Q67078: Different Warnings Appear Using Different Optimizations

Article: Q67078
Product(s): Microsoft C Compiler
Version(s): winnt:
Operating System(s): 
Keyword(s): kbcode kbCompiler
Last Modified: 22-JUL-2001

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The information in this article applies to:

- The C/C++ Compiler (CL.EXE), included with:
   - Microsoft C for MS-DOS, versions 6.0, 6.0a, 6.0ax 
   - Microsoft C for OS/2, versions 6.0, 6.0a 
   - Microsoft C/C++ for MS-DOS, version 7.0 
   - Microsoft Visual C++, versions 1.0, 1.5, 1.51, 1.52 
   - *EDITOR Please do not choose this product*Microsoft Visual C++ 32-bit Edition* use 241, 265, 225, versions 1.0, 2.0, 2.1, 2.2 
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SUMMARY
=======

The information in this article is included in the documentation starting with
Visual C++ 5.0. Look there for future revisions.

In Microsoft C/C++ compiler, different warning messages can appear during
separate compilations of the same code when different optimizations are used.
This is normal behavior that occurs because some warnings are found only while
optimizing.

MORE INFORMATION
================

If the sample code below is compiled with /W4 (all warnings) and the /Os
optimization (optimize for space), no warnings are found. However, when the code
is compiled with /W4 and /Osle (optimize for size, loop, and global-register
allocation), the following warning is generated:

  file.c(7) : warning C4202: unreachable code

With Visual C++ for Windows NT versions 1.0, 2.0, and 2.1, as well as Visual C++
for Windows version 1.5 and 1.51, the following warning is generated when
compiled with /W4 and /O2:

  file.c(7) : warning C4702: unreachable code

There are several other optimizations and code examples that will exhibit this
type of behavior. Once again, this is normal and serves to notify the developer
that there MAY be a problem.

Sample Code
-----------

    #include <stdio.h>

    void main(void)
    {
         goto bottom;

         printf("Not used\n");    /* Code that is not used */ 

  bottom:printf("Hello World!");
    }

Additional query words: 6.00 6.00a 6.00ax 7.00 8.00 8.00c 1.00 1.50 1.51 1.52 2.00 2.10 4.00

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Keywords          : kbcode kbCompiler 
Technology        : kbVCsearch kbAudDeveloper kbCVCComp
Version           : winnt:

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