Julia: Obscure regression in 32-bit Julia 0.6.3

[nalimilan]

In Julia 0.6.3, StatsBase.wmedian gives incorrect results on 32-bit. In the simplified function below, the result is 8.5 on 0.6.2 and 0.7.0-alpha, but 10.0 on 0.6.3. Everything is OK on 64-bit.

FWIW, the bug goes away just by adding @show statements.

function wmedian(v::AbstractVector{<:Real}, wt::AbstractVector{<:Real})
    midpoint = sum(wt)/2
    maxval, maxind = findmax(wt)
    if maxval > midpoint
        v[maxind]
    else
        permute = sortperm(v)
        cumulative_weight = zero(eltype(wt))
        i = 0
        for (_i, p) in enumerate(permute)
            i = _i
            if cumulative_weight == midpoint
                i += 1
                break
            elseif cumulative_weight > midpoint
                cumulative_weight -= wt[p]
                break
            end
            cumulative_weight += wt[p]
        end
        if cumulative_weight == midpoint
            middle(v[permute[i-2]], v[permute[i-1]])
        else
            middle(v[permute[i-1]])
        end
    end
end

julia> wmedian([1, 2, 4, 7, 10, 15], [1/3, 1/3, 1/3, 1, 1, 1])
10.0

[pkofod]

Our 32bit tests also fail on some weird last digit difference in LineSearches.jl. I can report details later (on mobile) but if I hardcode copies of the variables involved the results are correct.

edit:
I have not looked at making this self-contained, but to show the problem I had the following in a function

    a = ((phitest-phi_0)/alphatest - dphi_0)/alphatest  # quadratic fit
   @show a
   _a = 0.72
   _b = 2.0
   _c = 0.2
   _d = -8.0
   @show _a === phitest
   @show _b === phi_0
   @show _c === alphatest
   @show _d === dphi_0
   @show ((_a - _b)/_c - _d)/_c
   @show ((phitest-phi_0)/alphatest - dphi_0)/alphatest

with the output

a = 8.0
_a === phitest = true
_b === phi_0 = true
_c === alphatest = true
_d === dphi_0 = true
((_a - _b) / _c - _d) / _c = 8.000000000000002
((phitest - phi_0) / alphatest - dphi_0) / alphatest = 8.0

! Pretty weird that they can be identical ===, but the two calculations return different values!

[ChrisRackauckas]

Our 32-bit tests are failing because 0.5:0.1:0.7 is becoming 0.5:0.1:0.6. Maybe that's a small case to debug.

https://ci.appveyor.com/project/YingboMa/ordinarydiffeq-jl/build/1.0.62/job/o113qdrx1owjjfy2#L458

[ararslan]

Would someone with access to a 32-bit system be willing to run git bisect between v0.6.2 and v0.6.3? git bisect run with a minimal reproducing test case would make it easier. I'd do it myself but all of the machines I have access to are 64-bit.

Sorry this didn't get caught. PackageEvaluator runs on 64-bit Ubuntu so it doesn't catch OS- or architecture-specific issues in packages.

[nalimilan]

I have tried replicating the problem on a Fedora 32-bit VM, and while I see it with the official binaries, I don't when building from source. I'm using MARCH=pentium4, which AFAICT is what the official binaries use too. I'm not sure where the difference can come from.

For post-1.0 minor releases, we really need 32-bit PackageEvaluator runs (and ideally on several platforms, including Windows and OS X).

[KristofferC]

Any difference in the generated code? Julia lowering, llvm code, generated code?

[nalimilan]

Good question. There are lots of small differences in the native code, but none in the LLVM IR. So it seems it's a bug in LLVM? See this gist, where the first "commit" contains results with the official binaries, and the second one results with my custom build.

[ararslan]

See #27402. New 32-bit binaries for Windows and Linux have been uploaded. Let me know if this fixes things for you.

[nalimilan]

Thanks, that did the trick!