Recent failures with byte alignment

[sbromberger]

ERROR: LoadError: ArgumentError: reinterpret from alignment 1 bytes to alignment 4 bytes not allowed
Stacktrace:
 [1] reinterpret(::Type{UInt32}, ::Array{UInt8,1}, ::Tuple{Int64}) at ./array.jl:160
 [2] reinterpret(::Type{UInt32}, ::Array{UInt8,1}) at ./array.jl:139
 [3] include_from_node1(::String) at ./loading.jl:551
 [4] include(::String) at ./sysimg.jl:14
 [5] anonymous at ./<missing>:2
while loading /Users/seth/.julia/v0.7/JLD2/src/JLD2.jl, in expression starting on line 6

This appears to be all over the place via

const OBJECT_HEADER_SIGNATURE = reinterpret(UInt32, UInt8['O', 'H', 'D', 'R'])[1]

julia> versioninfo()
Julia Version 0.7.0-DEV.373
Commit 82e9638225 (2017-05-28 01:51 UTC)
Platform Info:
  OS: macOS (x86_64-apple-darwin16.5.0)
  CPU: Intel(R) Core(TM) i5-6267U CPU @ 2.90GHz
  WORD_SIZE: 64
  BLAS: libopenblas (USE64BITINT DYNAMIC_ARCH NO_AFFINITY Haswell)
  LAPACK: libopenblas64_
  LIBM: libopenlibm
  LLVM: libLLVM-3.9.1 (ORCJIT, skylake)

[timholy]

I wrote a package that may help fix this: https://github.com/JuliaArrays/UnalignedVectors.jl. Consequence of JuliaLang/julia#21831.

[yuyichao]

FWIW, that line would be better written as

const OBJECT_HEADER_SIGNATURE = htol(0x5244484f) # "OHDR"

Or with a helper function that does the reverse reinterpret.

A lot of packages abuse reinterpret this way and we could have base function to do that instead of casting through arrays.