Enable link time optimization (LTO) for s390x

[danbev]

There is currently a daily job that enables link time optimization (LT0) and we are wondering if it would be possible to create new CI jobs, or modify existing ones, to enable LTO for s390x and i686:

$ ./configure --enable-lto

The motivation for this is that at Red Hat/IBM we have run into issues with these platforms and it would be great if these could be discovered early.

[richardlau]

We don't build/test on i686 since Node.js 10 (see #885 for the whole discussion). 32-bit Linux binaries are built unofficially over in https://unofficial-builds.nodejs.org/ but those are not regularly checked and don't have tests run. We only have a few Linux -x86 platforms left in the CI (for node-addon-api and libuv, no Node.js builds run on them anymore) on older OS'es (Ubuntu 1404 and 1604) that I'd like to get rid of.

s390 would be reasonably easy to add without having to add a new CI job -- an extra call to https://ci.nodejs.org/job/node-test-commit-linuxone/ (with CONFIG_FLAGS build parameter set) from https://ci.nodejs.org/view/Node.js%20Daily/job/node-daily-master/.

[richardlau]

Ah the other problem might be is that we don't have later versions of gcc on the CI hosts. For s390x we have devtoolsets 6 and 8.

[mhdawson]

There is a dependency on the compiler in order to be able to enable LTO ?

[richardlau]

@mhdawson It was reported broken on gcc 9: nodejs/node#38570. Rereading it might be okay on gcc 8 -- I've started some CI runs with --enable-lto on s390x to check.

[richardlau]

https://ci.nodejs.org/job/node-test-commit-linuxone/28796/ passed with --enable-lto so compiler isn't an issue.

[mhdawson]

@richardlau so that's good, I wonder if it would make sense to add it as another target under the make PR testing job? linuxone is fast enough that it would likely not cause any issues.

[richardlau]

I would very much prefer not having to add a brand new job into the CI for testing LTO -- I'd rather call the existing job(s) (e.g. node-test-commit-linuxone) a second time with the CONFIG_FLAGS build parameter set to --enable-lto.

If we add to the PR testing job (as opposed to daily master) then we need to take into account all the release lines (as we use the same job to test backport PRs). Test builds with --enable-lto show that Node.js 12 and 14 are currently broken in that configuration:
Node.js 12: https://ci.nodejs.org/job/node-test-commit-linuxone/28835/
Node.js 14: https://ci.nodejs.org/job/node-test-commit-linuxone/28836/
Node.js 16 (passes): https://ci.nodejs.org/job/node-test-commit-linuxone/28838/

[mhdawson]

I'm thinking we must have a way to filter out jobs for particular node versions. Maybe using the version selector script we can filter it out by having a new platform, something like s390x-lto ?

That might also give us the way to run the same job with --enable-lto ?

[richardlau]

I've added an s390x LTO build into node-test-commit -- a second invocation of node-test-commit-linuxone with CONFIG_FLAGS set to --enable-lto run on condition ${NODE_MAJOR_VERSION} >= 16

Audit

• node-test-commit
  https://github.com/nodejs/jenkins-config-test/compare/c71e797545e055b55b603ee645397a1f8a6922e0...aff2e5f78e15b861a0e90c9fdeb549fc59e0e815
• node-test-commit-linuxone (labelling update)
  https://github.com/nodejs/jenkins-config-test/commit/c71e797545e055b55b603ee645397a1f8a6922e0 this allows the LTO build to show up as a separate entry in PR status
  

Test builds

• https://ci.nodejs.org/job/node-test-commit/48810/ is a Node.js master build with the LTO build running
  
• https://ci.nodejs.org/job/node-test-commit/48808/console is a Node.js 14 build which shows the LTO build skipped:

12:35:59 Skipping node-test-commit-linuxone (LTO). Condition was evaluated to false.

[mhdawson]

Looks great. @richardlau thanks !

[richardlau]

I've had to change the LTO build to be a second "platform" matrix configuration so that "resume build" will properly resume LTO builds -- it looks like the multijob plugin has some issues with multiple invocations of the same job with different parameters when resuming.

Audit

• https://github.com/nodejs/jenkins-config-test/commit/0838f5850227fe50d0622914db7889193d651749
• jenkins: skip LTO builds on Node.js < 16 #2764