[LLVMdev] Floating-point range checks

[Pete Cooper]

Hi Arch

Yeah, that seems reasonable to me.  LazyValueInfo seems to be more about what ranges of values control flow defines.  Knowing that 'fmul float %0, %0’ is >= 0 is perfectly reasonable for SimplifyFCmpInst to know.

Thanks,
Pete
> On Jan 13, 2015, at 11:27 AM, Robison, Arch <arch.robison at intel.com> wrote:
> 
> After writing a simple FPRange, I've hit a stumbling block.  I don't know what LLVM code should be extended to use it.  I was initially thinking of extending LazyValueInfo, but it appears to be used for passes that don’t address the case that I need for Julia.  I’m now wondering if I’m better off extending SimplifyFCmpInst to handle the few cases in question instead of trying to be more general.  Here’s an example case for Julia where a trivially true domain check should be removed:
>  
> %jl_value_t = type {}
> @jl_domain_exception = external global %jl_value_t*
> declare void @jl_throw_with_superfluous_argument(%jl_value_t*, i32)
> declare float @llvm.sqrt.f32(float %Val)
>  
> define float @julia_f_64805(float) {
> top:
>   %1 = fmul float %0, %0
>   %2 = fcmp uge float %1, 0.000000e+00
>   br i1 %2, label %pass, label %fail
>  
> fail:                                             ; preds = %top
>   %3 = load %jl_value_t** @jl_domain_exception, align 8
>   call void @jl_throw_with_superfluous_argument(%jl_value_t* %3, i32 2)
>   unreachable
>  
> pass:                                             ; preds = %top
>   %4 = call float @llvm.sqrt.f32(float %1)
>   ret float %4
> }
>  
> I just want to fold the branch.  Following Philip’s earlier suggestion, adding a routine CannotBeOrderedLessThanZero (that’s analogous to CannotBeNegativeZero) and making SimplifyFCmpInst use it would be enough to cover the cases of primary interest.  Comments?
>  
> - Arch

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