[LLVMdev] Floating-point range checks

[Hal Finkel]

----- Original Message -----
> From: "Arch Robison" <arch.robison at intel.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "Philip Reames" <listmail at philipreames.com>, llvmdev at cs.uiuc.edu
> Sent: Thursday, January 8, 2015 1:26:41 PM
> Subject: RE: [LLVMdev] Floating-point range checks
> 
> > Checks against 1.0 are also common. Why not just add a FP range
> > class, like our constant range, and go from there?
> 
> That's certainly another way to go.  My worry is that a more
> complicated lattice gets us deeper into rounding-mode issues and
> considerably more work for smaller gain.  I like the idea of
> creating an FPRange class.  We could start with a simple one and
> extend it as experience warrants.

I worry about that too ;) -- I think creating a simple one and extending from there makes sense.

 -Hal

> 
> - Arch
> 
> -----Original Message-----
> From: Hal Finkel [mailto:hfinkel at anl.gov]
> Sent: Thursday, January 8, 2015 1:03 PM
> To: Robison, Arch
> Cc: Philip Reames; llvmdev at cs.uiuc.edu
> Subject: Re: [LLVMdev] Floating-point range checks
> 
> ----- Original Message -----
> > From: "Arch Robison" <arch.robison at intel.com>
> > To: "Philip Reames" <listmail at philipreames.com>,
> > llvmdev at cs.uiuc.edu
> > Sent: Thursday, January 8, 2015 12:54:32 PM
> > Subject: Re: [LLVMdev] Floating-point range checks
> > 
> > 
> > Thanks for the pointers. Looks like LazyValueInfo has the sort of
> > infrastructure I had in mind. LVILatticeVal could be extended to
> > floating point. (The comment “this can be made a lot more rich in
> > the
> > future” is an invitation :-). I’m thinking a simple lattice would
> > address most cases of interest for floating-point checks. The
> > lattice
> > points for floating-point could be all subsets of the eight-element
> > set:
> > 
> > {-inf,<0,-0,+0,>0,+inf,-nan,+nan},
> > 
> > where <0 and >0 denote finite negative/positive numbers
> > respectively.
> > 
> 
> I'm not sure. Checks against 1.0 are also common. Why not just add a
> FP range class, like our constant range, and go from there?
> 
>  -Hal
> 
> > 
> > 
> > - Arch
> > 
> > 
> > 
> > 
> > 
> > From: Philip Reames [mailto:listmail at philipreames.com]
> > Sent: Wednesday, January 7, 2015 6:03 PM
> > To: Robison, Arch; llvmdev at cs.uiuc.edu
> > Subject: Re: [LLVMdev] Floating-point range checks
> > 
> > 
> > 
> > I don't believe we have much in this area currently.
> > 
> > Generally, something like this would existing in InstCombine and
> > ValueTracking.
> > 
> > Take a look at ComputeSignBit in ValueTracking.cpp. This doesn't
> > apply
> > (?) to floating point numbers, but we'd need something equivalent
> > for
> > them. It looks like there may already be a start in the form of:
> > CannotBeNegativeZero
> > 
> > Other places to look would be SimplifyFAdd and
> > InstCombine::visitFAdd.
> > 
> > For this particular example, you're probably going to want a
> > pattern
> > in SimplifyFCmp of the form:
> > matcher: sqrt_call( fadd(Value(X), SpecificValue(X)),
> > fadd(Value(Y),
> > SpecificValue(Y)))
> > && CannotBeNegativeZero(X) && CannotBeNegativeZero(Y)
> > 
> > You might also look at LazyValueInfo, but that's probably of
> > secondary
> > interest. It's purely in terms of constant integer ranges
> > currently.
> > 
> > Philip
> > 
> > 
> > 
> > 
> > 
> > On 01/07/2015 02:13 PM, Robison, Arch wrote:
> > 
> > 
> > 
> > 
> > The Julia language implements sqrt(x) with conditional branch taken
> > if
> > x<0. Alas this prevents vectorization of loops with sqrt. Often the
> > argument can be proven to be non-negative. E.g., sqrt(x*x+y*y).
> > Is there an existing LLVM pass or analysis that does floating-point
> > range propagation to eliminate such unnecessary checks?
> > 
> > 
> > 
> > 
> > 
> > Arch D. Robison
> > 
> > 
> > Intel Corporation
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > _______________________________________________ LLVM Developers
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> > 
> > 
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> 
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
> 

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