first, before manually adding <syntaxhighlight lang="pascal" inline>inline</syntaxhighlight> hints.
  
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<syntaxhighlight lang=pascal>
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== see also ==
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procedure subDemo(); inline;
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* [https://www.freepascal.org/docs-html/ref/refsu77.html <syntaxhighlight lang="pascal" inline>inline</syntaxhighlight>] in the FPC Reference Guide
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  begin
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* [https://www.freepascal.org/docs-html/prog/progsu36.html <syntaxhighlight lang="text" inline>$INLINE</syntaxhighlight>: Allow inline code] in the FPC Programmers’ Guide
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  ...
 
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  end;
 
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</syntaxhighlight>
 

Latest revision as of 14:22, 8 November 2023

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The modifier inline requests the FPC to consider copying the definition of a routine to the call site (“inlining”). The modifier noinline prevents the FPC from ever inlining a routine, even automatically (since SVN revision 41198, as of 2022 only available in Trunk).

use

The use of inline routines is switched off by default. You can enable it with the ‑Si compiler switch or the {$inline on} local compiler directive. The inline directive is placed after a routine’s signature at its defining point. Example: 

function cube(const x: ALUSInt): ALUSInt; inline;
begin
	cube := sqr(x) * x;
end;

implications

Inlining means that a routine’s implementation exists at multiple places in the final executable file. There is no single address the program jumps to, but every time you invoke that routine there is dedicated copy in the source code.

advantages

  • Avoid call overhead for frequently invoked routines. This could increase the speed of the program.
  • Elimination of an extra level of indirection in the case of parameters passed by reference.

disadvantages

  • More difficult to debug: There is no extra frame on the stack indicating the subroutine.
  • Inlining requires space. You will necessarily have numerous copies of the same code at many places.
  • It is not possible to “fine tune” the use of inline: You cannot ask for inlining just at specific places (e. g. in a loop).

caveats

  • inline is a compiler hint. The compiler can ignore it. If the compiler warns you it has not inlined a certain code part marked as inline, you should remove the inline directive. This is not a bug; it is about code complexity.
  • Recursive routines cannot be inlined.
  • Regardless of the inline request there is one instance the routine exists in memory like usual.

application

If you want to optimize for speed, consider trying {$optimization autoInline} first, before manually adding inline hints.

see also

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