FIX: Incorrect Optimization of Left Shift Operator

ID: Q113427

The information in this article applies to:

Microsoft C/C++ Compiler (CL.EXE), included with:
- Microsoft Visual C++ 32-bit Edition, version 1.0

SYMPTOMS

Optimization of the left shift operator may generate incorrect code in loop operations when compiled with global optimizations on. In the code sample below, the "for" loop incorrectly evaluates the "count" variable. The resultant code causes the loop to execute only once with the following output:

0

RESOLUTION

Turn off global optimizations (/Og). In the code sample below, turning off global optimizations will correctly output:

0
40000000
80000000
C0000000

Global optimizations can be disabled by excluding /Og from the command-line switches or by the use of a pragma:


   #pragma optimize ("g", off)

STATUS

Microsoft has confirmed this to be a problem in the Microsoft C/C++ 32-bit compiler, version 8.0, which is included with Visual C++ 32-bit Edition. This problem was corrected in C/C++ compiler version 9.0, included with Visual C++ 32-bit Edition, version 2.0

Sample Code


/*
* Compile options needed: /Og
*/ 

#include <stdio.h>

void main( void )
{
    unsigned count;
     for ( count = 0 ; count < 4 ; count++ )
         printf(" %X \n", count << 30);
}

Additional query words: 1.00 8.00

Keywords : kbCompiler
Version : :1.0
Platform : NT WINDOWS
Issue type :