Microsoft KB Archive/234511

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

Article ID: 234511

Article Last Modified on 8/7/2007

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APPLIES TO


 * Microsoft Visual C++ 6.0 Enterprise Edition
 * Microsoft Visual C++ 6.0 Professional Edition
 * Microsoft Visual C++ 6.0 Standard Edition

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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.



RESOLUTION
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

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



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 INFO: Visual Studio 6.0 Service packs, What, Where, Why

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



Steps to reproduce the behavior
Compile the following code with /O2 in Visual C++ 6.0:
 * 1) include 
 * 2) include 

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.

Additional query words: sp4 code generation performance slow codegen

Keywords: kbhotfixserver kbqfe kbqfe kbbug kbcompiler kbfix kbvs600sp4fix kbvs600sp5fix KB234511

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