llvm: Add support for selecting runtime library

This patch adds Kconfig options to select either GNU libgcc or LLVM
compiler-rt. The 'rtlib' flag is provided in a config file, so this
patch introduces 'clang_libgcc.cfg' and 'clang_compiler_rt.cfg' which
enable appropriate library. The file is selected by concatenating
the 'clang_' prefix with library name.

Signed-off-by: Patryk Duda <pdk@semihalf.com>

cmake/linker/ld/target_base.cmake

@@ -34,8 +34,14 @@ macro(toolchain_ld_base)
   endif()
 
   if (CONFIG_LLVM_USE_LD)
+    if(CONFIG_LIBGCC_RTLIB)
+      set(runtime_lib "libgcc")
+    elseif(CONFIG_COMPILER_RT_RTLIB)
+      set(runtime_lib "compiler_rt")
+    endif()
+
     zephyr_link_libraries(
-      --config ${ZEPHYR_BASE}/cmake/toolchain/llvm/clang.cfg
+      --config ${ZEPHYR_BASE}/cmake/toolchain/llvm/clang_${runtime_lib}.cfg
     )
   endif()
 

cmake/linker/lld/target_base.cmake

@@ -41,7 +41,13 @@ macro(toolchain_ld_base)
     )
   endif()
 
+  if(CONFIG_LIBGCC_RTLIB)
+    set(runtime_lib "libgcc")
+  elseif(CONFIG_COMPILER_RT_RTLIB)
+    set(runtime_lib "compiler_rt")
+  endif()
+
   zephyr_link_libraries(
-    --config ${ZEPHYR_BASE}/cmake/toolchain/llvm/clang.cfg
+    --config ${ZEPHYR_BASE}/cmake/toolchain/llvm/clang_${runtime_lib}.cfg
   )
 endmacro()

cmake/toolchain/llvm/clang_compiler_rt.cfg

@@ -0,0 +1,4 @@
+# Copyright (c) 2023 The ChromiumOS Authors
+# SPDX-License-Identifier: Apache-2.0
+
+--rtlib=compiler-rt

cmake/toolchain/llvm/generic.cmake

@@ -19,7 +19,4 @@ set(BINTOOLS llvm)
 
 set(TOOLCHAIN_HAS_NEWLIB OFF CACHE BOOL "True if toolchain supports newlib")
 
-list(APPEND TOOLCHAIN_C_FLAGS --config ${ZEPHYR_BASE}/cmake/toolchain/llvm/clang.cfg)
-list(APPEND TOOLCHAIN_LD_FLAGS --config ${ZEPHYR_BASE}/cmake/toolchain/llvm/clang.cfg)
-
 message(STATUS "Found toolchain: host (clang/ld)")

cmake/toolchain/llvm/target.cmake

@@ -42,3 +42,14 @@ if(DEFINED triple)
 
   unset(triple)
 endif()
+
+if(CONFIG_LIBGCC_RTLIB)
+  set(runtime_lib "libgcc")
+elseif(CONFIG_COMPILER_RT_RTLIB)
+  set(runtime_lib "compiler_rt")
+endif()
+
+list(APPEND TOOLCHAIN_C_FLAGS --config
+	${ZEPHYR_BASE}/cmake/toolchain/llvm/clang_${runtime_lib}.cfg)
+list(APPEND TOOLCHAIN_LD_FLAGS --config
+	${ZEPHYR_BASE}/cmake/toolchain/llvm/clang_${runtime_lib}.cfg)

lib/Kconfig

@@ -18,4 +18,6 @@ source "lib/open-amp/Kconfig"
 source "lib/smf/Kconfig"
 
 source "lib/acpi/Kconfig"
+
+source "lib/runtime/Kconfig"
 endmenu

lib/runtime/Kconfig

@@ -0,0 +1,26 @@
+# Copyright (c) 2023 The ChromiumOS Authors
+# SPDX-License-Identifier: Apache-2.0
+
+config COMPILER_RT_SUPPORTED
+	bool
+	default y
+	depends on "${ZEPHYR_TOOLCHAIN_VARIANT}" = "llvm"
+	help
+	  Selected when the compiler supports compiler-rt runtime library.
+
+choice RTLIB_IMPLEMENTATION
+	prompt "Runtime library implementation"
+	default LIBGCC_RTLIB
+
+config LIBGCC_RTLIB
+	bool "GNU Libgcc"
+	help
+	  Use libgcc as a runtime library.
+
+config COMPILER_RT_RTLIB
+	bool "LLVM compiler-rt"
+	depends on COMPILER_RT_SUPPORTED
+	help
+	  Use LLVM compiler-rt as a runtime library.
+
+endchoice

scripts/ci/check_compliance.py

@@ -631,6 +631,7 @@ flagged.
         "CDC_ACM_PORT_NAME_",
         "CLOCK_STM32_SYSCLK_SRC_",
         "CMU",
+        "COMPILER_RT_RTLIB",
         "BT_6LOWPAN",  # Defined in Linux, mentioned in docs
         "CMD_CACHE",  # Defined in U-Boot, mentioned in docs
         "COUNTER_RTC_STM32_CLOCK_SRC",
@@ -647,6 +648,7 @@ flagged.
         "FOO_SETTING_1",
         "FOO_SETTING_2",
         "LSM6DSO_INT_PIN",
+        "LIBGCC_RTLIB",
         "LLVM_USE_LD",   # Both LLVM_USE_* are in cmake/toolchain/llvm/Kconfig
         "LLVM_USE_LLD",  # which are only included if LLVM is selected but
                          # not other toolchains. Compliance check would complain,