gn/standalone/libc++/libc++.gni

# Copyright (C) 2017 The Android Open Source Project
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#      http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

import("//gn/standalone/sanitizers/sanitizers.gni")
import("//gn/standalone/wasm.gni")

declare_args() {
  # Background:
  # there are mainly two C++ libraries around in the world: (i) GNU's
  # libstdc++ and LLVM's libc++ (aka libcxx). By default Linux provides libstdc++
  # (even building with clang on Linux uses that by default) while Mac and
  # Android switched to libcxx.
  # buildtools/libcxx(abi) contains a fixed version of the libcxx, the same one
  # that Chrome uses on most production configurations (% lagging catching up
  # with our DEPS).
  # The variable use_custom_libcxx tells our build system to prefer the
  # aforementioned copy to the system one.
  #
  # Now, there are two reasons for using the checked in copy of libcxx:
  # 1) LLVM sanitizers require that the c++ library is built from sources,
  #    because they need to be instrumented with -fsanitize as well (see
  #    https://github.com/google/sanitizers/wiki/MemorySanitizerLibcxxHowTo).
  #    On top of this, they also require that the c++ library is dynamically
  #    linked to prevent duplicate symbol errors when linking (see Chrome's
  #    build/config/c++/c++.gni)
  # 2) The libstdc++ situation is too wild on Linux. Modern debian distros are
  #    fine but Ubuntu Trusty still ships a libstdc++ that doesn't fully
  #    support C++17. Hence we enable this flag on Linux by default.
  #    We still retain libstdc++ coverage on the CI bots by overriding
  #    use_custom_libcxx=false when we target a modern library (see the
  #    GCC target in infra/ci/config.py).
  use_custom_libcxx = is_linux && is_hermetic_clang && !is_wasm
  custom_libcxx_is_static = !using_sanitizer
}

libcxx_prefix = "//buildtools/libcxx"
libcxxabi_prefix = "//buildtools/libcxxabi"