xtensa: support for C library provided by Xtensa toolchain

arch/xtensa/core/thread.c

@@ -27,6 +27,10 @@ Z_THREAD_LOCAL uint32_t is_user_mode;
 
 #endif /* CONFIG_USERSPACE */
 
+#if defined(CONFIG_XTENSA_LIBC)
+#include <sys/reent.h>
+#endif
+
 /**
  * Initializes a stack area such that it can be "restored" later and
  * begin running with the specified function and three arguments.  The
@@ -125,6 +129,9 @@ void arch_new_thread(struct k_thread *thread, k_thread_stack_t *stack,
 	__ASSERT((((size_t)stack_ptr) % XCHAL_DCACHE_LINESIZE) == 0, "");
 	sys_cache_data_flush_and_invd_range(stack, (char *)stack_ptr - (char *)stack);
 #endif
+#if defined(CONFIG_XTENSA_LIBC)
+	_init_reent(&thread->arch.reent);
+#endif
 }
 
 #if defined(CONFIG_FPU) && defined(CONFIG_FPU_SHARING)

boards/cdns/xt-sim/xt-sim_defconfig

@@ -8,3 +8,5 @@ CONFIG_GEN_IRQ_VECTOR_TABLE=n
 CONFIG_SIMULATOR_XTENSA=y
 
 CONFIG_SYS_CLOCK_HW_CYCLES_PER_SEC=2000000
+
+CONFIG_XTENSA_LIBC=y

cmake/compiler/xt-clang/compiler_flags.cmake

@@ -11,7 +11,7 @@ set_compiler_property(PROPERTY nostdinc_include)
 # For C++ code, re-add the standard includes directory which was
 # cleared up from nostdinc_inlcude in above lines with no
 # "include-fixed" this time"
-if(CONFIG_CPP)
+if(CONFIG_CPP AND NOT CONFIG_XTENSA_LIBC)
   execute_process(
     COMMAND ${CMAKE_C_COMPILER} --print-file-name=include/stddef.h
     OUTPUT_VARIABLE _OUTPUT

cmake/linker/xt-ld/linker_flags.cmake

@@ -8,7 +8,11 @@ check_set_linker_property(TARGET linker PROPERTY base
 )
 
 if(NOT CONFIG_NATIVE_LIBRARY AND NOT CONFIG_EXTERNAL_MODULE_LIBCPP)
-  set_property(TARGET linker PROPERTY cpp_base -lstdc++)
+  if (CONFIG_XTENSA_LIBC)
+    set_property(TARGET linker PROPERTY cpp_base -lstdc++11)
+  else()
+    set_property(TARGET linker PROPERTY cpp_base -lstdc++)
+  endif()
 endif()
 
 check_set_linker_property(TARGET linker PROPERTY baremetal

cmake/linker/xt-ld/linker_libraries.cmake

@@ -6,4 +6,10 @@ set_linker_property(NO_CREATE PROPERTY c_library    "-lc")
 set_linker_property(NO_CREATE PROPERTY rt_library   "-lgcc")
 set_linker_property(NO_CREATE PROPERTY c++_library  "-lstdc++")
 set_linker_property(NO_CREATE PROPERTY hal_library  "-lhal")
-set_linker_property(PROPERTY link_order_library "c;rt;hal")
+if(CONFIG_XTENSA_LIBC)
+  set_linker_property(NO_CREATE PROPERTY m_library   "-lm")
+  set_linker_property(NO_CREATE PROPERTY sim_library "-lsim")
+  set_linker_property(PROPERTY link_order_library "m;c;rt;sim;hal")
+else()
+  set_linker_property(PROPERTY link_order_library "c;rt;hal")
+endif()

include/zephyr/arch/xtensa/thread.h

@@ -14,6 +14,10 @@
 #include <zephyr/arch/xtensa/mpu.h>
 #endif
 
+#ifdef CONFIG_XTENSA_LIBC
+#include <sys/reent.h>
+#endif
+
 /* Xtensa doesn't use these structs, but Zephyr core requires they be
  * defined so they can be included in struct _thread_base.  Dummy
  * field exists for sizeof compatibility with C++.
@@ -48,6 +52,9 @@ struct _thread_arch {
 	 */
 	uint32_t return_ps;
 #endif
+#ifdef CONFIG_XTENSA_LIBC
+	struct _reent reent;
+#endif
 };
 
 typedef struct _thread_arch _thread_arch_t;

lib/libc/CMakeLists.txt

@@ -10,5 +10,6 @@ add_subdirectory_ifdef(CONFIG_IAR_LIBC          iar)
 add_subdirectory_ifdef(CONFIG_MINIMAL_LIBC      minimal)
 add_subdirectory_ifdef(CONFIG_NEWLIB_LIBC       newlib)
 add_subdirectory_ifdef(CONFIG_PICOLIBC          picolibc)
+add_subdirectory_ifdef(CONFIG_XTENSA_LIBC       xclib)
 
 add_subdirectory(common)

lib/libc/Kconfig

@@ -143,6 +143,13 @@ config IAR_LIBC
 	  A reduced Zephyr minimal libc will be used for library functionality
 	  not provided by the IAR C Runtime Library.
 
+config XTENSA_LIBC
+	bool "Xtensa C library"
+	depends on !NATIVE_APPLICATION
+	depends on "$(ZEPHYR_TOOLCHAIN_VARIANT)" = "xt-clang"
+	help
+	  C library provided by Xtensa toolchain.
+
 endchoice # LIBC_IMPLEMENTATION
 
 config HAS_NEWLIB_LIBC_NANO

lib/libc/xclib/CMakeLists.txt

@@ -0,0 +1,8 @@
+# SPDX-License-Identifier: Apache-2.0
+
+zephyr_library()
+
+zephyr_library_sources(
+  libc-hooks.c
+  multithreading.c
+)

lib/libc/xclib/libc-hooks.c

@@ -0,0 +1,15 @@
+/*
+ * Copyright (c) 2025 Cadence Design Systems, Inc.
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+char __os_flag __attribute__((retain)); /* Needed by xclib to indicate the presense of an OS */
+
+int arch_printk_char_out(int c);
+static int (*_stdout_hook)(int c) = arch_printk_char_out;
+
+void __stdout_hook_install(int (*hook)(int))
+{
+	_stdout_hook = hook;
+}

lib/libc/xclib/multithreading.c

@@ -0,0 +1,67 @@
+/*
+ * Copyright (c) 2025 Cadence Design Systems, Inc.
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <zephyr/kernel.h>
+#include <stdlib.h>
+#include <sys/reent.h>
+
+int32_t _xclib_use_mt = 1; /* Enables xclib multithread support */
+
+typedef struct k_mutex *_Rmtx;
+
+/* Number of Locks needed by _Initlocks() without calling malloc()
+ * which would infinitely call _Initlocks() recursively.
+ */
+#define NLOCKS (_MAX_LOCK + 1)
+
+static struct k_mutex _Mtxinit_mtx[NLOCKS];
+
+void _Mtxinit(_Rmtx *mtx)
+{
+	static int i; /* static variable initially 0 */
+
+	if (i < NLOCKS) {
+		*mtx = &_Mtxinit_mtx[i];
+		++i;
+	} else {
+		*mtx = malloc(sizeof(struct k_mutex));
+	}
+	if (!*mtx) {
+		k_panic();
+	}
+	k_mutex_init(*mtx);
+}
+
+void _Mtxdst(_Rmtx *mtx)
+{
+	if (mtx != NULL && *mtx != NULL &&
+		(*mtx > &_Mtxinit_mtx[NLOCKS] || *mtx < &_Mtxinit_mtx[0])) {
+		free(*mtx);
+	}
+}
+
+void _Mtxlock(_Rmtx *mtx)
+{
+	if ((mtx != NULL) && (*mtx != NULL)) {
+		k_mutex_lock(*mtx, K_FOREVER);
+	}
+}
+
+void _Mtxunlock(_Rmtx *mtx)
+{
+	if ((mtx != NULL) && (*mtx != NULL)) {
+		k_mutex_unlock(*mtx);
+	}
+}
+
+#if defined(__DYNAMIC_REENT__)
+struct _reent *__getreent(void)
+{
+	return &k_current_get()->arch.reent;
+}
+#else
+#error __DYNAMIC_REENT__ support missing
+#endif