xtensa: support for C library provided by Xtensa toolchain

This change add an additional C library implementation
provided by Xtensa toolchain.

Signed-off-by: William Tambe <williamt@cadence.com>

Author: williamtcdns

arch/xtensa/core/crt1.S

@@ -188,6 +188,13 @@ _start:
 
 #endif /* !XCHAL_HAVE_BOOTLOADER */
 
+#if defined(CONFIG_XTENSA_LIBC)
+	rsr     a5, PRID
+	bnez    a5, 1f
+	CALL    _init_reent_bss
+1:
+#endif
+
 	/* Enter C domain, never returns from here */
 	CALL	z_prep_c
 

arch/xtensa/core/thread.c

@@ -27,6 +27,11 @@ Z_THREAD_LOCAL uint32_t is_user_mode;
 
 #endif /* CONFIG_USERSPACE */
 
+#if defined(CONFIG_XTENSA_LIBC) && defined(CONFIG_MULTITHREADING)
+#include <sys/reent.h>
+extern struct _reent *_reent_ptr;
+#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 +130,10 @@ 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) && defined(CONFIG_MULTITHREADING)
+	_init_reent(&thread->arch.reent);
+	_reent_ptr = &thread->arch.reent;
+#endif
 }
 
 #if defined(CONFIG_FPU) && defined(CONFIG_FPU_SHARING)

arch/xtensa/core/xtensa_asm2_util.S

@@ -331,6 +331,12 @@ noflush:
 	l32i a3, a2, ___xtensa_irq_bsa_t_a3_OFFSET
 	s32i a1, a3, 0
 
+#if defined(CONFIG_XTENSA_LIBC) && defined(CONFIG_MULTITHREADING)
+	rsr   a6, ZSR_CPU
+	l32i  a6, a6, ___cpu_t_current_OFFSET
+	call4 _xclib_update_reent_ptr
+#endif
+
 	/* Switch stack pointer and restore.  The jump to
 	 * _restore_context does not return as such, but we arrange
 	 * for the restored "next" address to be immediately after for

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/linker/xt-ld/linker_libraries.cmake

@@ -6,4 +6,9 @@ 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 sim_library  "-lsim")
+  set_linker_property(PROPERTY link_order_library "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;
+}