Strange M6201 Math -sqrt Domain Error in Release Version

[onkelhotte]

Hi there,
I have a strange problem with my project, Im using CVF6.6c, VS6, WinXP.

I get an M6201 Math -sqrt Domain Error, but this only appears in Release Mode. In Debug everything runs fine. So I tried certain things, and what I found out is very strange:

I use the subroutine (calcTorsion) where the M6201 Error comes fora few weeks, havent touched it since. The error appeared this morning when I changed a line in another subroutine that hasnt anything to do with calcTorsion.

Strangeness Nr.1: Changing that line and program runs fine.

So I looked in calcTorsion for x**0.5 and tested it

if (x.gt.0.)then
y=x**0.5
else
y=0.
end if

but M6201 was still there.

Next thing I tried was to find out where the program crashes. So I insertedthe line write(16,'(a,i2)') 'ix= ',ix after starting a do loop, where ix is the counter of that do loop where the torsions are calculated.

Strangeness Nr.2: Writing that damn ix in a file and program runs fine!

All variables are declared and initialised properly. Im using implicit none and no variable is used before its value is defined.

While writing these lines I changed the optimization settings. The error does not appear when using none or local optimizations. When I compile with global optimizatins, optimize for speed, full or maximum optimizations.

Thanks in advance,
Markus

[onkelhotte]

I found a "solution"...

I changed in the optimization setting Math Library to Fast, before it was set to Check*.
Now it works fine. But the "solution" with writing something in a file is very strange.

Markus

[TimP]

Not so strange, and similar to several situations discussed in past posts. Since all intermediate arithmetic in x87 code is promoted to double precision, optimization produces slightly different results. Inserting a write could prevent one of those optimizations. Such effects could occur even when you set /fltconsistency to prevent dangerous optimizations, and you didn't even mention whether you tried that.