1923255 - (clang-20) Update builders to clang 20

Status: RESOLVED FIXED

(Firefox Build System :: Toolchains, task)

Description

[Mike Hommey [:glandium] OoO until Dec 30]

(when it's released)

Comment 1

[Martin Bruse]

Attachment: Bug 1923255 - Enable Clang 20 support for Firefox with Windows plugin fixes r=glandium

This change adds support for building Firefox with Clang 20, including
critical fixes for the clang-plugin dynamic linking on Windows.

Clang 20 introduced two breaking changes that prevented the clang-plugin
from loading on Windows:

1. The CLANG_ABI macro was added to Attr classes, causing inline methods
   like classof() and getAnnotation() to generate dllimport references.
   This fails because these methods are defined inline in headers, not
   exported from the DLL.

2. The extract_symbols.py script became more aggressive in filtering
   symbols, blocking the Registry<PluginASTAction> static data members
   (Head/Tail) that don't match function signature patterns.

The fix consists of three patches for the Clang 20 build:

• add-plugin-symbols_clang_20.patch: Whitelists the Registry Head/Tail
  symbols in extract_symbols.py to bypass the filtering logic.

• force-plugin-symbols_clang_20.patch: Uses /INCLUDE linker flags to
  force the Registry add_node() and begin() symbols into clang.exe,
  since they exist in libraries but weren't being linked due to being
  unreferenced.

• remove-clang-abi-from-attrs_clang_20.patch: Removes the CLANG_ABI
  macro from Attr class generation to prevent dllimport issues with
  inline methods.

Additional changes:

• Fixed toolchain.configure to accept clang-cl as a valid compiler for
  the WASI target, deriving the clang.exe path when needed. Previously,
  only "clang" was accepted, causing configuration failures on Windows
  where clang-cl is the default.

• Modified build-clang.py to enable libxml2 for all Windows build stages,
  not just the final stage, ensuring it's available during intermediate
  builds.

• Fixed moz.build to apply extra_cxxflags even when LLVM_CXXFLAGS is
  empty, ensuring -GR- and -EHsc are always applied on Windows.

To test this locally, check out llvm 20.1.8 alongside firefox, then
build clang by running

MOZ_FETCHES_DIR=../firefox python3 ../firefox/build/build-clang/build-clang.py -c ../firefox/build/build-clang/use-clang-cl-artifact.json -c ../firefox/build/build-clang/win64.json -c ../firefox/build/build-clang/clang-20.json -c ../firefox/build/build-clang/2stages.json

After this, update mozconfig to

export CC=[THE LLVM CHECKOUT DIRECTORY]/build/stage2/clang/bin/clang-cl.exe
ac_add_options --enable-clang-plugin

Comment 2

[Martin Bruse]

Hey again Mike,

With some small patches to llvm 20.1.8 it behaves similarly enough to llvm 19 that the plugin works again.

Using patches also allowed me to minimize the changes in build/clang-plugin/moz.build.

Is this useful to you? Should I try to polish this into a submittable change, can you use it in a patch of your own?

Comment 3

[Martin Bruse]

I realize this patch should actually go in 1941482, I'll move it there.

Comment 4

[Phabricator Automation]

Comment on attachment 9519448 [details]
Bug 1923255 - Enable Clang 20 support for Firefox with Windows plugin fixes r=glandium

Revision D268255 was moved to bug 1941482. Setting attachment 9519448 [details] to obsolete.

Comment 5

[Mike Hommey [:glandium] OoO until Dec 30]

Attachment: Bug 1923255 - Update builders to clang 20.

Comment 6

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/53e5155d2f79
https://hg.mozilla.org/integration/autoland/rev/1d230cf52fb5
Update builders to clang 20. r=firefox-build-system-reviewers,ahochheiden

Comment 7

[pstanciu]

https://hg.mozilla.org/mozilla-central/rev/1d230cf52fb5

Comment 8

[Mike Hommey [:glandium] OoO until Dec 30]

This was backed out along bug 1948826

Comment 9

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/e1d8f0b1de4b
https://hg.mozilla.org/integration/autoland/rev/44d1c6b6b9f4
Update builders to clang 20. r=firefox-build-system-reviewers,ahochheiden

Comment 10

[Sandor Molnar[:smolnar]]

https://hg.mozilla.org/mozilla-central/rev/44d1c6b6b9f4