Add experimental ability to build LLVM libc.

This is extremely new and barely tested: I've compiled and run a
hello-world program, and that is literally all. But that's a start –
if you can do that, you can also investigate what else does and
doesn't compile or run successfully. So this is enough to allow users
to experiment.

As documented in the new `docs/llvmlibc.md`, this currently builds C
libraries only (not C++), for AArch32 only. I don't know how much
effort it will take to fix each of those: I only know that in each
case _at least one_ cause of build failure exists, but I don't know
how many more are hiding behind that one.

You can use the new `-DLLVM_TOOLCHAIN_C_LIBRARY` with or without the
option to build an overlay package. If you have both a newlib and an
llvm-libc overlay package, then it should be possible to install both
alongside each other on top of the same main toolchain.

Unlike picolibc and newlib, llvm-libc doesn't come with a crt0.o
startup file. Compiled for bare metal (as we're doing here), it also
leaves some support functions undefined (e.g. for writing to stdout or
stderr), expecting the user to provide them. So I've also added a
small source directory containing the missing pieces. That builds two
additional libraries, -lcrt0 and -lsemihost.

At present, I haven't set up a llvm-libc specific ld script, and
there's no code in my new crt0 that copies RW data out of ROM (as
there is in picolibc). The only definition required by the crt0 is
`__stack`, which you can define via an ld script of your own if you
want, or directly on the ld.lld command ilne if you prefer.

Author: statham-arm

CMakeLists.txt

@@ -140,7 +140,7 @@ set(LLVM_TOOLCHAIN_C_LIBRARY
     "Which C library to use."
 )
 set_property(CACHE LLVM_TOOLCHAIN_C_LIBRARY
-    PROPERTY STRINGS picolibc newlib)
+    PROPERTY STRINGS picolibc newlib llvmlibc)
 
 # Previously, the LLVM_TOOLCHAIN_LIBRARY_OVERLAY_INSTALL option was
 # called LLVM_TOOLCHAIN_NEWLIB_OVERLAY_INSTALL. Detect a setting of
@@ -153,7 +153,7 @@ else()
   set(overlay_install_default OFF)
 endif()
 option(LLVM_TOOLCHAIN_LIBRARY_OVERLAY_INSTALL
-    "Make cpack build an overlay package that can be unpacked over the main toolchain to install a secondary set of libraries based on newlib."
+    "Make cpack build an overlay package that can be unpacked over the main toolchain to install a secondary set of libraries based on newlib or llvm-libc."
     ${overlay_install_default})
 if(LLVM_TOOLCHAIN_LIBRARY_OVERLAY_INSTALL)
   if(LLVM_TOOLCHAIN_C_LIBRARY STREQUAL "picolibc")
@@ -268,7 +268,9 @@ function(read_repo_version output_variable_prefix repo)
     set(${output_variable_prefix}_SHALLOW "${shallow}" PARENT_SCOPE)
 endfunction()
 read_repo_version(llvmproject llvm-project)
-read_repo_version(${LLVM_TOOLCHAIN_C_LIBRARY} ${LLVM_TOOLCHAIN_C_LIBRARY})
+if(NOT (LLVM_TOOLCHAIN_C_LIBRARY STREQUAL llvmlibc)) # libc in a separate repo?
+  read_repo_version(${LLVM_TOOLCHAIN_C_LIBRARY} ${LLVM_TOOLCHAIN_C_LIBRARY})
+endif()
 
 # The patches are generated from custom branch, with followin command:
 #   git format-patch -k origin/main
@@ -319,7 +321,9 @@ FetchContent_Declare(newlib
 )
 
 FetchContent_MakeAvailable(llvmproject)
-FetchContent_MakeAvailable(${LLVM_TOOLCHAIN_C_LIBRARY})
+if(NOT (LLVM_TOOLCHAIN_C_LIBRARY STREQUAL llvmlibc)) # libc in a separate repo?
+  FetchContent_MakeAvailable(${LLVM_TOOLCHAIN_C_LIBRARY})
+endif()
 
 # We generally want to install to a local directory to see what the
 # output will look like rather than install into the system, so change
@@ -334,6 +338,10 @@ if(CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT)
     )
 endif()
 
+# Whether to try to build C++ libraries. (We can't currently do this
+# for all choices of C library.)
+set(CXX_LIBS ON)
+
 if(LLVM_TOOLCHAIN_C_LIBRARY STREQUAL newlib)
     install(
         FILES
@@ -350,6 +358,42 @@ install(
     COMPONENT llvm-toolchain-omax-cfg
 )
 
+if(LLVM_TOOLCHAIN_C_LIBRARY STREQUAL llvmlibc)
+    install(
+        FILES
+        ${CMAKE_CURRENT_SOURCE_DIR}/llvmlibc.cfg
+        DESTINATION bin
+        COMPONENT llvm-toolchain-llvmlibc-configs
+    )
+
+    # We aren't yet able to build C++ libraries to go with llvm-libc
+    set(CXX_LIBS OFF)
+
+    # We need to build libc-hdrgen
+    ExternalProject_Add(
+        libc_hdrgen
+        SOURCE_DIR ${llvmproject_SOURCE_DIR}/llvm
+        DEPENDS ${lib_tool_dependencies}
+        CMAKE_ARGS
+            -DLLVM_ENABLE_RUNTIMES=libc
+            -DLLVM_LIBC_FULL_BUILD=ON
+            -DCMAKE_BUILD_TYPE=Debug
+        STEP_TARGETS build install
+        BUILD_COMMAND ${CMAKE_COMMAND} --build . --target libc-hdrgen
+        INSTALL_COMMAND ${CMAKE_COMMAND} -E true
+        # Always run the build command so that incremental builds are correct.
+        BUILD_ALWAYS TRUE
+        CONFIGURE_HANDLED_BY_BUILD TRUE
+    )
+    ExternalProject_Get_property(libc_hdrgen BINARY_DIR)
+    set(LIBC_HDRGEN ${BINARY_DIR}/bin/libc-hdrgen${CMAKE_EXECUTABLE_SUFFIX})
+
+    # Add an empty check target, to simplify the logic below that expects to
+    # find one for every libc type. We have no current setup to run the LLVM
+    # libc test suite.
+    add_custom_target(check-llvmlibc)
+endif()
+
 add_subdirectory(
     ${llvmproject_SOURCE_DIR}/llvm llvm
 )
@@ -396,7 +440,7 @@ set(CPACK_ARCHIVE_COMPONENT_INSTALL TRUE)
 # Don't create a separate archive for each component.
 set(CPACK_COMPONENTS_GROUPING ALL_COMPONENTS_IN_ONE)
 # When extracting the files put them in an ArmCompiler-.../ directory.
-# Exception: the newlib overlay package does not do this, because it has
+# Exception: the overlay packages do not do this, because they have
 # to be able to unpack over the top of an existing installation on all
 # platforms, and each platform has a different top-level directory name.
 if(LLVM_TOOLCHAIN_LIBRARY_OVERLAY_INSTALL)
@@ -479,7 +523,7 @@ add_custom_target(
     -DLLVMEmbeddedToolchainForArm_VERSION=${LLVMEmbeddedToolchainForArm_VERSION}
     -DLLVMEmbeddedToolchainForArm_SOURCE_DIR=${CMAKE_CURRENT_SOURCE_DIR}
     -Dllvmproject_SOURCE_DIR=${llvmproject_SOURCE_DIR}
-    # only one of picolibc and newlib source dirs is needed, but easiest to
+    # at most one of picolibc and newlib source dirs is needed, but easiest to
     # specify both definitions
     -Dpicolibc_SOURCE_DIR=${picolibc_SOURCE_DIR}
     -Dnewlib_SOURCE_DIR=${newlib_SOURCE_DIR}
@@ -817,6 +861,112 @@ function(
     )
 endfunction()
 
+function(
+    add_llvmlibc
+    directory
+    variant
+    target_triple
+    flags
+    test_executor_params
+    default_boot_flash_addr
+    default_boot_flash_size
+    default_flash_addr
+    default_flash_size
+    default_ram_addr
+    default_ram_size
+    default_stack_size
+)
+    get_runtimes_flags("${directory}" "${flags}")
+
+    set(runtimes_flags "${runtimes_flags} -Wno-error=atomic-alignment")
+
+    set(common_cmake_args
+        -DCMAKE_AR=${LLVM_BINARY_DIR}/bin/llvm-ar${CMAKE_EXECUTABLE_SUFFIX}
+        -DCMAKE_ASM_COMPILER=${LLVM_BINARY_DIR}/bin/clang${CMAKE_EXECUTABLE_SUFFIX}
+        -DCMAKE_ASM_COMPILER_TARGET=${target_triple}
+        -DCMAKE_ASM_FLAGS=${runtimes_flags}
+        -DCMAKE_BUILD_TYPE=Release
+        -DCMAKE_CXX_COMPILER=${LLVM_BINARY_DIR}/bin/clang++${CMAKE_EXECUTABLE_SUFFIX}
+        -DCMAKE_CXX_COMPILER_TARGET=${target_triple}
+        -DCMAKE_CXX_FLAGS=${runtimes_flags}
+        -DCMAKE_C_COMPILER=${LLVM_BINARY_DIR}/bin/clang${CMAKE_EXECUTABLE_SUFFIX}
+        -DCMAKE_C_COMPILER_TARGET=${target_triple}
+        -DCMAKE_C_FLAGS=${runtimes_flags}
+        -DCMAKE_INSTALL_MESSAGE=${CMAKE_INSTALL_MESSAGE}
+        -DCMAKE_INSTALL_PREFIX=<INSTALL_DIR>
+        -DCMAKE_NM=${LLVM_BINARY_DIR}/bin/llvm-nm${CMAKE_EXECUTABLE_SUFFIX}
+        -DCMAKE_RANLIB=${LLVM_BINARY_DIR}/bin/llvm-ranlib${CMAKE_EXECUTABLE_SUFFIX}
+        # Let CMake know we're cross-compiling
+        -DCMAKE_SYSTEM_NAME=Generic
+        -DCMAKE_TRY_COMPILE_TARGET_TYPE=STATIC_LIBRARY
+    )
+
+    ExternalProject_Add(
+        llvmlibc_${variant}
+        SOURCE_DIR ${llvmproject_SOURCE_DIR}/runtimes
+        PREFIX llvmlibc/${variant}
+        INSTALL_DIR llvmlibc/${variant}/install
+        DEPENDS ${lib_tool_dependencies} ${libc_target} libc_hdrgen
+        CMAKE_ARGS
+        ${common_cmake_args}
+        -DLIBC_TARGET_TRIPLE=${target_triple}
+        -DLIBC_HDRGEN_EXE=${LIBC_HDRGEN}
+        -DLIBC_TARGET_OS=baremetal
+        -DLLVM_CMAKE_DIR=${LLVM_BINARY_DIR}/lib/cmake/llvm
+        -DLLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON
+        -DLLVM_ENABLE_RUNTIMES=libc
+        -DLLVM_INCLUDE_TESTS=OFF # I haven't yet got the tests to build
+        -DLLVM_LIBC_FULL_BUILD=ON
+        STEP_TARGETS build install
+        USES_TERMINAL_CONFIGURE FALSE
+        USES_TERMINAL_BUILD TRUE
+        USES_TERMINAL_INSTALL TRUE
+        USES_TERMINAL_TEST TRUE
+        LIST_SEPARATOR ,
+        # Always run the build command so that incremental builds are correct.
+        BUILD_ALWAYS TRUE
+        CONFIGURE_HANDLED_BY_BUILD TRUE
+        INSTALL_COMMAND ${CMAKE_COMMAND} --install .
+        # Copy llvm-libc lib directory, moving libraries out of their
+        # target-specific subdirectory.
+        COMMAND
+            ${CMAKE_COMMAND}
+            -E copy_directory
+            <INSTALL_DIR>/lib/${target_triple}
+            "${LLVM_BINARY_DIR}/${directory}/lib"
+        # And copy the include directory, which is already arranged right.
+        COMMAND
+            ${CMAKE_COMMAND}
+            -E copy_directory
+            <INSTALL_DIR>/include
+            "${LLVM_BINARY_DIR}/${directory}/include"
+    )
+
+    ExternalProject_Add(
+        llvmlibc-support_${variant}
+        SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/llvmlibc-support
+        PREFIX llvmlibc-support/${variant}
+        INSTALL_DIR "${LLVM_BINARY_DIR}/${directory}"
+        DEPENDS ${lib_tool_dependencies} llvmlibc_${variant}-install
+        CMAKE_ARGS ${common_cmake_args}
+        STEP_TARGETS build install
+        USES_TERMINAL_CONFIGURE FALSE
+        USES_TERMINAL_BUILD TRUE
+        USES_TERMINAL_INSTALL TRUE
+        USES_TERMINAL_TEST TRUE
+        LIST_SEPARATOR ,
+        # Always run the build command so that incremental builds are correct.
+        BUILD_ALWAYS TRUE
+        CONFIGURE_HANDLED_BY_BUILD TRUE
+    )
+
+    add_dependencies(
+        llvm-toolchain-runtimes
+        llvmlibc_${variant}
+        llvmlibc-support_${variant}
+    )
+endfunction()
+
 macro(
     add_libc
     directory
@@ -869,6 +1019,21 @@ macro(
             "${default_ram_size}"
             "${default_stack_size}"
         )
+    elseif(LLVM_TOOLCHAIN_C_LIBRARY STREQUAL llvmlibc)
+        add_llvmlibc(
+            "${directory}"
+            "${variant}"
+            "${target_triple}"
+            "${flags}"
+            "${test_executor_params}"
+            "${default_boot_flash_addr}"
+            "${default_boot_flash_size}"
+            "${default_flash_addr}"
+            "${default_flash_size}"
+            "${default_ram_addr}"
+            "${default_ram_size}"
+            "${default_stack_size}"
+        )
     endif()
 endmacro()
 
@@ -1231,25 +1396,29 @@ function(add_library_variant target_arch)
             "${VARIANT_COMPILE_FLAGS}"
             "${lit_test_executor}"
             "${LLVM_TOOLCHAIN_C_LIBRARY}_${variant}-install"
-        )
-        add_libcxx_libcxxabi_libunwind(
-            "${directory}"
-            "${variant}"
-            "${target_triple}"
-            "${VARIANT_COMPILE_FLAGS}"
-            "${lit_test_executor}"
-            "${LLVM_TOOLCHAIN_C_LIBRARY}_${variant}-install"
-            "${${LLVM_TOOLCHAIN_C_LIBRARY}_specific_runtimes_options}"
-            ${VARIANT_ENABLE_EXCEPTIONS}
-            ${VARIANT_ENABLE_RTTI}
-        )
+            )
+        if(CXX_LIBS)
+            add_libcxx_libcxxabi_libunwind(
+                "${directory}"
+                "${variant}"
+                "${target_triple}"
+                "${VARIANT_COMPILE_FLAGS}"
+                "${lit_test_executor}"
+                "${LLVM_TOOLCHAIN_C_LIBRARY}_${variant}-install"
+                "${${LLVM_TOOLCHAIN_C_LIBRARY}_specific_runtimes_options}"
+                ${VARIANT_ENABLE_EXCEPTIONS}
+                ${VARIANT_ENABLE_RTTI}
+            )
+        endif()
         if(VARIANT_COMPILE_FLAGS MATCHES "-march=armv8")
             message("C++ runtime libraries tests disabled for ${variant}")
         else()
             add_custom_target(check-llvm-toolchain-runtimes-${variant})
             add_dependencies(check-llvm-toolchain-runtimes check-llvm-toolchain-runtimes-${variant})
             add_compiler_rt_tests("${variant}")
-            add_libcxx_libcxxabi_libunwind_tests("${variant}")
+            if(CXX_LIBS)
+                add_libcxx_libcxxabi_libunwind_tests("${variant}")
+            endif()
         endif()
     endif()
 
@@ -1333,21 +1502,25 @@ set(multilib_yaml_content "")
 # For most variants, the "flash" memory is placed in address range, where
 # simulated boards have RAM. This is because code for some tests does not fit
 # the real flash.
-add_library_variants_for_cpu(
-    aarch64
-    COMPILE_FLAGS "-march=armv8-a"
-    MULTILIB_FLAGS "--target=aarch64-unknown-none-elf"
-    PICOLIBC_BUILD_TYPE "release"
-    QEMU_MACHINE "virt"
-    QEMU_CPU "cortex-a57"
-    BOOT_FLASH_ADDRESS 0x40000000
-    BOOT_FLASH_SIZE 0x1000
-    FLASH_ADDRESS 0x40001000
-    FLASH_SIZE 0xfff000
-    RAM_ADDRESS 0x41000000
-    RAM_SIZE 0x1000000
-    STACK_SIZE 8K
-)
+if(NOT (LLVM_TOOLCHAIN_C_LIBRARY STREQUAL llvmlibc))
+    # llvm-libc doesn't have a bare-metal configuration for AArch64, so we
+    # leave out the AArch64 library if we're building that libc.
+    add_library_variants_for_cpu(
+        aarch64
+        COMPILE_FLAGS "-march=armv8-a"
+        MULTILIB_FLAGS "--target=aarch64-unknown-none-elf"
+        PICOLIBC_BUILD_TYPE "release"
+        QEMU_MACHINE "virt"
+        QEMU_CPU "cortex-a57"
+        BOOT_FLASH_ADDRESS 0x40000000
+        BOOT_FLASH_SIZE 0x1000
+        FLASH_ADDRESS 0x40001000
+        FLASH_SIZE 0xfff000
+        RAM_ADDRESS 0x41000000
+        RAM_SIZE 0x1000000
+        STACK_SIZE 8K
+    )
+endif()
 # For AArch32, clang uses different defaults for FPU selection than GCC, both
 # when "+fp" or "+fp.dp" are used and when no FPU specifier is provided in
 # "-march=". Using "-mfpu=" explicitly.

README.md

@@ -7,7 +7,7 @@ bare-metal LLVM based toolchain targeting Arm based on:
 * libc++abi
 * libc++
 * compiler-rt
-* picolibc, or optionally newlib
+* picolibc, or optionally newlib or LLVM's libc
 
 ## Goal
 

cmake/generate_version_txt.cmake

@@ -23,12 +23,14 @@ execute_process(
     OUTPUT_STRIP_TRAILING_WHITESPACE
     COMMAND_ERROR_IS_FATAL ANY
 )
-execute_process(
-    COMMAND git -C ${${LLVM_TOOLCHAIN_C_LIBRARY}_SOURCE_DIR} rev-parse HEAD
-    OUTPUT_VARIABLE ${LLVM_TOOLCHAIN_C_LIBRARY}_COMMIT
-    OUTPUT_STRIP_TRAILING_WHITESPACE
-    COMMAND_ERROR_IS_FATAL ANY
-)
+if(NOT (LLVM_TOOLCHAIN_C_LIBRARY STREQUAL llvmlibc)) # libc in a separate repo?
+    execute_process(
+        COMMAND git -C ${${LLVM_TOOLCHAIN_C_LIBRARY}_SOURCE_DIR} rev-parse HEAD
+        OUTPUT_VARIABLE ${LLVM_TOOLCHAIN_C_LIBRARY}_COMMIT
+        OUTPUT_STRIP_TRAILING_WHITESPACE 
+        COMMAND_ERROR_IS_FATAL ANY
+    )
+endif()
 
 configure_file(
     ${CMAKE_CURRENT_LIST_DIR}/VERSION.txt.in