Q122265: BUG: Incorrect Calculations with Both /Og and /G5

Article: Q122265
Product(s): Microsoft C Compiler
Version(s): 1.0,2.0,2.1
Operating System(s): 
Keyword(s): 
Last Modified: 29-JUL-2001

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The information in this article applies to:

- Microsoft Visual C++, versions 1.0, 2.0, 2.1 
- Microsoft C/C++ Compiler (CL.EXE) 
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SYMPTOMS
========

Code compiled with both /G5 and the /Og options generate incorrect floating
point results.

RESOLUTION
==========

To work around the problem, do not use both the /G5 and the /Og compiler
switches together; use either /G5 or /Og. Alternatively, you can disable the
global optimization (provided by /Og) for the area of code generated incorrectly
with the #pragma optimize.

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article. We are researching this bug and will post new
information here in the Microsoft Knowledge Base as it becomes available.

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

The /G5 compiler switch tells the compiler to place the assembly instructions
generated into an order that will be more beneficial to the pipelined
architecture of the Pentium processor. The /Og switch enables global
optimizations.

The following sample code illustrates the problem. Remove the comment slashes
(//) from the #pragma line to optimze the line and illustrate a workaround to
the problem.

Sample Code
-----------

  /* Compile options needed: /G5 /Og
  */ 

  #include <stdio.h>

  //#pragma optimize("g", off)    //uncomment to fix the problem
  void main()
  {
     double   polylgd[5] = {0.0, 0.0, 0.0, 1.0, 1.0};
     double   valpol,an,an1,temp,dtemp;
     double   *polyptr;
     int      i;

     polyptr = &polylgd[4];
     an  = *polyptr--;
     an1 = *polyptr--;

     for (i = 4; i >= 2; i--)
     {
        dtemp = (double) (i - 1.0) / (double) i ;
        temp  =  dtemp * an;
        temp  = *polyptr - temp;
        *polyptr--;
        an    = an1 + (dtemp + 1.0) * (-0.5) * an;
        an1   = temp;
     }

     valpol = an1 + (-0.5) * an;
     printf("value should be:  0.14843750\n");
     printf("value this run is %.8lf\n", valpol);
  }

Additional query words: 8.00 9.00 1.00 2.00 2.10 buglist1.00 buglist2.00

======================================================================
Keywords          :  
Technology        : kbVCsearch kbAudDeveloper kbvc100 kbCVCComp kbVC200 kbVC210
Version           : :1.0,2.0,2.1

=============================================================================

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