Re: Effects of newly introduced -mpcX 80387 precision flag

[Uros Bizjak <ubizjak at gmail dot com>]

Bradley Lucier wrote:

> I just (re-)discovered these tables giving maximum known errors in 
> some libm functions when extended precision is enabled:
>
> http://people.inf.ethz.ch/gonnet/FPAccuracy/linux/summary.html
>
> and when the precision of the mantissa is set to 53 bits (double 
> precision):
>
> http://people.inf.ethz.ch/gonnet/FPAccuracy/linux64/summary.html
>
> This is from 2002, and indeed, some of the errors in double-precision 
> results are hundreds or thousands of times bigger when the precision 
> is set to 53 bits.
>
> I think the warning in the documentation is very mild considering the 
> possible effects.
>
> Perhaps the manual should also mention that sometimes this option 
> brings a 2% improvement in the speed of FP-intensive code along with 
> massive increases in the error of some libm functions, and then people 
> could decide if they want to use it.  (I'm not opposed to a switch 
> like this, my favorite development environment sets the precision to 
> 53 bits globally just as this switch does, but I think the 
> documentation should be more clear about the trade-offs.)

Could you please post a patch with suggested wording about this option 
(I was trying to write something similar to the warning that icc has in 
its documentation about precision settings).

Thanks,
Uros.