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Apr 2, 2021
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Update the "LLVM updates" section #1102

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20 changes: 12 additions & 8 deletions src/backend/updating-llvm.md
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Expand Up @@ -67,15 +67,15 @@ Example PRs look like:
## Feature updates

> Note that this information is as of the time of this writing <!-- date:
2018-12 --> (December 2018). The process for updating LLVM changes with
2021-04 --> (April 2021). The process for updating LLVM changes with
practically all LLVM updates, so this may be out of date!

Unlike bugfixes, updating to pick up a new feature of LLVM typically requires a
lot more work. This is where we can't reasonably cherry-pick commits backwards
so we need to do a full update. There's a lot of stuff to do here, so let's go
through each in detail.

1. Create a new branch in the rust-lang/llvm-project repository. This branch
1. Create a new branch in the [rust-lang/llvm-project repository]. This branch
should be named `rustc/a.b-yyyy-mm-dd` where `a.b` is the current version
number of LLVM in-tree at the time of the branch and the remaining part is
today's date. Move this branch to the commit in LLVM that you'd like, which
Expand Down Expand Up @@ -119,12 +119,12 @@ through each in detail.
Ideally, build LLVM and test it on a few platforms:

* Linux
* OSX
* macOS
* Windows

and afterwards run some docker containers that CI also does:

* `./src/ci/docker/run.sh wasm32-unknown`
* `./src/ci/docker/run.sh wasm32`
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Why did this change?

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The dir name is changed, it should be matched here: https://github.com/rust-lang/rust/tree/master/src/ci/docker/host-x86_64

* `./src/ci/docker/run.sh arm-android`
* `./src/ci/docker/run.sh dist-various-1`
* `./src/ci/docker/run.sh dist-various-2`
Expand All @@ -136,15 +136,17 @@ through each in detail.
`src/llvm-project` and will likely also change [`llvm-wrapper`] as well.

For prior art, previous LLVM updates look like
[#55835](https://github.com/rust-lang/rust/pull/55835)
[#47828](https://github.com/rust-lang/rust/pull/47828)
[#62474](https://github.com/rust-lang/rust/pull/62474)
[#62592](https://github.com/rust-lang/rust/pull/62592). Note that sometimes it's
[#62592](https://github.com/rust-lang/rust/pull/62592)
[#67759](https://github.com/rust-lang/rust/pull/67759)
[#73526](https://github.com/rust-lang/rust/pull/73526)
[#81451](https://github.com/rust-lang/rust/pull/81451). Note that sometimes it's
easiest to land [`llvm-wrapper`] compatibility as a PR before actually updating
`src/llvm-project`. This way while you're working through LLVM issues others
interested in trying out the new LLVM can benefit from work you've done to
update the C++ bindings.

[rust-lang/llvm-project repository]: https://github.com/rust-lang/llvm-project
[`llvm-wrapper`]: https://github.com/rust-lang/rust/tree/master/compiler/rustc_llvm/llvm-wrapper

### Caveats and gotchas
Expand All @@ -155,10 +157,12 @@ keep in mind while going through them:
* LLVM bugs are hard to find, don't hesitate to ask for help! Bisection is
definitely your friend here (yes LLVM takes forever to build, yet bisection is
still your friend)
* If you've got general questions, @alexcrichton can help you out.
* If you've got general questions, [wg-llvm] can help you out.
* Creating branches is a privileged operation on GitHub, so you'll need someone
with write access to create the branches for you most likely.

[wg-llvm]: https://rust-lang.zulipchat.com/#narrow/stream/187780-t-compiler.2Fwg-llvm

## New LLVM Release Updates

Updating to a new release of LLVM is very similar to the "feature updates"
Expand Down