Bug 1941482 - 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 breaking changes that prevented the clang-plugin
from building on Windows:

1. Clang is built with '/Zc:dllexportInlines-' which excludes inlined
   members like some methods of Attr classes from being dllexported.

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

The fix consists of one change to the build configuration of clang-plugin,
and one patch for the Clang 20 and trunk builds:

- plugin-registry-symbols-llvm-pr-163391.patch:

  Make extract_symbols.py recognize and export Registry<T>::Head and
  ::Tail static members for any Registry instantiation.

  See llvm/llvm-project#163367 and
  llvm/llvm-project#163391.

- Add '/Zc:dllexportInlines-' to the CXX flags when building
  clang-plugin, which will match the clang build configuration and
  inline the members instead of referencing the symbols.

Differential Revision: https://phabricator.services.mozilla.com/D268255

Author: zond

build/build-clang/clang-20.json

@@ -13,6 +13,7 @@
         "llvmorg-22-init-4745-gbe179d069664.patch",
         "android-hardware-buffer-header-workaround.patch",
         "arm64e-hack.patch",
-        "compiler-rt-rss-limit-heap-profile.patch"
+        "compiler-rt-rss-limit-heap-profile.patch",
+        "plugin-registry-symbols-llvm-pr-163391.patch"
     ]
 }

build/build-clang/clang-trunk.json

@@ -10,6 +10,7 @@
         "revert-llvmorg-15-init-13446-g7524fe962e47.patch",
         "android-hardware-buffer-header-workaround_clang_21.patch",
         "arm64e-hack.patch",
-        "compiler-rt-rss-limit-heap-profile.patch"
+        "compiler-rt-rss-limit-heap-profile.patch",
+        "plugin-registry-symbols-llvm-pr-163391.patch"
     ]
 }

build/build-clang/plugin-registry-symbols-llvm-pr-163391.patch

@@ -0,0 +1,35 @@
+Fix plugin registry symbols not exported in static builds with plugin support.
+
+When building LLVM statically (without BUILD_SHARED_LIBS) on Windows with
+LLVM_EXPORT_SYMBOLS_FOR_PLUGINS=ON, external plugins cannot register through
+llvm::Registry<clang::PluginASTAction> because:
+
+Static data members (Head, Tail) are filtered out during symbol export by
+extract_symbols.py because they don't match the function signature patterns
+that the script looks for.
+
+This patch fixes the issue by adding pattern matching to extract_symbols.py
+to recognize and export Registry static data members.
+
+Note: When both LLVM and the plugins are built with /Zc:dllexportInlines-,
+the static methods (add_node(), begin()) are inlined by the plugin at call
+sites, so only the data symbols need to be exported.
+
+See https://github.com/llvm/llvm-project/issues/163367
+
+diff --git a/llvm/utils/extract_symbols.py b/llvm/utils/extract_symbols.py
+index 388723421d66..72f992f560c7 100755
+--- a/llvm/utils/extract_symbols.py
++++ b/llvm/utils/extract_symbols.py
+@@ -105,6 +105,11 @@ def should_keep_microsoft_symbol(symbol, calling_convention_decoration):
+     # Skip X86GenMnemonicTables functions, they are not exposed from llvm/include/.
+     elif re.match(r"\?is[A-Z0-9]*@X86@llvm", symbol):
+         return None
++    # Keep Registry<T>::Head and Registry<T>::Tail static members for plugin support.
++    # Pattern matches: ?Head@?$Registry@<template_args>@llvm@@ or ?Tail@?$Registry@...
++    elif ("?$Registry@" in symbol and "@llvm@@" in symbol and
++          (symbol.startswith("?Head@") or symbol.startswith("?Tail@"))):
++        return symbol
+     # Keep mangled llvm:: and clang:: function symbols. How we detect these is a
+     # bit of a mess and imprecise, but that avoids having to completely demangle
+     # the symbol name. The outermost namespace is at the end of the identifier

build/clang-plugin/moz.build

@@ -134,6 +134,9 @@ if CONFIG["HOST_OS_ARCH"] == "WINNT":
     # but as of writing, it's not necessary for the plugin code, so enable
     # the escape hatch, at least until we generally upgrade to VS 2019.
     HOST_DEFINES["LLVM_FORCE_USE_OLD_TOOLCHAIN"] = True
+    # Clang 20+ needs /Zc:dllexportInlines- to work around CLANG_ABI issues with inline methods
+    if CONFIG["HOST_CC_VERSION"] and int(CONFIG["HOST_CC_VERSION"].split(".")[0]) >= 20:
+        extra_cxxflags += ["/Zc:dllexportInlines-"]
 else:
     extra_cxxflags = ["-fno-rtti", "-fno-exceptions"]