-ffast-math: Math critical applications?

[Jim Gettys]

On Wed, 2006-10-04 at 22:13 -0400, John Richard Moser wrote: 
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> This sheds a lot more light on the situation than I previously had
> access to.  I was considering blacklisting breakable applications if
> feasible; clearly this is not a maintainable plan of attack.  Possibly,
> though, it may be acceptable to attack specific points of interest.
> 
> Considerations thus lead me to ask:  Can the JPEG and PNG decoders used
> by the Web page renderer, and the renderer itself, be -ffast-math
> compiled safely?  As far as I am aware, these only calculate visual
> elements such as alpha blend for PNG with alpha channel, and discrete
> cosine transforms for JPEG, so pretty much failures here should result
> in slightly fudged visual elements.

You have to look at the algorithms on a case by case basis,
individually.

It defaults "off" for a very good reason.

If you truncate rather than gradual underflow, you can cause results to
become unpredictable in many algorithms.


> 
> This leaves me with the question of whether outright failures will occur
> due to -ffast-math inherently.  If it just spits out bad values that can
> be used, that's fine; if it spits out results that when accessed cause
> an error (i.e. {a = invalidly_calculated_float;} causes segfaults... I
> can't imagine how), this is not feasible.
> 
> 

Bad values is a mis-nomer: typically, this option is turning off gradual
underflow and similar features of IEEE.

And yes, you can get outright failure in algorithms that depend on
correct IEEE floating point behavior.
                                       - Jim


> 
> 
> > For openssl, mcrypt, mpfr and gmp, the applications had decent
> > testsuites, and the bug was easy to catch, but in the other cases it
> > wasn't caught until much later.
> > 
> > 
> > 
> 
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Jim Gettys
One Laptop Per Child