quantile gives wrong gradient on Julia 1.6

[jgreener64]

On Julia 1.9 and Enzyme main (6692fad) this works:

using Enzyme, Statistics
f(x) = sum(quantile([1.0, x], [0.5, 0.7]))
autodiff(Reverse, f, Active, Active(2.0))[1][1]

1.2

But on Julia 1.6.7 it gives the wrong answer:

0.7

autodiff(Forward, f, Duplicated(2.0, 1.0))[1] also gives 0.7.

A possibly simpler variant: sum(quantile([1.0, x], 0.7)) gives the correct 0.7 on both versions but sum(quantile([1.0, x], [0.7])) gives 0.7 on Julia 1.9 (correct) and 0.0 on Julia 1.6 (incorrect).

[wsmoses]

Looks like the primal computation itself got messed up:

julia> autodiff(Forward, f, Duplicated,Duplicated(2.0, 1.0))
(0.0, 0.0)

julia> f(x)=sum(quantile([1.0, x], [0.7]))^C

julia> x=1.0
1.0

julia> quantile([1.0, x], [0.7])
1-element Vector{Float64}:
 1.0

julia> f(2.0)
1.7

julia> f(x)=sum(quantile([1.0, x], [0.7]))
f (generic function with 1 method)

[wsmoses]

using Enzyme, Statistics

Enzyme.API.printall!(true)

@inline function myquantile(v::AbstractVector, p::Real; alpha::Real=1.0, beta::Real=alpha)
    n = length(v)
    
    m = alpha + p * (one(alpha) - alpha - beta)
    aleph = n*p + oftype(p, m)
    j = clamp(trunc(Int, aleph), 1, n-1)
    γ = clamp(aleph - j, 0, 1)

    if n == 1
        a = v[1]
        b = v[1]
    else
        a = v[j]
        b = v[j + 1]
    end
    
    return a + γ*(b-a)
end

function f(x)
    v = [1.0, x]
    return first(map(x->myquantile(v, x, alpha=1., beta=1.), [0.7]))
end

@show f(2.0)
@show autodiff(Forward, f, Duplicated,Duplicated(2.0, 1.0))

[wsmoses]

This turns out to be a bug in the attributor which changes a store double ... into a store double undef

https://godbolt.org/z/Pa7Eejcq4

@jdoerfert any insights here?

[wsmoses]

The bug appears to occur in LLVM 11 (above) and 12 (https://godbolt.org/z/YehKx37h8), but not 13+

[wsmoses]

This is effectively fixed by disabling the attributor in #986 for lower LLVM versions, but is still quite unsatisfying. Either way closing as now resolved and the attributor bug is independent of Enzyme.jl