[LLVMdev] Floating-point range checks
Robison, Arch
arch.robison at intel.com
Thu Jan 8 10:54:32 PST 2015
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.
- 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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