BUG: /Ox or /Oei Generates Incorrect Floating Point Results
Q131314
1.00 1.50 1.51 1.52
WINDOWS
kbtool kbbuglist
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The information in this article applies to:
- The Microsoft C/C++ Compiler (CL.EXE), included with:
Microsoft Visual C++ for Windows, versions 1.0, 1.5, 1.51, 1.52
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SYMPTOMS
========
Multiple floating point calculations may generate incorrect code when
compiled with either the /Ox or /Oei optimizations. For example, the sample
code shown at the end of this article should print 0.498418 but with either
the /Ox or /Oei optimizations enabled, it prints -0.000880.
RESOLUTION
==========
Use either of the following workarounds:
- Disable the optimizations for the entire file or to use the optimize
pragma to turn them off for just one function. For example:
#pragma optimize("",off)
function()
#pragma optimize("",on)
-or-
- Simplify the calculations by breaking complex statements into multiple
lines of code or by using temporary variables to hold intermediate
results.
STATUS
======
Microsoft has confirmed this to be a bug in the Microsoft products listed
at the beginning of this article. We are researching this problem and will
post new information here in the Microsoft Knowledge Base as it becomes
available.
NOTE: this is not a problem in 32-bit versions of Microsoft Visual C++.
MORE INFORMATION
================
Sample Code to Demonstrate Problem
----------------------------------
/* Compile options needed: /Oei or /Ox
*/
#include <stdio.h>
#include <math.h>
float func(float x) {
float c,c1,c2,c3,c4,c5,c6,c7,c8,c9,c0,d,t,z;
c=.7071068; c1=-.82215223; c2=.17087277; c3=-1.13520398;
c4=1.48851587; c5=.09678418; c6=-.18628806; c7=.27886807;
c8=-1.26551223; c9=1.00002368; c0=.37409196;
z=c*x;
if (z<0)
z=-z;
t=1/(1+z/2);
d=t*(c1+t*c2);
d=t*(c3+t*(c4+d));
d=t*(c5+t*(c6+t*(c7+d)));
d=-z*z+c8+t*(c9+t*(c0+d));
d=t * (float) exp(d);
if (x<0)
return d/2;
else
return 1-d/2;
}
void main () {
const float x=-0.00396531;
float y;
y = func(x);
if (y>0.498417 && y<0.498419)
printf("Successful: func(%f) = %f\n", x, y);
else
printf("Failed: func(%f) = %f, should be 0.498418\n", x, y);
}
Additional reference words: 1.00 1.50 8.00 8.00c 8.0 8.0c
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KBSubcategory: CLIss