`vec(::Matrix)` allocates

[gdalle]

As pointed out by @mcabbott in #564, vec(::Matrix) is not free. Neither is reshape for that matter. He suggested directly using ReshapedArray

[amontoison]

Are you sure that it's not free for Matrix?
For other types maybe, but for Array it should be free.

[gdalle]

julia> using BenchmarkTools

julia> @benchmark vec(M) setup=(M=rand(2, 3))
BenchmarkTools.Trial: 10000 samples with 999 evaluations.
 Range (min … max):  12.624 ns …  5.056 μs  ┊ GC (min … max):  0.00% … 98.51%
 Time  (median):     14.606 ns              ┊ GC (median):     0.00%
 Time  (mean ± σ):   20.292 ns ± 98.337 ns  ┊ GC (mean ± σ):  14.24% ±  3.10%

  ▃█▅▁▇▇▁        ▁▂▃▃▄▅▄▂▂▃▄▂              ▂▃▃▃▂▁▁            ▂
  ███████▇▇▆▆▆▆▇▇████████████▇▆▃▃▃▁▄▁▅████████████▇▆▅▅▅▅▅▆▅▆▆ █
  12.6 ns      Histogram: log(frequency) by time        36 ns <

 Memory estimate: 32 bytes, allocs estimate: 1.

[amontoison]

Don't you need to use vec($M) to not take into account the storage related to the pointer?

[amontoison]

https://juliaci.github.io/BenchmarkTools.jl/stable/

[gdalle]

No because I create M in the setup phase, it is not a global variable. The interpolation of a global matrix variable would give the same result