ERL-1302: Many "case"s trigger slowdown on ssa_opt_dead

[OTP-Maintainer]

Original reporter: josevalim
Affected version: OTP-22.0
Fixed in version: OTP-24.0
Component: compiler
Migrated from: https://bugs.erlang.org/browse/ERL-1302

---

Someone reported a scenario where compiling some code got slower from Erlang/OTP 21 to Erlang/OTP 23. I have isolated the code to the following Erlang file:

[https://gist.github.com/josevalim/2c175678a0f85568eb001dcb97024ef5]

In my machine, it is about 7x faster to compile on Erlang/OTP 21 compared to Erlang/OTP 22. It is a bit faster on Erlang/OTP 23 (and even faster in master), but it is still 4x to 5x slower than what it was before.

Using the +time flag, we can see the majority of the time is spent on "ssa_opt_dead". I am not sure if there is something that could be done to speed it up but I thought I would open up a report in case it does reveal some pathological case. Feel free to close otherwise. :)

Thank you!

[OTP-Maintainer]

josevalim said:

If you want more data samples to play with, here is another gist also generated by Elixir's template engine compiled down to Erlang: [https://gist.github.com/josevalim/acac154996e1e61058f367a5b3a161aa]

This one puts more load on the live_intervals pass rather than ssa dead. I have attached some profiling results to the gist. It takes roughly 24s on my machine.

Curiously, this also triggered a slow down on Elixir passes too. In our case, we were generating really long variable chains where `var1 -> var2 -> var3 -> varN -> integer`. In our case we could fix it by pointing to the first variable, so it become `var1 -> integer`, `var2 -> var1 -> integer`, `var3 -> var1 -> integer`, etc.

[OTP-Maintainer]

bjorn said:

[https://github.com/erlang/otp/pull/2894]