BUG: Bad Code Generated by Global Optimization

ID: Q236349

The information in this article applies to:

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

SYMPTOMS

For builds that use global optimization (/Og), the condition of an if or while statement in which two pointers are compared and incremented, can appear to be false when in fact the condition is true. This problem occurs only under a fairly narrow set of circumstances. See the sample code in the "More Information" section for an example. Note that both the "maximize speed" (/O2) and "minimize size" (/O1) optimizations are composite optimization switches that include /Og.

CAUSE

The optimizer has incorrectly removed a CMP instruction.

RESOLUTION

Disable the global optimizations for the function, source file, or project in question. Global optimizations can be disabled on a function-by-function basis by using #pragma optimize. Global optimizations can be disabled for a source file (or project) by adding /Og- to the file's (or project's) settings. See "workaround #2" in the "More Information" section for an example of the #pragma approach.

Another workaround is to move the increment (or decrement) of the pointers outside the comparison. See "workaround #3" in the "More Information" section for an example of this approach.

STATUS

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

MORE INFORMATION



 /****************************************************/ 
 /* test.c - demonstrates Global Optimization Problem*/ 
 /* cl /c /FAs /Fa /Og test.c                        */ 
 /****************************************************/ 
// #define WORKAROUND3 // Uncomment this line for workaround #3.

struct MyStruct   {
       unsigned long m_len;
       unsigned char *m_lpData;   };
   typedef struct MyStruct *LPMyStruct;

// #pragma optimize("g",off)  // Uncomment this line for workaround #2.
   short MyStructComp ( struct MyStruct *Arg1,struct MyStruct *Arg2)
   {
       unsigned long dwLen;
       unsigned char *str1 ,  *str2;
       dwLen = Arg1->m_len;
       str1 = Arg1->m_lpData;
       str2 = Arg2->m_lpData;
	   while (dwLen--)
           if (*str1 < *str2)
               return (-1);
#ifndef WORKAROUND3 
		   else if (*(str1++) > *(str2++))
               return (1);
#else
		   else if (*str1 > *str2)
		   {
			   str1++;
			   str2++;
		   }
#endif
       return (0);
 }
// #pragma optimize("g",on) // Uncomment this line for workaround #2.

When the code is compiled with /Og, the MyStructComp function may return 0 (zero); the function should return 1. The code path for the "else if" condition may not be taken, even when the condition is true. To see the reason why, examine the following mixed assembly and source listing, generated by using /FAs /Fa, (hand annotated):


Test.asm generated by cl /c /FAs /Fa /Og Test.c:
------------------------------------------------

; 16   : 	   while (dwLen--)

	mov	edx, edi
	dec	edi
	test	edx, edx
	je	SHORT $L234
	sub	esi, eax
$L233:

; 17   :            if (*str1 < *str2)

	mov	cl, BYTE PTR [eax]
	mov	dl, BYTE PTR [esi+eax]
	cmp	cl, dl
	jb	SHORT $L242

; 19   : #ifndef WORKAROUND3 
; 20   : 		   else if (*(str1++) > *(str2++))

	inc	eax        ;
	ja	SHORT $L243 ; <== Note that this depends upon the zero flag
	mov	ecx, edi    ; remaining from the cmp cl,dl above,  
	dec	edi         ;but the inc eax instruction just before it may 
	test	ecx, ecx    ;have affected the zero flag...
	jne	SHORT $L233
$L234:
	pop	edi

; 22   : #else
; 23   : 		   else if (*str1 > *str2)
; 24   : 		   {
; 25   : 			   str1++;
; 26   : 			   str2++;
; 27   : 		   }
; 28   : #endif
; 29   :        return (0);

	xor	ax, ax
	pop	esi

; 30   :  }

	pop	ebp
	ret	0
$L242:
	pop	edi

; 18   :                return (-1);

	or	ax, -1
	pop	esi

; 30   :  }

	pop	ebp
	ret	0
$L243:
	pop	edi

; 21   :                return (1);

	mov	ax, 1
	pop	esi

; 30   :  }

Additional query words: Og CodeGen

Keywords : kbCodeGen kbVC kbVC600 kbVC600bug kbDSupport
Version : winnt:6.0
Platform : winnt
Issue type : kbbug