Re: O2 optimization differences

[Dale Johannesen <dalej at apple dot com>]

On Sep 29, 2004, at 9:14 AM, Joe Buck wrote:

> On Wed, Sep 29, 2004 at 03:10:12PM +0530, Sujith R. wrote:
>
> > 	I am finding the output differences between O2 flagged , optimized
> > version and a -g3 -gdwarf-2 flagged debugable version of a C++
> > application , which does some very heavy , floating point , number
> > crunching.  The differences are with very small numbers (close to 0)
> > like 1.234E-11 being treated as 0, but some subsequent calculations go
> > for a toss.
>
> If you're running x86 hardware, you can see things like this.  The
> floating point registers have extra precision (80 bits worth), but this
> precision is lost when a register is spilled to memory.  Since this
> differs depending on optimization level, you will see effects like 
> this.

Indeed.  If this is the problem -ffloat-store may help.