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 because 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. However, 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.
+      "_LIBCPP_OVERRIDABLE_FUNC_VIS=__attribute__((__visibility__(\"default\")))",
     ]
   }
   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 ]
 }