BUG: /Og Generates Bad Code for a Compare/Branch

ID: Q234602

The information in this article applies to:

Microsoft Visual C++, 32-bit Editions, version 6.0

SYMPTOMS

When using the /Og compiler switch for a compare/branch that targets another compare/branch such as the following:


if (!count--) return 0;
if (!count) return 1;

The code generated for the second statement above may be incorrectly removed by the compiler.

CAUSE

This is caused by a bug in C2.dll.

RESOLUTION

As a workaround, the problem can be avoided by modifying the code so that post-decrement is performed after the first compare/branch, such as:
```
if (!count) return 0;
count--;
if (!count) return 1; 
```
The correct code is generated even with /Og optimization.

Alternatively, 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/support/supportnet/default.asp

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

Name Size Date Time Version# Platform

c2.dll 721KB 6/18/99 10:14 PM 12.00.8534.0 x86

NOTE: If this product was already installed on your computer when you purchased it from the Original Equipment Manufacturer (OEM) and you need this fix, please call the Pay Per Incident number listed on the above Web site. If you contact Microsoft to obtain this fix, a fee may be charged. This fee is refundable if it is determined that you only require the fix you requested. However, this fee is non-refundable if you request additional technical support, if your no-charge technical support period has expired, or if you are not eligible for standard no-charge technical support.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article.

MORE INFORMATION

The sample code below can be used to reproduce the problem.


int main(int count)
{
    do 
    {
        if (count > 10) return -1;

        /* start - code snippet */ 

        if (!count--) return 0;
        /* Missing instructions for the below statement with /Og */ 
        if (!count) return 1;

        /* end - code snippet */ 

    } while (1);
}

When compiled with /Og, the following incorrect assembly instructions are generated:


; 7    : 		/* start - code snippet */ 
; 8    : 
; 9    : 		if (!count--) return 0;

  0000b	8b c8		 mov	 ecx, eax
  0000d	48		 dec	 eax
  0000e	85 c9		 test	 ecx, ecx
  00010	74 0a		 je	 SHORT $L44

; 3    : 	do 
; 4    : 	{
; 5    : 		if (count > 10) return -1;

  00012	83 f8 0a	 cmp	 eax, 10			; 0000000aH
  00015	7e f4		 jle	 SHORT $L33
$L43:
  00017	83 c8 ff	 or	 eax, -1

; 10   : 		/* Missing instructions for the below statement with /Og */ 
; 11   : 		if (!count) return 1;
; 12   : 
; 13   : 		/* end - code snippet */<BR/>

The assembly instruction for line eleven above is incorrectly removed in the above optimized code.

The instructions generated with /Od are given as follows:


; 7    : 		/* start - code snippet */ 
; 8    : 
; 9    : 		if (!count--) return 0;

  0000e	8b 45 08	 mov	 eax, DWORD PTR _count$[ebp]
  00011	8b 4d 08	 mov	 ecx, DWORD PTR _count$[ebp]
  00014	83 e9 01	 sub	 ecx, 1
  00017	89 4d 08	 mov	 DWORD PTR _count$[ebp], ecx
  0001a	85 c0		 test	 eax, eax
  0001c	75 04		 jne	 SHORT $L37
  0001e	33 c0		 xor	 eax, eax
  00020	eb 16		 jmp	 SHORT $L35
$L37:

; 10   : 		/* Missing instructions for the below statement with /Og */ 
; 11   : 		if (!count) return 1;

  00022	83 7d 08 00	 cmp	 DWORD PTR _count$[ebp], 0
  00026	75 07		 jne	 SHORT $L38
  00028	b8 01 00 00 00	 mov	 eax, 1
  0002d	eb 09		 jmp	 SHORT $L35
$L38:

; 12   : 
; 13   : 		/* end - code snippet */

The assembly instructions for line eleven above is not removed, which is the correct behavior.

By using resolution one (workaround) of moving post-decrement to a separate statement, the correct code is generated even with /Og optimization, as show here:


; 7    : 		/* start - code snippet */ 
; 8    : 
; 9    : 		if (!count) return 0;

  0000b	85 c0		 test	 eax, eax
  0000d	74 0d		 je	 SHORT $L44

; 10   : 		count--;  /* workaround */ 

  0000f	48		 dec	 eax

; 11   : 		/* No missing instructions for the below statement with /Og */ 
; 12   : 		if (!count) return 1;

  00010	74 0e		 je	 SHORT $L45

; 3    : 	do 
; 4    : 	{
; 5    : 		if (count > 10) return -1;

  00012	83 f8 0a	 cmp	 eax, 10			; 0000000aH
  00015	7e f4		 jle	 SHORT $L33
$L43:
  00017	83 c8 ff	 or	 eax, -1

; 13   : 
; 14   : 		/* end - code snippet */

By using resolution two (fix) of the fixed C2.dll, the correct code is generated with /Og optimization, as shown here:


; 7    : 		/* start - code snippet */ 
; 8    : 
; 9    : 		if (!count--) return 0;

  0000b	8b c8		 mov	 ecx, eax
  0000d	48		 dec	 eax
  0000e	85 c9		 test	 ecx, ecx
  00010	74 0e		 je	 SHORT $L44

; 10   : 		/* No missing instructions for the below statement with /Og */ 
; 11   : 		if (!count) return 1;

  00012	85 c0		 test	 eax, eax
  00014	74 0e		 je	 SHORT $L45

; 3    : 	do 
; 4    : 	{
; 5    : 		if (count > 10) return -1;

  00016	83 f8 0a	 cmp	 eax, 10			; 0000000aH
  00019	7e f0		 jle	 SHORT $L33
$L43:
  0001b	83 c8 ff	 or	 eax, -1

; 12   : 
; 13   : 		/* end - code snippet */

Additional query words: /Og assembly code removed

Keywords : kbCodeGen kbCompiler kbVC600QFE kbDSupport kbGrpVCCompiler
Version : winnt:6.0
Platform : winnt
Issue type : kbbug