optimisation -O1 changes floating-point results

[Florian Weimer]

On 12/18/2013 03:23 PM, Diggory Hardy wrote:

> Is this a known issue?

Yes, it's the excess precision problem on i386:

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323

> Is it the same with later GCC versions?

GCC 4.5 has some fixes.

> Is there a way I can turn on most optimisations while still getting exactly
> the same results as without optimisation?

You could compile with -mfpmath=sse -msse2, or you could set the 
floating point control word to restrict precision to 53 bits.  The 
latter will cover doubles only, the issue can still surface with floats.

-- 
Florian Weimer / Red Hat Product Security Team