[libc][libcxx] Support for building libc++ against LLVM libc

clang/cmake/caches/Fuchsia-stage2.cmake

@@ -339,6 +339,7 @@ foreach(target armv6m-unknown-eabi;armv7m-unknown-eabi;armv8m.main-unknown-eabi)
   set(RUNTIMES_${target}_LIBCXX_ENABLE_SHARED OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_STATIC ON CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_NEW_DELETE_DEFINITIONS ON CACHE BOOL "")
+  set(RUNTIMES_${target}_LIBCXX_LIBC "llvm-libc" CACHE STRING "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_FILESYSTEM OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_RANDOM_DEVICE OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_LOCALIZATION OFF CACHE BOOL "")
@@ -389,6 +390,7 @@ foreach(target riscv32-unknown-elf)
   set(RUNTIMES_${target}_LIBCXX_ENABLE_SHARED OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_STATIC ON CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_NEW_DELETE_DEFINITIONS ON CACHE BOOL "")
+  set(RUNTIMES_${target}_LIBCXX_LIBC "llvm-libc" CACHE STRING "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_FILESYSTEM OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_RANDOM_DEVICE OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_LOCALIZATION OFF CACHE BOOL "")

libcxx/CMakeLists.txt

@@ -212,6 +212,14 @@ set(LIBCXX_ABI_DEFINES "" CACHE STRING "A semicolon separated list of ABI macros
 set(LIBCXX_EXTRA_SITE_DEFINES "" CACHE STRING "Extra defines to add into __config_site")
 option(LIBCXX_USE_COMPILER_RT "Use compiler-rt instead of libgcc" OFF)
 
+# C Library options -----------------------------------------------------------
+
+set(LIBCXX_SUPPORTED_C_LIBRARIES system llvm-libc)
+set(LIBCXX_LIBC "system" CACHE STRING "Specify C library to use. Supported values are ${LIBCXX_SUPPORTED_C_LIBRARIES}.")
+if (NOT "${LIBCXX_LIBC}" IN_LIST LIBCXX_SUPPORTED_C_LIBRARIES)
+  message(FATAL_ERROR "Unsupported C library: '${LIBCXX_CXX_ABI}'. Supported values are ${LIBCXX_SUPPORTED_C_LIBRARIES}.")
+endif()
+
 # ABI Library options ---------------------------------------------------------
 if (MSVC)
   set(LIBCXX_DEFAULT_ABI_LIBRARY "vcruntime")
@@ -495,6 +503,7 @@ endif()
 # Setup Compiler Flags
 #===============================================================================
 
+include(HandleLibC) # Setup the C library flags
 include(HandleLibCXXABI) # Setup the ABI library flags
 
 # FIXME(EricWF): See the FIXME on LIBCXX_ENABLE_PEDANTIC.

libcxx/cmake/Modules/HandleLibC.cmake

@@ -0,0 +1,39 @@
+#===============================================================================
+# Define targets for linking against the selected C library
+#
+# After including this file, the following targets are defined:
+# - libcxx-libc-headers: An interface target that allows getting access to the
+#                        headers of the selected C library.
+# - libcxx-libc-shared: A target representing the selected shared C library.
+# - libcxx-libc-static: A target representing the selected static C library.
+#===============================================================================
+
+# Link against a system-provided libc
+if (LIBCXX_LIBC STREQUAL "system")
+  add_library(libcxx-libc-headers INTERFACE)
+
+  add_library(libcxx-libc-static INTERFACE)
+  add_library(libcxx-libc-shared INTERFACE)
+
+# Link against the in-tree LLVM libc
+elseif (LIBCXX_LIBC STREQUAL "llvm-libc")
+  add_library(libcxx-libc-headers INTERFACE)
+  target_link_libraries(libcxx-libc-headers INTERFACE libc-headers)
+  if(CXX_SUPPORTS_NOSTDLIBINC_FLAG)
+    target_compile_options(libcxx-libc-headers INTERFACE "-nostdlibinc")
+  endif()
+
+  add_library(libcxx-libc-static INTERFACE)
+  if (TARGET libc)
+    target_link_libraries(libcxx-libc-static INTERFACE libc)
+  endif()
+  if (TARGET libm)
+    target_link_libraries(libcxx-libc-static INTERFACE libm)
+  endif()
+  if (CXX_SUPPORTS_NOLIBC_FLAG)
+    target_link_options(libcxx-libc-static INTERFACE "-nolibc")
+  endif()
+
+  # TODO: There's no support for building LLVM libc as a shared library yet.
+  add_library(libcxx-libc-shared INTERFACE)
+endif()

libcxx/cmake/config-ix.cmake

@@ -27,6 +27,9 @@ if (NOT LIBCXX_USE_COMPILER_RT)
   endif()
 endif()
 
+check_cxx_compiler_flag(-nostdlibinc CXX_SUPPORTS_NOSTDLIBINC_FLAG)
+check_cxx_compiler_flag(-nolibc CXX_SUPPORTS_NOLIBC_FLAG)
+
 # libc++ is using -nostdlib++ at the link step when available,
 # otherwise -nodefaultlibs is used. We want all our checks to also
 # use one of these options, otherwise we may end up with an inconsistency between

libcxx/include/CMakeLists.txt

@@ -1041,7 +1041,7 @@ list(APPEND _all_includes "${LIBCXX_GENERATED_INCLUDE_DIR}/libcxx.imp")
 add_custom_target(generate-cxx-headers ALL DEPENDS ${_all_includes})
 
 add_library(cxx-headers INTERFACE)
-target_link_libraries(cxx-headers INTERFACE libcxx-abi-headers)
+target_link_libraries(cxx-headers INTERFACE libcxx-libc-headers libcxx-abi-headers)
 add_dependencies(cxx-headers generate-cxx-headers)
 # It's important that the arch directory be included first so that its header files
 # which interpose on the default include dir be included instead of the default ones.

libcxx/src/CMakeLists.txt

@@ -199,7 +199,7 @@ split_list(LIBCXX_LINK_FLAGS)
 if (LIBCXX_ENABLE_SHARED)
   add_library(cxx_shared SHARED ${exclude_from_all} ${LIBCXX_SOURCES} ${LIBCXX_HEADERS})
   target_include_directories(cxx_shared PRIVATE ${CMAKE_CURRENT_SOURCE_DIR})
-  target_link_libraries(cxx_shared PUBLIC cxx-headers
+  target_link_libraries(cxx_shared PUBLIC cxx-headers libcxx-libc-shared
                                    PRIVATE ${LIBCXX_LIBRARIES})
   set_target_properties(cxx_shared
     PROPERTIES
@@ -292,7 +292,7 @@ set(CMAKE_STATIC_LIBRARY_PREFIX "lib")
 if (LIBCXX_ENABLE_STATIC)
   add_library(cxx_static STATIC ${exclude_from_all} ${LIBCXX_SOURCES} ${LIBCXX_HEADERS})
   target_include_directories(cxx_static PRIVATE ${CMAKE_CURRENT_SOURCE_DIR})
-  target_link_libraries(cxx_static PUBLIC cxx-headers
+  target_link_libraries(cxx_static PUBLIC cxx-headers libcxx-libc-static
                                    PRIVATE ${LIBCXX_LIBRARIES}
                                    PRIVATE libcxx-abi-static)
   set_target_properties(cxx_static