mac/arm: Add a document describing the current state.

Describes both building for and on arm macs.

Bug: 1098899,1103236
Change-Id: Iac5becb22ba07453765b10c854b47182d96c98e5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2287899
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Auto-Submit: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#786345}

docs/mac_arm64.md

@@ -0,0 +1,63 @@
+Chromium for arm Macs
+=====================
+
+Apple is planning on selling Macs with arm chips by the end of 2020.
+This document describes the state of native binaries for these Macs.
+
+Building _for_ arm Macs
+-----------------------
+
+We're currently bringing up the build. At the moment, not everything compiles.
+You can follow the [tracking bug](https://crbug.com/1098899) to get updates on
+progress.
+
+There's a [bot](https://ci.chromium.org/p/chromium/builders/ci/mac-arm64) that
+builds for arm that you can look at to get an idea of the current state.
+
+To build for arm64, you have to do 2 things:
+
+1. use the `MacOSX11.0.sdk` that comes with
+   Xcode 12 beta. If you're on Google's corporate network, edit your `.gclient`
+   file and add this `custom_vars`:
+
+       "custom_vars": { "mac_xcode_version": "xcode_12_beta" },
+
+   Then just `gclient sync` and you'll automatically get that SDK and will build
+   against it. Else, manually download and install Xcode 12 beta and make it the
+   active xcode with `xcode-select`.
+
+2. Add `target_cpu = "arm64"` to your `args.gn`.
+
+Then build normally.
+
+Building _on_ arm Macs
+-----------------------
+
+Building _on_ arm Macs means that all the tools we use to build chrome need
+to either work under Rosetta or have a native arm binary.
+
+We think it makes sense to use arch-specific binaries for stuff that's
+downloaded in a hook (things pulled from cipd and elsewhere in gclient hooks --
+I think this includes clang, gn, clang-format, python3 in depot\_tools, ...),
+and fat binaries for things where we'd end up downloading both binaries anyways
+(mostly ninja-mac in depot\_tools). There's a
+[tracking bug](https://crbug.com/1103236) for eventually making native arm
+binaries available for everything.
+
+Go does [not yet](https://github.com/golang/go/issues/38485) support building
+binaries for arm macs, so all our go tooling needs to run under Rosetta for
+now.
+
+`cipd` defaults to downloading Intel binaries on arm macs for now, so that
+they can run under Rosetta.
+
+If a binary runs under Rosetta, the subprocesses it spawns by default also
+run under rosetta, even if they have a native slice. The `arch` tool
+can be used to prevent this ([example cl](https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2287751)),
+which can be useful to work around Rosetta bugs.
+
+As of today, it's possible to install depot\_tools and then run
+`fetch chromium`, and it will download Chromium and its dependencies,
+but it will die in `runhooks`.
+
+`ninja`, `gn`, and `gomacc` all work fine under Rosetta.