[LLVMdev] Floating-point range checks

[Philip Reames]

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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