[PATCH] The LoopVectorizer and libm sqrt

[Hal Finkel]

----- Original Message -----
> 
> On Sep 12, 2013, at 2:52 PM, Hal Finkel <hfinkel at anl.gov> wrote:
> 
> > ----- Original Message -----
> >> On 09/12/2013 09:34 PM, Hal Finkel wrote:
> >>> ----- 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?
> >> 
> >> It seems only 2) would allow to mix fast-math and non-fast-math
> >> modes,
> >> which in fact my be very helpful in case of LTO and/or math
> >> libraries
> >> that provide fast as well as precise versions of a function.
> > 
> > I don't think that's right. For one thing, the 'fast math' flags
> > are now stored in function-level attributes (specifically for this
> > reason).
> 
> Except that the inliner ignores them afaik :).
> 
> I believe that floating point function level attributes are in a
> somewhat broken state, they work because I think LTO will remove
> them if they mismatch (at least I hope so :)

Okay :) -- Maybe the frontend approach is better then (at least for now).

 -Hal


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