FIX: Optimization Causes Code Generation Error in a Conditional Statement (236119)


The information in this article applies to:

  • Microsoft Visual C++, 32-bit Enterprise Edition 5.0
  • Microsoft Visual C++, 32-bit Enterprise Edition 6.0
  • Microsoft Visual C++, 32-bit Professional Edition 5.0
  • Microsoft Visual C++, 32-bit Professional Edition 6.0
  • Microsoft Visual C++, 32-bit Learning Edition 6.0
This article was previously published under Q236119

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 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 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;

}
Modification Type:MajorLast Reviewed:12/1/2003
Keywords:kbBug kbCodeGen kbfix kbNoUpdate KB236119
©2003 Microsoft Corporation. All rights reserved.