Microsoft KB Archive/122265

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

Article ID: 122265

Article Last Modified on 7/5/2005

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APPLIES TO


 * Microsoft Visual C++ 1.0 Professional Edition
 * Microsoft Visual C++ 2.0 Professional Edition
 * Microsoft Visual C++ 2.1
 * Microsoft C++ 32-bit Compiler 1.0

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This article was previously published under Q122265



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


 * 1) include 

//#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: KB122265

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