arch: riscv: Fix warning when C++ is enabled

When compiling with C++ enabled (CONFIG_CPP), add an unused member to
prevent an empty struct; this makes the struct size the same for both C
and C++.

Fixes the following warnings:

In file included from include/zephyr/drivers/gpio.h:22:
In file included from include/zephyr/tracing/tracing.h:9:
In file included from include/zephyr/kernel.h:17:
In file included from include/zephyr/kernel_includes.h:32:
In file included from include/zephyr/kernel_structs.h:29:
In file included from include/zephyr/arch/structs.h:29:
include/zephyr/arch/riscv/structs.h:11:1: error: empty struct has size 0
in C, size 1 in C++ [-Werror,-Wextern-c-compat]
   11 | struct _cpu_arch {
      | ^

In file included from include/zephyr/drivers/gpio.h:22:
In file included from include/zephyr/tracing/tracing.h:9:
In file included from include/zephyr/kernel.h:17:
In file included from include/zephyr/kernel_includes.h:36:
In file included from include/zephyr/arch/cpu.h:25:
In file included from include/zephyr/arch/riscv/arch.h:18:
include/zephyr/arch/riscv/thread.h:68:1: error: empty struct has size 0
in C, size 1 in C++ [-Werror,-Wextern-c-compat]
   68 | struct _thread_arch {
      | ^

Signed-off-by: Tom Hughes <[email protected]>

Author: thughes

include/zephyr/arch/riscv/structs.h

@@ -22,6 +22,15 @@ struct _cpu_arch {
 	atomic_ptr_val_t fpu_owner;
 	uint32_t fpu_state;
 #endif
+#if defined(CONFIG_CPP) && !defined(CONFIG_USERSPACE) &&                                           \
+	!(defined(CONFIG_SMP) || (CONFIG_MP_MAX_NUM_CPUS > 1)) && !defined(CONFIG_FPU_SHARING)
+	/* Empty struct has size 0 in C, size 1 in C++. Force them to be the same. */
+	uint8_t unused_cpp_size_compatibility;
+#endif
 };
 
+#if defined(CONFIG_CPP)
+BUILD_ASSERT(sizeof(struct _cpu_arch) >= 1);
+#endif
+
 #endif /* ZEPHYR_INCLUDE_RISCV_STRUCTS_H_ */

include/zephyr/arch/riscv/thread.h

@@ -84,8 +84,17 @@ struct _thread_arch {
 	unsigned long m_mode_pmpaddr_regs[PMP_M_MODE_SLOTS];
 	unsigned long m_mode_pmpcfg_regs[PMP_M_MODE_SLOTS / sizeof(unsigned long)];
 #endif
+#if defined(CONFIG_CPP) && !defined(CONFIG_FPU_SHARING) && !defined(CONFIG_USERSPACE) &&           \
+	!defined(CONFIG_PMP_STACK_GUARD)
+	/* Empty struct has size 0 in C, size 1 in C++. Force them to be the same. */
+	uint8_t unused_cpp_size_compatibility;
+#endif
 };
 
+#if defined(CONFIG_CPP)
+BUILD_ASSERT(sizeof(struct _thread_arch) >= 1);
+#endif
+
 typedef struct _thread_arch _thread_arch_t;
 
 #endif /* _ASMLANGUAGE */