Q250887: BUG: Bad Code Generated by Global Optimizations w/ Bitwise Shift

Article: Q250887
Product(s): Microsoft C Compiler
Version(s): 6.0SP3
Operating System(s): 
Keyword(s): kbCodeGen kbCompiler kbVC600 kbVS600sp3bug kbDSupport kbGrpDSVCCompiler
Last Modified: 07-MAY-2001

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The information in this article applies to:

- Microsoft Visual C++, 32-bit Enterprise Edition, version 6.0sp3 
- Microsoft Visual C++, 32-bit Professional Edition, version 6.0sp3 
- Microsoft Visual C++, 32-bit Learning Edition, version 6.0sp3 
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SYMPTOMS
========

For builds that use global optimizations (/Og), an if statement may never be
executed when the bitwise shift operator, >> or <<, is called after
the if statement.

NOTE: 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
==========

To resolve this problem, disable the global optimizations.

Global optimizations can be turned off on a function-by-function basis by using
the #pragma optimize("g",off) directive.

Global optimizations can also be turned off by adding /Og- to a source file or to
a project's settings.

STATUS
======

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

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

Steps to Reproduce Behavior
---------------------------

  //test.cpp
  // Compiler options are cl /GX /Og test.cpp.

  #include <stdio.h>

  //Uncomment the following line to turn off global optimizations:
  //#pragma optimize("g",off) 
  void ShiftValues (const char * pEncoded)
  {
  	size_t nCount = 0;
  	size_t nBits = 0;
  	size_t x = 0;
  	while (pEncoded[nCount] != '=')
  	{
  		nBits += 6;
  		printf("added 6, nBits now %d\n", nBits);
  		if (nBits >= 8)
  		{
  			printf("%x\n", (0 << (nBits - 8)));
  			nBits -= 8;
  			printf("subtracted 8, nBits now %d\n", nBits);
  		}
  		nCount++;
  	}
  }
  // Uncomment the following line to turn global optimizations back on:
  //#pragma optimize("g",on) 

  void main()
  {
  	ShiftValues ("AA=");
  }

  Correct output with global optimizations turn off:<BR/>
  added 6, nBits now 6
  added 6, nBits now 12
  0
  subtracted 8, nBits now 4

  Incorrect output with global optimizations turn on:
  added 6, nBits now 6
  0
  subtracted 8, nBits now -2
  added 6, nBits now 4
  0
  subtracted 8, nBits now -4

Additional query words:

======================================================================
Keywords          : kbCodeGen kbCompiler kbVC600 kbVS600sp3bug kbDSupport kbGrpDSVCCompiler 
Technology        : kbVCsearch kbAudDeveloper kbVC32bitSearch kbVC600SP3
Version           : :6.0SP3
Issue type        : kbbug

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