Re: turning off optimization on a code segment

[Tim Prince <TimothyPrince at sbcglobal dot net>]

John Fine wrote:
> In my first reply, I forgot to include the example I meant to give of 
> where I have seen that behavior.  It was a different compiler (not GCC) 
> but the issue occurs in any efficient use of the x86 floating point 
> stack, so I expect it occurs in GCC as well.
>
> do
> {
>   *pdTime = *pdTnext;
>   invoke some external function
>  *pdTnext= some computation
> } while ( *pdTime < *pdTnext)
>
> On two successive passes, the exact same 80 bit value is computed for 
> *pdTnext, but the loop doesn't end.  The value stored in each of 
> *pdTnext  and *pdTime is the 64 bit round of the computed 80 bit value, 
> and the value used for *pdTime in the comparison is that 64 bit value, 
> but the value used for *pdTnext is the just computed 80 bit value that 
> is still available in a register (instead of reading back the value just 
> written).  The external function keeps the compiler from tracking the 
> value of *pdTime in a register.
>
> I've debugged the above infinite loop a few times in different programs.
>
> Frédéric BOIS wrote:
> > pifn->bOn = (*pdTime < *pdTnext);
> >   

Such a problem could be avoided by several well-known methods:
a) -mfpmath=sse
b) long double *pdTime, *pdTnext
c) -ffloat-store   (more in line with your desire to disable optimization)