arch: kernel: lib: toolchain: Standardize TLS keyword

Up until now, the `__thread` keyword has been used for declaring
variables as Thread local storage. However, `__thread` is a GNU
specific keyword which thus limits compatibility with other
toolchains (for instance IAR).

This PR intoduces a new macro `Z_THREAD_LOCAL` which expands to the
corresponding C11, C23 or C++11 standard keyword based on the standard
that is specified during compilation, else it uses the old `__thread`
keyword.

Signed-off-by: Daniel Flodin <daniel.flodin@iar.com>

Author: danielfl-iar

arch/riscv/core/thread.c

@@ -15,7 +15,7 @@
 /*
  * Per-thread (TLS) variable indicating whether execution is in user mode.
  */
-__thread uint8_t is_user_mode;
+Z_THREAD_LOCAL uint8_t is_user_mode;
 #endif
 
 void arch_new_thread(struct k_thread *thread, k_thread_stack_t *stack,

arch/xtensa/core/thread.c

@@ -22,7 +22,7 @@ LOG_MODULE_DECLARE(os, CONFIG_KERNEL_LOG_LEVEL);
 /*
  * Per-thread (TLS) variable indicating whether execution is in user mode.
  */
-__thread uint32_t is_user_mode;
+Z_THREAD_LOCAL uint32_t is_user_mode;
 #endif
 
 #endif /* CONFIG_USERSPACE */

doc/kernel/services/other/thread_local_storage.rst

@@ -29,25 +29,25 @@ making a system call.
 Declaring and Using Thread Local Variables
 ******************************************
 
-The keyword ``__thread`` can be used to declare thread local variables.
+The macro ``Z_THREAD_LOCAL`` can be used to declare thread local variables.
 
 For example, to declare a thread local variable in header files:
 
 .. code-block:: c
 
-   extern __thread int i;
+   extern Z_THREAD_LOCAL int i;
 
 And to declare the actual variable in source files:
 
 .. code-block:: c
 
-   __thread int i;
+   Z_THREAD_LOCAL int i;
 
 Keyword ``static`` can also be used to limit the variable within a source file:
 
 .. code-block:: c
 
-   static __thread int j;
+   static Z_THREAD_LOCAL int j;
 
 Using the thread local variable is the same as using other variable, for example:
 

include/zephyr/toolchain/common.h

@@ -39,6 +39,24 @@
 #endif
 #endif
 
+/*
+ * Thread local variables are declared with different keywords depending on
+ * which C/C++ standard that is used. C++11 and C23 uses "thread_local" whilst
+ * C11 uses "_Thread_local". Previously the GNU "__thread" keyword was used
+ * which is the same in both gcc and g++.
+ */
+#ifndef Z_THREAD_LOCAL
+#if defined(__cplusplus) && (__cplusplus) >= 201103L /* C++11 */
+#define Z_THREAD_LOCAL thread_local
+#elif defined(__STDC_VERSION__) && (__STDC_VERSION__) >= 202311L /* C23 */
+#define Z_THREAD_LOCAL thread_local
+#elif defined(__STDC_VERSION__) && (__STDC_VERSION__) >= 201112L /* C11 */
+#define Z_THREAD_LOCAL _Thread_local
+#else /* Default back to old behavior which used the GNU keyword. */
+#define Z_THREAD_LOCAL __thread
+#endif
+#endif /* Z_THREAD_LOCAL */
+
 /*
  * Generate a reference to an external symbol.
  * The reference indicates to the linker that the symbol is required

kernel/compiler_stack_protect.c

@@ -47,7 +47,7 @@ void _StackCheckHandler(void)
  * The canary value gets initialized in z_cstart().
  */
 #ifdef CONFIG_STACK_CANARIES_TLS
-__thread volatile uintptr_t __stack_chk_guard;
+Z_THREAD_LOCAL volatile uintptr_t __stack_chk_guard;
 #elif CONFIG_USERSPACE
 K_APP_DMEM(z_libc_partition) volatile uintptr_t __stack_chk_guard;
 #else

kernel/errno.c

@@ -27,7 +27,7 @@ const int _k_neg_eagain = -EAGAIN;
 #if defined(CONFIG_LIBC_ERRNO)
 /* nothing needed here */
 #elif defined(CONFIG_ERRNO_IN_TLS)
-__thread int z_errno_var;
+Z_THREAD_LOCAL int z_errno_var;
 #else
 
 #ifdef CONFIG_USERSPACE

kernel/init.c

@@ -291,7 +291,7 @@ void z_bss_zero_pinned(void)
 
 #ifdef CONFIG_STACK_CANARIES
 #ifdef CONFIG_STACK_CANARIES_TLS
-extern __thread volatile uintptr_t __stack_chk_guard;
+extern Z_THREAD_LOCAL volatile uintptr_t __stack_chk_guard;
 #else
 extern volatile uintptr_t __stack_chk_guard;
 #endif /* CONFIG_STACK_CANARIES_TLS */

kernel/xip.c

@@ -12,7 +12,7 @@
 
 #ifdef CONFIG_STACK_CANARIES
 #ifdef CONFIG_STACK_CANARIES_TLS
-extern __thread volatile uintptr_t __stack_chk_guard;
+extern Z_THREAD_LOCAL volatile uintptr_t __stack_chk_guard;
 #else
 extern volatile uintptr_t __stack_chk_guard;
 #endif /* CONFIG_STACK_CANARIES_TLS */

lib/os/thread_entry.c

@@ -15,11 +15,11 @@
 #ifdef CONFIG_CURRENT_THREAD_USE_TLS
 #include <zephyr/random/random.h>
 
-__thread k_tid_t z_tls_current;
+Z_THREAD_LOCAL k_tid_t z_tls_current;
 #endif
 
 #ifdef CONFIG_STACK_CANARIES_TLS
-extern __thread volatile uintptr_t __stack_chk_guard;
+extern Z_THREAD_LOCAL volatile uintptr_t __stack_chk_guard;
 #endif /* CONFIG_STACK_CANARIES_TLS */
 
 /*

tests/kernel/mem_protect/stackprot/src/main.c

@@ -143,7 +143,7 @@ ZTEST(stackprot, test_create_alt_thread)
 }
 
 #ifdef CONFIG_STACK_CANARIES_TLS
-extern __thread volatile uintptr_t __stack_chk_guard;
+extern Z_THREAD_LOCAL volatile uintptr_t __stack_chk_guard;
 #else
 extern volatile uintptr_t __stack_chk_guard;
 #endif

tests/kernel/threads/tls/src/main.c

@@ -51,14 +51,14 @@ K_APP_BMEM(part_common) static k_tid_t tls_tid[NUM_THREADS];
 K_APP_BMEM(part_common) static enum test_result tls_result[NUM_THREADS];
 
 /* Thread data with initialized values */
-static uint8_t  __thread thread_data8  = STATIC_DATA8;
-static uint32_t __thread thread_data32 = STATIC_DATA32;
-static uint64_t __thread thread_data64 = STATIC_DATA64;
+static uint8_t Z_THREAD_LOCAL thread_data8 = STATIC_DATA8;
+static uint32_t Z_THREAD_LOCAL thread_data32 = STATIC_DATA32;
+static uint64_t Z_THREAD_LOCAL thread_data64 = STATIC_DATA64;
 
 /* Zeroed thread data */
-static uint8_t  __thread thread_bss8;
-static uint32_t __thread thread_bss32;
-static uint64_t __thread thread_bss64;
+static uint8_t Z_THREAD_LOCAL thread_bss8;
+static uint32_t Z_THREAD_LOCAL thread_bss32;
+static uint64_t Z_THREAD_LOCAL thread_bss64;
 
 static void tls_thread_entry(void *p1, void *p2, void *p3)
 {