Q234511: FIX: VC++ 6.0 Compiler Does Not Remove Unneeded Instructions
Article: Q234511
Product(s): Microsoft C Compiler
Version(s): 6.0
Operating System(s):
Keyword(s): kbCompiler kbVC600QFE kbDSupport kbVS600sp4fix kbVS600sp5fix
Last Modified: 20-MAY-2002
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The information in this article applies to:
- Microsoft Visual C++, 32-bit Enterprise Edition, version 6.0
- Microsoft Visual C++, 32-bit Professional Edition, version 6.0
- Microsoft Visual C++, 32-bit Learning Edition, version 6.0
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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, please go to the following
address on the World Wide Web:
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:
+------------------------------------------------------------------+
| Name | Size | Date | Time | Version# | Platform |
+------------------------------------------------------------------+
| c1xx.dll | 1,169KB | 6/4/99 | 11:01 PM | 12.00.8520.0 | x86 |
+------------------------------------------------------------------+
STATUS
======
Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article. This bug was corrected in the next service pack for
Visual Studio 6.0.
For additional information about Visual Studio service packs, click the article
numbers below to view the articles in the Microsoft Knowledge Base:
Q194022 INFO: Visual Studio 6.0 Service Packs, What, Where, Why
Q194295 HOWTO: Tell That a Visual Studio Service Pack Is Installed
You can download the latest Visual Studio service pack from the following
Microsoft Web site:
Visual Studio Product Updates
(http://msdn.microsoft.com/vstudio/downloads/updates.asp)
MORE INFORMATION
================
Steps to Reproduce 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.
Additional query words: sp4 code generation performance slow codegen
======================================================================
Keywords : kbCompiler kbVC600QFE kbDSupport kbVS600sp4fix kbVS600sp5fix
Technology : kbVCsearch kbAudDeveloper kbVC600 kbVC32bitSearch
Version : :6.0
Issue type : kbbug
Solution Type : kbfix
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