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Ruby 2.6.0 ships with a JIT (just in time compiler) that utilizes GCC (or any other `cc` such as clang) at runtime. This version of Ruby will be released on December 25th, 2018 and Heroku needs to provide support for this feature out of the box. ## Background The design alternatives can be found in this document https://docs.google.com/document/d/1q5yEpRddUU2JPynESmffI_BpLDW4XaBgO7iyJE-mwio/edit?usp=sharing ## This PR This PR adds the `gcc` package to Heroku-16 and Heroku-18 runtime images. ## Size cost Adding the `gcc` package increases the runtime stack image by about 84mb: Before adding `gcc` ``` -----> Size breakdown... # ... ubuntu:18.04 85.8MB heroku/heroku:18 428MB heroku/heroku:18-build 805MB ``` After adding `gcc` ``` -----> Size breakdown... # ... ubuntu:18.04 85.8MB heroku/heroku:18 512MB heroku/heroku:18-build 805MB ``` It looks like this works with only `gcc`, however if we end up needing `build-essential` then the cost would increase by 64MB to 576MB. ## Testing JIT on the stack images testing On a currently running Heroku-18 dyno with Ruby 2.6: ``` $ hs run bash -a infinite-everglades-58502 ~ $ ruby -v ruby 2.6.0preview3 (2018-11-06 trunk 65578) [x86_64-linux] ~ $ ruby -e "def a; end; def b; end; a; a; a; a; a; a; a; puts 'done'" --jit --jit-verbose=1 MJIT: Error in execv: /usr/bin/gcc MJIT warning: Making precompiled header failed on compilation. Stopping MJIT worker... done ``` > Note: The warnings in the output indicate gcc could not be found On a local docker image with `gcc` available: ``` $ bin/build.sh heroku-18 heroku/heroku:18 heroku/heroku:18-build # ... $ docker run -i -t heroku/heroku:18 /bin/bash root@4437f834d502:/# mkdir -p vendor/ruby-2.6.0 root@4437f834d502:/# cd vendor/ruby-2.6.0 root@4437f834d502:/vendor/ruby-2.6.0# curl https://heroku-buildpack-ruby.s3.amazonaws.com/heroku-18/ruby-2.6.0.tgz -s -o - | tar zxf - root@4437f834d502:/vendor/ruby-2.6.0# bin/ruby -e "def a; end; def b; end; a; a; a; a; a; a; a; puts 'done'" --jit --jit-verbose=1 done Successful MJIT finish ``` > Note: The output indicates that MJIT was successful For more output you can also apply `--jit-wait` which will force mjit to run synchronously (instead of in a background thread). cc/ @hone @hunterloftis
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