[LLVMdev] Optimize away sqrt in simple cases?

[Christoph Grenz]

Am Dienstag, 23. April 2013, 22:50:51 schrieben Sie:
> [...]
> Giving more-than-expected precision can be just as bad for the user as less.
>  It tends to come up in situations where the optimization would break some
> symmetry, the same way that aggressively forming FMAs can break user code. 
> [...]
> 
> It boils down to the fact that giving excess precision in
> some-places-but-not-others can lead to bad behavior.

Ok, I didn't think about excess precision resulting in problems.
Now it's clear to me that fast-math is neccessary for this optimization.

BTW: Is there a way to only get fast-math optimizations that don't change 
behaviour on NaNs and Infs? I think it would be an interesting possibility to 
allow arithmetic tranformations but only those preserving NaN and infinity 
bahavior to catch errors/corner cases.

At least the pow(sqrt(x),2)=>x optimization falls into this category as I 
wrote before.

> --Owen

Christoph