stop running functions in init

[KristofferC]

Zygote currently does the same thing as CSV used to do (JuliaData/CSV.jl#324) which is to run some representative functions in __init__ to make the first call look faster. In reality, this just shifts the latency in the first call to package load time. The problem is that Zygote can be loaded in a Julia session without necessarily getting called. In those scenarios, users have to pay the compilation cost anyway which makes it more costly to have Zygote as a dependency.

[DhairyaLGandhi]

If we are removing the code to be called, then does precompile.jl need to be removed as well?

[IanButterworth]

This makes sense, and would address the issue that motivated me to look into #606
Also, on reflection, the contents of precompile.jl is probably what the SnoopCompile bot should be running in #606 rather than the unit tests

[IanButterworth]

As a case study, loading is currently quite painful on a modern aarch64 system

julia> @time using Zygote
[ Info: Precompiling Zygote [e88e6eb3-aa80-5325-afca-941959d7151f]
155.025100 seconds (76.22 M allocations: 3.948 GiB, 2.12% gc time)

julia> exit()
ian@nx1:~/Documents/julia-1.4.1/bin$ ./julia
               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.4.1 (2020-04-14)
 _/ |\__'_|_|_|\__'_|  |  Official https://julialang.org/ release
|__/                   |

julia> @time using Zygote
 90.218599 seconds (56.06 M allocations: 2.980 GiB, 2.39% gc time)

This PR improves things a lot (but aarch64 does seem disproportionately slow at loading for some reason..)

julia> @time using Zygote
[ Info: Precompiling Zygote [e88e6eb3-aa80-5325-afca-941959d7151f]
 58.374538 seconds (23.72 M allocations: 1.380 GiB, 0.99% gc time)

julia> exit()
ian@nx1:~/Documents/julia-1.4.1/bin$ ./julia
               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.4.1 (2020-04-14)
 _/ |\__'_|_|_|\__'_|  |  Official https://julialang.org/ release
|__/                   |

julia> @time using Zygote
 19.383780 seconds (22.75 M allocations: 1.336 GiB, 2.19% gc time)

And adding the SnoopCompile bot to run the precompile.jl script and provide baked in precompile statements should restore the benefit while keeping load time lower. New PR incoming, @aminya is working in upcoming changes to the SnoopCompile bot api

[CarloLucibello]

so this definitely seems worth doing, and we can keep precompile.jl for SnoopCompile

bors r+

[bors]

Build succeeded:

• ci/gitlab/gitlab.com

[MikeInnes]

Yes, we may as well remove precompile.jl here too.

[aminya]

@MikeInnes this is done in #615