Commits on Nov 18, 2018

1. Add GCC to runtime dependencies …

   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

   

   schneems committed Nov 18, 2018
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