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

clang/cmake/caches/Fuchsia-stage2.cmake

@@ -338,6 +338,7 @@ foreach(target armv6m-unknown-eabi;armv7m-unknown-eabi;armv8m.main-unknown-eabi)
   set(RUNTIMES_${target}_LIBCXX_CXX_ABI none CACHE STRING "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_SHARED OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_STATIC ON CACHE BOOL "")
+  set(RUNTIMES_${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_${target}_LIBCXX_ENABLE_LOCALIZATION OFF CACHE BOOL "")
@@ -388,6 +389,7 @@ foreach(target riscv32-unknown-elf)
   set(RUNTIMES_${target}_LIBCXX_CXX_ABI none CACHE STRING "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_SHARED OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_STATIC 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 "")

libc/cmake/modules/CheckCompilerFeatures.cmake

@@ -102,3 +102,6 @@ check_cxx_compiler_flag("-nostdlib++" LIBC_CC_SUPPORTS_NOSTDLIBPP)
 
 # clang-3.0+
 check_cxx_compiler_flag("-nostdlibinc" LIBC_CC_SUPPORTS_NOSTDLIBINC)
+
+# clang-3.0+
+check_cxx_compiler_flag("-nolibc" LIBC_CC_SUPPORTS_NOLIBC)

libc/include/CMakeLists.txt

@@ -664,6 +664,9 @@ get_all_install_header_targets(all_install_header_targets ${TARGET_PUBLIC_HEADER
 add_library(libc-headers INTERFACE)
 add_dependencies(libc-headers ${all_install_header_targets})
 target_include_directories(libc-headers SYSTEM INTERFACE ${LIBC_INCLUDE_DIR})
+if(LIBC_CC_SUPPORTS_NOSTDLIBINC)
+  target_compile_options(libc-headers INTERFACE "-nostdlibinc")
+endif()
 
 foreach(target IN LISTS all_install_header_targets)
   get_target_property(header_file ${target} HEADER_FILE_PATH)

libc/lib/CMakeLists.txt

@@ -30,7 +30,10 @@ foreach(archive IN ZIP_LISTS
       ARCHIVE_OUTPUT_NAME ${archive_0}
   )
   if(LLVM_LIBC_FULL_BUILD)
-    target_link_libraries(${archive_1} PUBLIC libc-headers)
+    target_link_libraries(${archive_1} INTERFACE libc-headers)
+    if(LIBC_CC_SUPPORTS_NOLIBC)
+      target_link_options(${archive_1} INTERFACE "-nolibc")
+    endif()
     if(TARGET libc-startup)
       add_dependencies(${archive_1} libc-startup)
     endif()

libcxx/CMakeLists.txt

@@ -226,6 +226,15 @@ 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_DEFAULT_C_LIBRARY system)
+set(LIBCXX_SUPPORTED_C_LIBRARIES system llvm-libc)
+set(LIBCXX_LIBC "${LIBCXX_DEFAULT_C_LIBRARY}" 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")
@@ -509,6 +518,7 @@ 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.
@@ -784,6 +794,10 @@ config_define_if_not(LIBCXX_ENABLE_WIDE_CHARACTERS _LIBCPP_HAS_NO_WIDE_CHARACTER
 config_define_if_not(LIBCXX_ENABLE_TIME_ZONE_DATABASE _LIBCPP_HAS_NO_TIME_ZONE_DATABASE)
 config_define_if_not(LIBCXX_ENABLE_VENDOR_AVAILABILITY_ANNOTATIONS _LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS)
 
+if (LIBCXX_LIBC STREQUAL "llvm-libc")
+  config_define(ON _LIBCPP_HAS_LLVM_LIBC)
+endif()
+
 if (LIBCXX_ENABLE_ASSERTIONS)
   message(DEPRECATION "LIBCXX_ENABLE_ASSERTIONS is deprecated and will be removed in LLVM 20. Please use LIBCXX_HARDENING_MODE instead.")
   set(LIBCXX_HARDENING_MODE "extensive")

libcxx/cmake/Modules/HandleLibC.cmake

@@ -0,0 +1,34 @@
+#===============================================================================
+# 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-libm-shared: A target representing the selected shared C math library.
+# - libcxx-libc-static: A target representing the selected static C library.
+# - libcxx-libm-static: A target representing the selected static C math 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-libm-static INTERFACE)
+
+  add_library(libcxx-libc-shared INTERFACE)
+  add_library(libcxx-libm-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)
+
+  add_library(libcxx-libc-static ALIAS libc)
+  add_library(libcxx-libm-static ALIAS libm)
+
+  # TODO: There's no support for building LLVM libc as a shared library yet.
+  add_library(libcxx-libc-shared INTERFACE)
+  add_library(libcxx-libm-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

@@ -1042,7 +1042,7 @@ 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/include/__config_site.in

@@ -16,6 +16,7 @@
 #cmakedefine _LIBCPP_HAS_NO_THREADS
 #cmakedefine _LIBCPP_HAS_NO_MONOTONIC_CLOCK
 #cmakedefine _LIBCPP_HAS_MUSL_LIBC
+#cmakedefine _LIBCPP_HAS_LLVM_LIBC
 #cmakedefine _LIBCPP_HAS_THREAD_API_PTHREAD
 #cmakedefine _LIBCPP_HAS_THREAD_API_EXTERNAL
 #cmakedefine _LIBCPP_HAS_THREAD_API_WIN32

libcxx/include/__locale_dir/locale_base_api.h

@@ -25,6 +25,8 @@
 #  include <__locale_dir/locale_base_api/fuchsia.h>
 #elif defined(__wasi__) || defined(_LIBCPP_HAS_MUSL_LIBC)
 #  include <__locale_dir/locale_base_api/musl.h>
+#elif defined(_LIBCPP_HAS_LLVM_LIBC)
+#  include <__locale_dir/locale_base_api/llvm_libc.h>
 #elif defined(__APPLE__) || defined(__FreeBSD__)
 #  include <xlocale.h>
 #endif

libcxx/include/__locale_dir/locale_base_api/llvm_libc.h

@@ -0,0 +1,17 @@
+// -*- C++ -*-
+//===-----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___LOCALE_LOCALE_BASE_API_LLVM_LIBC_H
+#define _LIBCPP___LOCALE_LOCALE_BASE_API_LLVM_LIBC_H
+
+#include <__support/xlocale/__nop_locale_mgmt.h>
+#include <__support/xlocale/__strtonum_fallback.h>
+#include <cstdlib>
+
+#endif // _LIBCPP___LOCALE_LOCALE_BASE_API_LLVM_LIBC_H

libcxx/src/CMakeLists.txt

@@ -200,7 +200,8 @@ 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
-                                   PRIVATE ${LIBCXX_LIBRARIES})
+                                   PRIVATE ${LIBCXX_LIBRARIES}
+                                   PRIVATE libcxx-libc-shared libcxx-libm-shared)
   set_target_properties(cxx_shared
     PROPERTIES
       COMPILE_FLAGS "${LIBCXX_COMPILE_FLAGS}"
@@ -294,7 +295,7 @@ if (LIBCXX_ENABLE_STATIC)
   target_include_directories(cxx_static PRIVATE ${CMAKE_CURRENT_SOURCE_DIR})
   target_link_libraries(cxx_static PUBLIC cxx-headers
                                    PRIVATE ${LIBCXX_LIBRARIES}
-                                   PRIVATE libcxx-abi-static)
+                                   PRIVATE libcxx-libc-static libcxx-libm-static libcxx-abi-static)
   set_target_properties(cxx_static
     PROPERTIES
       COMPILE_FLAGS "${LIBCXX_COMPILE_FLAGS}"