[LLVMdev] Floating-point range checks

[Hal Finkel]

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