FIX: Visual C++ 6.0 compiler does not remove certain unneeded temporary instructions

View products that this article applies to.

This article was previously published under Q234511

SYMPTOMS

The Visual C++ 6.0 compiler does not remove certain unneeded temporary instructions, which can slow down code execution.

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A supported fix for Visual C++ 6.0 that corrects this problem is now available from Microsoft, but it has not been fully regression tested and should be applied only to systems experiencing this specific problem.

To resolve this problem immediately, contact Microsoft Product Support Services to obtain the fix. For a complete list of Microsoft Product Support Services phone numbers and information on support costs, visit the following Web site:

http://support.microsoft.com/default.aspx?scid=fh;EN-US;CNTACTMS (http://support.microsoft.com/default.aspx?scid=fh;en-us;cntactms)

The English version of this fix should have the following file attributes or later.

Collapse this tableExpand this table

Name	Size	Date	Time	Version#	Platform
c1xx.dll	1,169KB	6/4/99	11:01 PM	12.00.8520.0	x86

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STATUS

Microsoft has confirmed that this is a bug in the Microsoft products that are listed in the "Applies to" section. This bug was corrected in the next service pack for Visual Studio 6.0.

For more information about Visual Studio service packs, click the following article numbers to view the articles in the Microsoft Knowledge Base:

194022 (http://support.microsoft.com/kb/194022/ ) INFO: Visual Studio 6.0 Service packs, What, Where, Why

194295 (http://support.microsoft.com/kb/194295/ ) HOWTO: Tell That a Visual Studio service Pack Is Installed

To download the latest Visual Studio service pack, visit the following Microsoft Web site:

http://msdn2.microsoft.com/en-us/vstudio/Aa718353.aspx (http://msdn2.microsoft.com/en-us/vstudio/Aa718353.aspx)

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MORE INFORMATION

Steps to reproduce the behavior

Compile the following code with /O2 in Visual C++ 6.0:

#include <time.h>
#include <stdio.h> 

class boo
{
 	int a;
 	int b;
 	int c;
 	int d;

public:
 	boo():
	  a(0), b(0), c(0), d(0) {}

 	boo(int x):
	  a(x), b(x), c(x), d(x) {}
};

boo fun(int x)
{
 	if (x)
 		return boo(x);
 	else
 		return boo();
}

void main()
{
	time_t   start, finish; 
	double   elapsed_time;
         
 	int i;
	 time( &start );
 	for (i=0; i<200000000; i++)
 	{
 		fun(0);
 	}
 	for (i=0; i<200000000; i++)
 	{
 		fun(10);
 	}
	time( &finish );
	elapsed_time = difftime( finish, start );
	printf( "\nProgram takes %6.0f seconds.\n", elapsed_time );
}

And compare the following snippet of the generated code:

; 21   :  	if (x)

	mov	ecx, DWORD PTR _x$[esp-4]
	test	ecx, ecx
	je	SHORT $L622

; 22   :  		return boo(x);

	mov	eax, DWORD PTR ___$ReturnUdt$[esp-4]
	mov	edx, eax
	mov	DWORD PTR [edx], ecx
	mov	DWORD PTR [edx+4], ecx
	mov	DWORD PTR [edx+8], ecx
	mov	DWORD PTR [edx+12], ecx

; 25   : }

	ret	0
$L622:

; 23   :  	else
; 24   :  		return boo();

	mov	eax, DWORD PTR ___$ReturnUdt$[esp-4]
	push	ebx
	mov	ebx, eax
	xor	ecx, ecx
	xor	edx, edx
	push	esi
	mov	DWORD PTR [ebx], ecx
	xor	esi, esi
	push	edi
	xor	edi, edi
	mov	DWORD PTR [ebx+4], edx
	mov	DWORD PTR [ebx+8], esi
	mov	DWORD PTR [ebx+12], edi
	pop	edi
	pop	esi
	pop	ebx

; 25   : }

	ret	0

With that generated by Visual C++ 5.0 when compiled with /O2:

; 21   :  	if (x)

	mov	ecx, DWORD PTR _x$[esp-4]

; 22   :  		return boo(x);

	mov	eax, DWORD PTR ___$ReturnUdt$[esp-4]
	xor	edx, edx
	cmp	ecx, edx
	je	SHORT $L547
	mov	DWORD PTR [eax], ecx
	mov	DWORD PTR [eax+4], ecx
	mov	DWORD PTR [eax+8], ecx
	mov	DWORD PTR [eax+12], ecx

; 25   : }

	ret	0
$L547:

; 23   :  	else
; 24   :  		return boo();

	mov	DWORD PTR [eax], edx
	mov	DWORD PTR [eax+4], edx
	mov	DWORD PTR [eax+8], edx
	mov	DWORD PTR [eax+12], edx

; 25   : }

	ret	0

The Visual C++ 6.0 version of the code contains more instructions, which can slow down program execution. The executable built with Visual C++ 6.0 runs slower than the one built with Visual C++ 5.0 on a comparable system.

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