2 files changed, 50 insertions, 0 deletions
diff --git a/build/pure_virtual/moz.build b/build/pure_virtual/moz.build
new file mode 100644
index 0000000000..2e5169fec6
--- /dev/null
+++ b/build/pure_virtual/moz.build
@@ -0,0 +1,23 @@
+# -*- 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
+
+# LTO can mess things up.
+if CONFIG["CC_TYPE"] == "clang-cl":
+    CFLAGS += ["-clang:-fno-lto"]
+else:
+    CFLAGS += ["-fno-lto"]
diff --git a/build/pure_virtual/pure_virtual.c b/build/pure_virtual/pure_virtual.c
new file mode 100644
index 0000000000..23b18f2aae
--- /dev/null
+++ b/build/pure_virtual/pure_virtual.c
@@ -0,0 +1,27 @@
+/* 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 <mozilla/Assertions.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 we can use MOZ_CRASH
+// instead of the system's abort.
+#ifdef _MSC_VER
+int __cdecl _purecall() { MOZ_CRASH("pure virtual call"); }
+#else
+__attribute__((visibility("hidden"))) void __cxa_pure_virtual() {
+  MOZ_CRASH("pure virtual call");
+}
+#endif