BUG: Optimization Causes Code Generation Error In a Conditional Statement

• Microsoft Visual C++, 32-bit Editions, versions 5.0, 6.0

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 3 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.

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 kbGrpVCCompiler
Version : winnt:5.0,6.0
Platform : winnt
Issue type : kbbug