Optimisation prevents overflow?

[Daniel Franke]

On Thursday 19 July 2007 23:05:06 Davide Mancusi wrote:
> Is optimisation preventing overflow? What is really happening here?

In other mail:
> ...and yes, -ffloat-store fixes it. What does it do? I could not find
> documentation about it.

I'd guess, that's the (in)famous extended precision, 80 vs. 64 bits (iirc). 
Try an external function to force the usage of the default precision, like 
this:

	program fk
	double precision a, olda
	integer i

	a=1.d0
	i=0
	do
	  i=i+1
	  olda=a
	  a=f(a)
	  if(a.le.olda) exit
	end do

	print *, "when not printing, overflow at step ", i
	end program fk

	function f(a)
	  double precision, intent(in) :: a
	  double precision f
	  f = a * 1.5d18
	end function

$> gfortran-svn -O3 ml.f90 &&./a.out
 when not printing, overflow at step           18