Thanks for the work!

@junaruga this is now passing:

I'm going to try merging this.

Hopefully #8393 continues to be okay.

Using gcc-9 makes sense for now, because we use gcc (version 9.4.0) on Ruby CI Ubuntu ppc64le and s390x. So, I agree using the gcc-9.

And the PR's Travis CI is passing.
https://app.travis-ci.com/github/ruby/ruby/builds/265789165

However, I see the used gcc version is actually not gcc-9 but gcc (= version 9.4.0) in the Travis CI log. Because while the &gcc-10 exists in .travis.yml, the &gcc-9 doesn't exist in it.
https://app.travis-ci.com/github/ruby/ruby/jobs/609434805#L139

$ gcc --version
gcc (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

So, I think a possible fix is to add the &gcc-9 referred by *gcc-9 to explicitly showing the used gcc is gcc-9.

$ git diff .travis.yml
diff --git a/.travis.yml b/.travis.yml
index c6054ae672..3cbe921762 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -72,6 +72,24 @@ env:
         openssl
         zlib1g-dev
 
+  - &gcc-9
+    compiler: gcc-9
+    before_install:
+      - tool/travis_retry.sh sudo bash -c "rm -rf '${TRAVIS_ROOT}/var/lib/apt/lists/'* && exec apt-get update -yq"
+      - >-
+        tool/travis_retry.sh sudo -E apt-get $travis_apt_get_options install
+        ccache
+        gcc-9
+        g++-9
+        libffi-dev
+        libncurses-dev
+        libncursesw5-dev
+        libreadline-dev
+        libssl-dev
+        libyaml-dev
+        openssl
+        zlib1g-dev
+
   # --------
 
   - &arm64-linux

Or to use the default gcc (= version 9.4.0). I like this way, because it's simple and is aligning with the current RubyCI Ubuntu ppc64le and s390x.

$ git diff
diff --git a/.travis.yml b/.travis.yml
index c6054ae672..11ae8a9b45 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -82,12 +82,12 @@ env:
   - &ppc64le-linux
     name: ppc64le-linux
     arch: ppc64le
-    <<: *gcc-10
+    compiler: gcc
 
   - &s390x-linux
     name: s390x-linux
     arch: s390x
-    <<: *gcc-10
+    compiler: gcc
 
   - &arm32-linux
     name: arm32-linux

What do you think? Could you change the code?

When we fix on the 2nd way, I think the PR title can be like this.

"Try the system gcc 9.4.0 to see if it exhibits the same compiler bugs."

or

"Try the default gcc 9.4.0 to see if it exhibits the same compiler bugs."

Can you change the commit message in the 2 commits to change the gcc-9 to gcc, including the keyword "Travis" and "ppc64le" and "s390x" and explaining the context and reasons? I think squashing the 2 commits to 1 commit is better, because it comes from the same reason, right?

The message can be like this. You can check how the commit messages are written in the past in these kind of cases by the $ git log .travis.yml.

.travis.yml: Try gcc 9.4.0 instead of gcc-10 in the ppc64le and s390x cases.

Use gcc 9.4.0 instead of gcc-10 to avoid failures in the ppc64le and s390x cases and align with RubyCI Ubuntu ppc64le and s390x.

I think you can commit to my branch if you want. Let me give you access.

I think you can commit to my branch if you want. Let me give you access.

I am a bit sick today. So, if you want to merge this PR today, I hope you change the PR by yourself. If you are fine to merge this PR next week, I try to update your branch. I am not sure if I can update your branch today.

@ioquatix I changed my mind. I am working to update your branch now.

I rebased the PR. Let's see the results of the Travis CI

I confirmed the rebased commit uses the default gcc 9.4.0 as I expected in my forked repository's Travis in advance.
https://app.travis-ci.com/github/junaruga/ruby/builds/265795672

All right. I executed Travis 2 times for this PR on ruby/ruby Travis. And I see ppc64le and 390x cases pass in the both 1st and 2nd time.

https://app.travis-ci.com/github/ruby/ruby/builds/265796147
https://app.travis-ci.com/github/ruby/ruby/builds/265795840