src/docs/updating_clang.md

# Updating clang

We distribute prebuilt packages of LLVM binaries, including clang and lld, that
all developers and bots pull at `gclient runhooks` time. These binaries are
just regular LLVM binaries built at a fixed upstream revision. This document
describes how to build a package at a newer revision and update Chromium to it.
An archive of all packages built so far is at https://is.gd/chromeclang

1.  Check that https://ci.chromium.org/p/chromium/g/chromium.clang/console
    looks reasonably green. Red bots with seemingly normal test failures are
    usually ok, that likely means the test is broken with the stable Clang as
    well.
1.  Sync your Chromium tree to the latest revision to pick up any plugin
    changes.
1.  Run [go/chrome-push-clang-to-goma](https://goto.google.com/chrome-push-clang-to-goma).
    This takes a recent dry run CL to update clang, and if the trybots were
    successful it will copy the binaries from the staging bucket to the
    production one. Writing to this bucket requires special permissions. File a
    bug at g.co/bugatrooper if you don't have these already (e.g.,
    https://crbug.com/1034081). Then it will push the packages to goma. If you
    do not have the necessary credentials to do the upload, ask
    clang@chromium.org to find someone who does.
    *   Alternatively, to create your own roll CL, you can manually run
	`tools/clang/scripts/upload_revision.py` with a recent upstream LLVM
	commit hash as the argument. After the `*_upload_clang` trybots are
	successfully finished, run
	[go/chrome-promote-clang](https://goto.google.com/chrome-promote-clang)
	on the new Clang package name.
1.  Run an exhaustive set of try jobs to test the new compiler. The CL
    description created previously by upload_revision.py includes
    `Cq-Include-Trybots:` lines for all needed bots, so it's sufficient to just
    run `git cl try` (or hit "CQ DRY RUN" on gerrit).
1.  Commit the roll CL from the previous step.
1.  The bots will now pull the prebuilt binary, and goma will have a matching
    binary, too.

## Performance regressions

After doing a clang roll, you may get a performance bug assigned to you
([example](https://crbug.com/1094671)). Some performance noise is expected
while doing a clang roll.

You can check all performance data for a clang roll via
`https://chromeperf.appspot.com/group_report?rev=XXXXXX`, where `XXXXXX` is the
revision number, e.g. `778090` for the example bug (look in the first message
of the performance bug to find this). Click the checkboxes to display graphs.
Hover over points in the graph to see the value and error.

Serious regressions require bisecting upstream commits (TODO: how to repro?).
If the regressions look insignificant and there is green as well as red, you
can close the bug as "WontFix" with an explanation.

## Adding files to the clang package

The clang package is downloaded unconditionally by all bots and devs. It's
called "clang" for historical reasons, but nowadays also contains other
mission-critical toolchain pieces besides clang.

We try to limit the contents of the clang package. They should meet these
criteria:

- things that are used by most developers use most of the time (e.g. a
  compiler, a linker, sanitizer runtimes)
- things needed for doing official builds

If you want to add something to the clang package that doesn't (yet?) meet
these criteria, you can make package.py upload it to a separate zip file
and then download it on an opt-in basis by using update.py's --package option.

If you're adding a new feature that you expect will meet the inclusion criteria
eventually but doesn't yet, start by having your things in a separate zip
and move it to the main zip once the criteria are met.