[PATCH] The LoopVectorizer and libm sqrt

[Hal Finkel]

----- Original Message -----
> ----- Original Message -----
> > I did not write this code but I assume this was done on purpose
> > because our llvm.sqrt intrinsics has a slightly different
> > semantics:
> > 
> > The ‘llvm.sqrt‘ intrinsics return the sqrt of the specified
> > operand,
> > returning the same value as the libm ‘sqrt‘ functions would. Unlike
> > sqrt in libm, however, llvm.sqrt has undefined behavior for
> > negative
> > numbers other than -0.0 (which allows for better optimization,
> > because there is no need to worry about errno being set).
> > llvm.sqrt(-0.0) is defined to return -0.0 like IEEE sort.
> 
> Hrmm... okay; I'll send a revised patch where we explicitly check for
> fast-math mode.

Or perhaps not; the TargetOptions are not available at the IR-level right now, and so this seems to leave us with two options:

 1. Feed something through TTI

 2. Have Clang generate the intrinsic directly in fast-math mode

I'm leaning toward (1), because I'd like to give the target the ability to declare the availability of a vectorized sqrt that is suitable as a libm sqrt replacement.

What do you think?

 -Hal

> 
>  -Hal
> 
> > 
> >  
> > On Sep 12, 2013, at 1:39 PM, Hal Finkel <hfinkel at anl.gov> wrote:
> > 
> > > Nadav, Arnold, et al.,
> > > 
> > > It seems as though libm sqrt was accidentally omitted from the
> > > list
> > > of functions with associated vectorizable intrinsics. The
> > > attached
> > > patch fixes this. Please review.
> > > 
> > > Thanks again,
> > > Hal
> > > 
> > > --
> > > Hal Finkel
> > > Assistant Computational Scientist
> > > Leadership Computing Facility
> > > Argonne National Laboratory<lv-libm-sqrt.patch>
> > 
> > 
> 
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
> 
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> 

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