Make operator new/delete symbols have default visibility

third_party/libc++/BUILD.gn

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/toolchain/toolchain.gni")
 
 # Sanitizer builds need to override operator new, operator delete, and
 # some exception handling symbols, so libc++ must be a shared library
 # to prevent duplicate symbol errors when linking.
 libcpp_is_static = !is_component_build && !using_sanitizer
 
 # Used by libc++ and libc++abi.
 config("config") {
   defines = [ "LIBCXX_BUILDING_LIBCXXABI" ]
   if (libcpp_is_static) {
     # Don't leak any symbols on a static build.
     defines += [
       "_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS",
       "_LIBCXXABI_DISABLE_VISIBILITY_ANNOTATIONS",
+
+      # This resets the visibility to default only for the various
+      # flavors of operator new and operator delete.  These symbols
+      # are weak and get overriden by Chromium-provided ones, but if
+      # these symbols had hidden visibility, this would make the
+      # Chromium symbols hidden too.
+      "_LIBCPP_OVERRIDABLE_FUNC_VIS=__attribute__((__visibility__(\"default\")))",

[Nico, 2017/06/19 19:22:41]

Can you add a comment here along the lines of "
elf visibility rules require that linkers use the least visible form when
merging, and if this is hidden, then when we merge it with tcmalloc's operator
new, hidden visibility would win. Hoever, tcmalloc needs a visible operator new
to also override operator new references from system libraries.
TODO(lld): Ask lld for a --force-public-visibility flag or similar to that
overrides the default elf merging rules, and make tcmalloc's gn config pass that
to all its dependencies, then remove this override here."

[Tom Anderson, 2017/06/19 19:27:40]

Done.

     ]
   }
   cflags = [
     "-fPIC",
     "-fstrict-aliasing",
   ]
   cflags_cc = [
     "-nostdinc++",
     "-isystem" + rebase_path("trunk/include", root_build_dir),
     "-isystem" + rebase_path("../libc++abi/trunk/include", root_build_dir),
@@ skipping 104 matching lines @@
       # Normally the generator takes care of RPATH. Our case is special because
       # the generator is unaware of the libc++.so dependency. Note that setting
       # RPATH here is a potential security issue. See the following for another
       # example of this issue: https://code.google.com/p/gyp/issues/detail?id=315
       "-Wl,-rpath,\$ORIGIN/",
     ]
   }
 
   lib_dirs = [ root_build_dir ]
 }