Bug 1844484 - Override the symbol used by compilers in vtables for pure virtual methods. r=firefox-build-system-reviewers,nalexander

In bug 1839743, we made the build system prefer packed relative
relocations to elfhack when both the system libc and linker support
them. Unfortunately, while that covers most of the benefits from
elfhack, it doesn't cover bug 651892.

To cover it, we make every C++ executable contain its own copy of
the symbol, so that all relocations related to it become relative.

And because this is actually (slightly) beneficial on macos, and because
it's also an advantage to have our own abort called rather than the
system's, we apply the same to all platforms.

Differential Revision: https://phabricator.services.mozilla.com/D184068

build/moz.build

@@ -18,6 +18,8 @@ if CONFIG["OS_ARCH"] == "WINNT":
 else:
     DIRS += ["unix"]
 
+DIRS += ["pure_virtual"]
+
 if CONFIG["MOZ_STDCXX_COMPAT"]:
     DIRS += ["unix/stdc++compat"]
 

build/pure_virtual/moz.build

@@ -0,0 +1,17 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+Library("pure_virtual")
+
+SOURCES += ["pure_virtual.c"]
+
+FORCE_STATIC_LIB = True
+
+USE_STATIC_LIBS = True
+
+# Build a real library so that the linker can remove it if the symbol
+# is never used.
+NO_EXPAND_LIBS = True

build/pure_virtual/pure_virtual.c

@@ -0,0 +1,25 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include <stdlib.h>
+
+// This function is used in vtables to point at pure virtual methods.
+// The implementation in the standard library usually aborts, but
+// the function is normally never called (a call would be a bug).
+// Each of these entries in vtables, however, require an unnecessary
+// dynamic relocation. Defining our own function makes the linker
+// point the vtables here instead of the standard library, replacing
+// the dynamic relocations with relative relocations.
+//
+// On Windows, it doesn't really make a difference, but on macOS it
+// can be packed better, saving about 10KB in libxul, and on 64-bits
+// ELF systems, with packed relative relocations, it saves 140KB.
+//
+// Another advantage of having our own is that the abort() will fall
+// back to our own abort() rather than the system's.
+#ifdef _MSC_VER
+int __cdecl _purecall() { abort(); }
+#else
+__attribute__((visibility("hidden"))) void __cxa_pure_virtual() { abort(); }
+#endif

python/mozbuild/mozbuild/frontend/emitter.py

@@ -388,6 +388,8 @@ class TreeMetadataEmitter(LoggingMixin):
                     context, obj, variable, self.STDCXXCOMPAT_NAME[obj.KIND]
                 )
             if obj.KIND == "target":
+                if "pure_virtual" in self._libs:
+                    self._link_library(context, obj, variable, "pure_virtual")
                 for lib in context.config.substs.get("STLPORT_LIBS", []):
                     obj.link_system_library(lib)