FIX: Incorrect Code Generated with /Og and /Oa or /Ow

View products that this article applies to.

This article was previously published under Q195376

SYMPTOMS

The compiler might generate incorrect code when building with /Og (Global Optimizations) and /Oa (Assume No Aliasing) or /Og and /Ow (Assume Cross-function Aliasing).

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RESOLUTION

Turn off /Oa and /Ow to work around the bug.

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Microsoft has confirmed that this is a bug in the Microsoft products that are listed at the beginning of this article. This bug was corrected in Visual Studio 6.0 Service Pack 3. For more information about Visual Studio service packs, please see the following articles in the Microsoft Knowledge Base:

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

194295 (http://support.microsoft.com/kb/194295/EN-US/ ) HOWTO: Tell That Visual Studio 6.0 Service Packs Are Installed

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MORE INFORMATION

Please refer to Help for more information about the compiler options /Og, /Oa, and /Ow.

The following sample demonstrates the bug. In this case, the problem appears to be related to the final attempt to access an array in a function when that access involves doubling the variable used to index it.

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Sample Code

   //compiler options : cl /Og /Oa (or /Ow)
   #include <stdio.h>
   typedef struct C
   {
     float x,y,z;
     long  index;
     }C;

   float gdIn[3]={1.0,2.0,3.0};

   void init(C *gc)
   {
     float fd[3]={1.0,2.0,3.0};
     long   i = gc->index;
     float *pfd = gdIn;
     gc->x = *(pfd+0);
     gc->y = *(pfd+i);
     gc->z = *(pfd+i*2);
   }

   int main()
   {
     C c;
     c.index = 1;
     init(&c);
     printf("Should be 3.0, is %f \n", c.z);
     return 0;
   }

The program output should be: 3.0.

The program output is: 2.000000.

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