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)
