Failing offsetarrays test

[martinholters]

https://freebsdci.julialang.org/#/builders/1/builds/11271 (master):

Error in testset offsetarray:
Test Failed at /home/julia/ci/worker/11rel-amd64/build/test/offsetarray.jl:365
  Expression: maximum(A, dims=1) == OffsetArray(maximum(parent(A), dims=1), A.offsets)
   Evaluated: [0.949121 0.374514 0.839718 0.883561] == [0.949121 0.170196 0.839718 0.883561]

https://freebsdci.julialang.org/#builders/1/builds/11275 (#28079):

Error in testset offsetarray:
Test Failed at /usr/home/julia/julia-fbsd-buildbot/worker/11rel-amd64/build/test/offsetarray.jl:365
  Expression: maximum(A, dims=1) == OffsetArray(maximum(parent(A), dims=1), A.offsets)
   Evaluated: [0.933049 0.721069 0.480884 0.692188] == [0.933049 0.721069 0.369098 0.692188]

https://freebsdci.julialang.org/#/builders/1/builds/11274 (#28099):

Error in testset offsetarray:
Test Failed at /home/julia/ci/worker/11rel-amd64/build/test/offsetarray.jl:365
  Expression: maximum(A, dims=1) == OffsetArray(maximum(parent(A), dims=1), A.offsets)
   Evaluated: [0.658427 0.962348 0.890035 0.473921] == [0.658427 0.962348 0.890035 0.372744]

https://travis-ci.org/JuliaLang/julia/jobs/403462714 (master):

Error in testset offsetarray:
Test Failed at /tmp/julia/share/julia/test/offsetarray.jl:365
  Expression: maximum(A, dims=1) == OffsetArray(maximum(parent(A), dims=1), A.offsets)
   Evaluated: [0.814057 0.837049 0.789705 0.40065] == [0.814057 0.837049 0.789705 0.385899]

[andreasnoack]

Unfortunately that very much looks like something that #27845 could have caused. Maybe something has changed since the PR CI ran. I can't look into it so maybe we should revert for now.

[andreasnoack]

Just tried locally though and the tests pass.

[martinholters]

Worth noting: The differing value on the LHS is always (well, in these four cases at least) larger than on the RHS. So it might pick up a wrong initial value which, only with a certain chance, is too large. E.g. a value from another column.

[martinholters]

julia> A = OffsetArray(reshape(16:-1:1, (4, 4)), (-3,5))
OffsetArray{Int64,2,Base.ReshapedArray{Int64,2,StepRange{Int64,Int64},Tuple{}}} with indices -2:1×6:9:
 16  12  8  4
 15  11  7  3
 14  10  6  2
 13   9  5  1

julia> @test maximum(A, dims=1) == OffsetArray(maximum(parent(A), dims=1), A.offsets)
Test Failed at REPL[9]:1
  Expression: maximum(A, dims=1) == OffsetArray(maximum(parent(A), dims=1), A.offsets)
   Evaluated: [16 13 13 13] == [16 12 8 4]
ERROR: There was an error during testing

[andreasnoack]

Thanks. It's the wrong slice I'm using. Let's revert and I'll come up with something better.

[andreasnoack]

There seems to be something weird about views of OffsetArrays which is the root cause of this issue here.

julia> A = OffsetArray(reshape(16:-1:1, (4, 4)), (-3,5))
OffsetArray{Int64,2,Base.ReshapedArray{Int64,2,StepRange{Int64,Int64},Tuple{}}} with indices -2:1×6:9:
 16  12  8  4
 15  11  7  3
 14  10  6  2
 13   9  5  1

julia> idx = (Base.Slice(-2:-2), Base.Slice(6:9))
(Base.Slice(-2:-2), Base.Slice(6:9))

julia> A[idx...]
OffsetArray{Int64,2,Array{Int64,2}} with indices -2:-2×6:9:
 16  12  8  4

julia> view(A, idx...)
view(::OffsetArray{Int64,2,Base.ReshapedArray{Int64,2,StepRange{Int64,Int64},Tuple{}}}, :, :) with eltype Int64 with indices -2:-2×6:9:
 16  15  14  13

julia> view(A, idx[1].indices, idx[2].indices)
1×4 view(::OffsetArray{Int64,2,Base.ReshapedArray{Int64,2,StepRange{Int64,Int64},Tuple{}}}, -2:-2, 6:9) with eltype Int64:
 16  12  8  4

cc: @mbauman @timholy

[mbauman]

This is due to Slice playing double-duty — it both represents an idempotent vector such that s[i] == i AND it represents an entire axis.

Views exploit the latter for performance. That's what you're running into here. Looks like this problem is more urgent than I had thought. I'll see what I can do here.