[LLVMdev] [RFC] How to fix sqrt vs llvm.sqrt optimization asymmetry

[Hal Finkel]

----- Original Message -----
> On Nov 11, 2013, at 10:44 PM, Hal Finkel <hfinkel at anl.gov> wrote:
> >> "Unlike sqrt in libm, however, llvm.sqrt has undefined behavior
> >> for
> >> ..."
> >> 
> >> to "... produces an undefined value", with a link back to
> >> ##undefined-values.
> > 
> > I'm not sure that helps, because it will prevents sqrt +
> > -fno-math-errno (a readnone sqrt) -> llvm.sqrt -- and thus still
> > prevents the use of llvm.sqrt to vectorize sqrt. I think what will
> > help is just saying something like this:
> > 
> >  "Unlike sqrt in libm, after calling llvm.sqrt with a negative
> >  argument (except for -0.0), the state of errno, and any other
> >  portions of the floating-point environment that are used to
> >  record errors, is undefined.”
> 
> Makes sense to me.
> 
> > 
> > FWIW, we may want to make a similar change for the other llvm.<libm
> > function> intrinsics, at least for the vector versions (as vector
> > ops on many ISAs don't affect the FP state bits like the scalar
> > ops do).
> 
> Yep.

Okay, good. I'll construct a documentation patch for review shortly.

> 
> > 
> >> 
> >> Transformations that apply to sqrt() should generally apply to
> >> llvm.sqrt as well.
> > 
> > And the intent is for this to be true for all of the other
> > llvm.<libm function> intrinsics as well, right?
> 
> Yep!

Okay; I'll work on refactoring the existing optimizations to apply to both.

 -Hal

> 
> -Chris

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory