[stabbles]

A little bit late to the party here, but the number of allocations is really just 0 bytes. It shows 128 bytes because the benchmark is creating new references to A, B and C. To correct this use either interpolation with $A, $B and $C or initialize A, B and C in the setup phase:

    > @benchmark C .= A .+ B setup = (A = rand(1000, 1000); B = rand(1000, 1000); C = rand(1000, 1000))
    BenchmarkTools.Trial:
      memory estimate:  0 bytes
      allocs estimate:  0
      --------------
      minimum time:     2.048 ms (0.00% GC)

This is showing 0 bytes indeed.