When does the compiler listen to the inline specifier

**Dang** · 09-16-2001

This simple program does not expand the inline functions. After the code there is a listing of masm and source code showing its not inlining. Rather it calling the function doin' the whole epilog then function then prolog rather than expanding the code in place. I've tried without the inline specifier which defaults to inline when defined in the class (which it is).

Code:

#include "stdafx.h"

class tstring
{

	long m_nTStringLength;

public:

	inline tstring( int nLength = 100 ) : m_nTStringLength( nLength )
	{	}

	inline int GetTString( )
	{
		return m_nTStringLength;
	}
};

int main(int argc, char* argv[])
{

	tstring t1;

	long i = t1.GetTString( );

	return 0;

}

Generates:

Code:

	TITLE	C:\Practice\tstringTest\tstringTest.cpp
PUBLIC	??0tstring@@QAE@H@Z				; tstring::tstring
PUBLIC	?GetTString@tstring@@QAEHXZ			; tstring::GetTString
PUBLIC	_main
EXTRN	__chkesp:NEAR
;	COMDAT _main
_TEXT	SEGMENT
_t1$ = -4
_i$ = -8
_main	PROC NEAR					; COMDAT

; 26   : {

	push	ebp
	mov	ebp, esp
	sub	esp, 72					; 00000048H
	push	ebx
	push	esi
	push	edi
	lea	edi, DWORD PTR [ebp-72]
	mov	ecx, 18					; 00000012H
	mov	eax, -858993460				; ccccccccH
	rep stosd

; 27   : 
; 28   : 	tstring t1;

	push	100					; 00000064H
	lea	ecx, DWORD PTR _t1$[ebp]
	call	??0tstring@@QAE@H@Z			; tstring::tstring

; 29   : 
; 30   : 	long i = t1.GetTString( );

	lea	ecx, DWORD PTR _t1$[ebp]
	call	?GetTString@tstring@@QAEHXZ		; tstring::GetTString
	mov	DWORD PTR _i$[ebp], eax

; 31   : 
; 32   : 	return 0;

	xor	eax, eax

; 33   : 
; 34   : }

	pop	edi
	pop	esi
	pop	ebx
	add	esp, 72					; 00000048H
	cmp	ebp, esp
	call	__chkesp
	mov	esp, ebp
	pop	ebp
	ret	0
_main	ENDP
_TEXT	ENDS
;	COMDAT ??0tstring@@QAE@H@Z
_TEXT	SEGMENT
_nLength$ = 8
_this$ = -4
??0tstring@@QAE@H@Z PROC NEAR				; tstring::tstring, COMDAT

; 17   : 	{	}

	push	ebp
	mov	ebp, esp
	sub	esp, 68					; 00000044H
	push	ebx
	push	esi
	push	edi
	push	ecx
	lea	edi, DWORD PTR [ebp-68]
	mov	ecx, 17					; 00000011H
	mov	eax, -858993460				; ccccccccH
	rep stosd
	pop	ecx
	mov	DWORD PTR _this$[ebp], ecx
	mov	eax, DWORD PTR _this$[ebp]
	mov	ecx, DWORD PTR _nLength$[ebp]
	mov	DWORD PTR [eax], ecx
	mov	eax, DWORD PTR _this$[ebp]
	pop	edi
	pop	esi
	pop	ebx
	mov	esp, ebp
	pop	ebp
	ret	4
??0tstring@@QAE@H@Z ENDP				; tstring::tstring
_TEXT	ENDS
;	COMDAT ?GetTString@tstring@@QAEHXZ
_TEXT	SEGMENT
_this$ = -4
?GetTString@tstring@@QAEHXZ PROC NEAR			; tstring::GetTString, COMDAT

; 20   : 	{

	push	ebp
	mov	ebp, esp
	sub	esp, 68					; 00000044H
	push	ebx
	push	esi
	push	edi
	push	ecx
	lea	edi, DWORD PTR [ebp-68]
	mov	ecx, 17					; 00000011H
	mov	eax, -858993460				; ccccccccH
	rep stosd
	pop	ecx
	mov	DWORD PTR _this$[ebp], ecx

; 21   : 		return m_nTStringLength;

	mov	eax, DWORD PTR _this$[ebp]
	mov	eax, DWORD PTR [eax]

; 22   : 	}

	pop	edi
	pop	esi
	pop	ebx
	mov	esp, ebp
	pop	ebp
	ret	0
?GetTString@tstring@@QAEHXZ ENDP			; tstring::GetTString
_TEXT	ENDS
END

thanks

dang

**Zach L.** · 09-16-2001

It should be inlining those functions. The only thing I can think of that you might try is to make 'm_nTStringLength' an int so that no type-casting goes on in those functions.

**Dang** · 09-16-2001

Thanks, but still won't inline. I've even fooled around with the /obx compiler optimization command for inline expansion and no luck.

thanks

dang

**Stoned_Coder** · 09-16-2001

if you are using visual c (and i guess you are) then check out __forceinline.

inline, __inline, __forceinline
inline

function_declarator; // C++ Specific

__inline

function_declarator;// Microsoft Specific

__forceinline

function_declarator;// Microsoft Specific

The inline and __inline keywords allow the compiler to insert a copy of the function body into each place the function is called. The insertion occurs only if the compiler's cost/benefit analysis show it to be profitable. The __forceinline keyword overrides the cost/benefit analysis and relies on the judgement of the programmer instead. Exercise caution when using __forceinline. Indiscriminate use of __forceinline can result in larger code with only marginal performance gains or, in some cases, even performance losses (due to increased paging of a larger executable, for example).

You cannot force the compiler to inline a function when conditions other than cost/benefit analysis prevent it. You cannot inline a function if:

The function or its caller is compiled with /Ob0 (the default option for debug builds).

The function and the caller use different types of exception handling (C++ exception handling in one, structured exception handling in the other).

The function has a variable argument list.

The function uses inline assembly and is not compiled with /Og, /Ox, /O1, or /O2).

Function returns an unwindable object by value and is not compiled with /GX, /EHs, or /EHa).

The function receives a copy-constructed object passed by value, when compiled with /GX, /EHs,, or /EHa.

The function is recursive and is not accompanied by #pragma(inline_recursion, on). With the pragma, recursive functions can be inlined to a default depth of eight calls. To change the inlining depth, use #pragma(inline_depth, n).
If the compiler cannot inline a function declared __forceinline, it generates a level 1 warning (4714).

The inline keyword is available only in C++. The __inline and __forceinline keywords are available in both C and C++. For compatibility with previous versions, _inline is a synonym for __inline.

Using inline functions can make your program faster because they eliminate the overhead associated with function calls. Functions expanded inline are subject to code optimizations not available to normal functions.

The /Ob compiler optimization option determines whether inline function expansion actually occurs.

For related information, see auto_inline.

Example 1

inline int max( int a , int b )
{
if( a > b ) return a;
return b;
}
A class's member functions can be declared inline either by using the inline keyword or by placing the function definition within the class definition.

Example 2

class MyClass
{
public:
void print() { cout << i << ''; } // Implicitly inline
private:
int i;
};

**Dang** · 09-16-2001

Thank Stoned_Coder that did the trick.

Thanks alot!

dang

**zen** · 09-17-2001

Are you compiling the debug build? The compiler can't inline functions if the /ZI compiler option is enabled. I compiled your code in release build and by changing the debug option to /Zi (and enabled inling in optimisations) and in both cases even without the inline specifiers in the source code the functions were inlined in the asm output.

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