Q236119: FIX: Optimization Causes Code Error In Conditional Statement

Article: Q236119
Product(s): Microsoft C Compiler
Version(s): 5.0,6.0
Operating System(s): 
Keyword(s): kbCodeGen kbVC500bug kbVC600bug kbDSupport kbGrpDSVCCompiler kbNoUpdate kbgrpdsvc
Last Modified: 11-FEB-2002

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The information in this article applies to:

- Microsoft Visual C++, 32-bit Enterprise Edition, versions 5.0, 6.0 
- Microsoft Visual C++, 32-bit Professional Edition, versions 5.0, 6.0 
- Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
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SYMPTOMS
========

The optimizer may generate incorrect code for an if statement that contains
identical expressions in both conditional statements.

CAUSE
=====

The optimizer makes a mistake when trying to move common sub-expressions to a
single location.

RESOLUTION
==========

There are three potential ways to work around this bug:

1. Turn off global optimization for that function.

2. Make the function inline.

3. Move the common expression before the if block.

STATUS
======

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

This bug was corrected in Visual Studio 6 Service Pack 4.

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

The following code demonstrates the problem and resolutions 1 and 3.

  // compiler options: cl /Og

  #include <iostream>

  static long llll = 0;

  double dTry1(double d) {
   
     if (d < 0.0) {
          llll++;
          return(0.0);
     } else {
          llll++;
          return(100.0);
     }
  }

  #pragma optimize("g",off)
  double dTry2(double d) {

     if (d < 0.0) {
          llll++;
          return(0.0);
     } else {
          llll++;
          return(100.0);
     }
  }
  #pragma optimize("",on)

  double dTry3(double d) {

     llll++;
     if (d < 0.0)
        return(0.0);
     else
        return(100.0);
  }

  void main(){
     using namespace std;
     cout << "Result with Optimizer: " << dTry1(100.0) <<endl;
     cout << "Result without Optimizer: " << dTry2(100.0) <<endl;
     cout << "Result with moved sub-expression: " << dTry3(100.0) <<endl;

  }

Additional query words:

======================================================================
Keywords          : kbCodeGen kbVC500bug kbVC600bug kbDSupport kbGrpDSVCCompiler kbNoUpdate kbgrpdsvc 
Technology        : kbVCsearch kbAudDeveloper kbVC500 kbVC600 kbVC32bitSearch kbVC500Search
Version           : :5.0,6.0
Issue type        : kbbug
Solution Type     : kbfix

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