score: Add _CPU_Context_switch_no_return()

The __builtin_unreachable() cannot be used with current GCC versions to
tell the compiler that a function does not return to the caller, see:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99151

Add a no return variant of _CPU_Context_switch() to avoid generation of
dead code in _Thread_Start_multitasking() if RTEMS was built with SMP
support enabled.

bsps/powerpc/shared/cpu_asm.S

@@ -258,7 +258,9 @@ PROC (_CPU_Context_restore_fp):
 
 	ALIGN (PPC_CACHE_ALIGNMENT, PPC_CACHE_ALIGN_POWER)
 	PUBLIC_PROC (_CPU_Context_switch)
+	PUBLIC_PROC (_CPU_Context_switch_no_return)
 PROC (_CPU_Context_switch):
+PROC (_CPU_Context_switch_no_return):
 
 #ifdef BSP_USE_SYNC_IN_CONTEXT_SWITCH
 	sync

cpukit/score/cpu/arm/cpu_asm.S

@@ -54,6 +54,9 @@
  */
 
 DEFINE_FUNCTION_ARM(_CPU_Context_switch)
+	.globl	_CPU_Context_switch_no_return
+	.set	_CPU_Context_switch_no_return, _CPU_Context_switch
+
 /* Start saving context */
 	GET_SELF_CPU_CONTROL	r2
 	ldr	r3, [r2, #PER_CPU_ISR_DISPATCH_DISABLE]

cpukit/score/cpu/arm/include/rtems/score/cpu.h

@@ -465,6 +465,11 @@ void _CPU_ISR_install_vector(
  */
 void _CPU_Context_switch( Context_Control *run, Context_Control *heir );
 
+RTEMS_NO_RETURN void _CPU_Context_switch_no_return(
+  Context_Control *executing,
+  Context_Control *heir
+);
+
 RTEMS_NO_RETURN void _CPU_Context_restore( Context_Control *new_context );
 
 #if defined(ARM_MULTILIB_ARCH_V7M)

cpukit/score/cpu/no_cpu/include/rtems/score/cpu.h

@@ -1054,22 +1054,46 @@ void _CPU_ISR_install_vector(
 void *_CPU_Thread_Idle_body( uintptr_t ignored );
 
 /**
+ * @brief Performs a context switch from the executing thread to the heir
+ *   thread.
+ *
  * @addtogroup RTEMSScoreCPUExampleContext
  * 
- * This routine switches from the run context to the heir context.
+ * This routine switches from the executing context to the heir context.
  *
- * @param[in] run points to the context of the currently executing task
- * @param[in] heir points to the context of the heir task
+ * @param[out] executing points to the context of the currently executing task.
+ *
+ * @param[in, out] heir points to the context of the heir task.
  *
  * Port Specific Information:
  *
  * XXX document implementation including references if appropriate
  */
 void _CPU_Context_switch(
-  Context_Control  *run,
+  Context_Control  *executing,
   Context_Control  *heir
 );
 
+/**
+ * @brief Performs a context switch from the executing thread to the heir
+ *   thread and does not return.
+ *
+ * @addtogroup RTEMSScoreCPUExampleContext
+ *
+ * This routine shall be a strong alias to _CPU_Context_switch().  It shall be
+ * provided for all target architectures which support an SMP build
+ * configuration (RTEMS_SMP).  The purpose is help to compiler to avoid
+ * generation of dead code in _Thread_Start_multitasking().
+ *
+ * @param[out] executing points to the context of the currently executing task.
+ *
+ * @param[in, out] heir points to the context of the heir task.
+ */
+RTEMS_NO_RETURN void _CPU_Context_switch_no_return(
+  Context_Control *executing,
+  Context_Control *heir
+);
+
 /**
  * @addtogroup RTEMSScoreCPUExampleContext
  * 

cpukit/score/cpu/powerpc/include/rtems/score/cpu.h

@@ -914,6 +914,11 @@ void _CPU_Context_switch(
   Context_Control  *heir
 );
 
+RTEMS_NO_RETURN void _CPU_Context_switch_no_return(
+  Context_Control *executing,
+  Context_Control *heir
+);
+
 /*
  *  _CPU_Context_restore
  *

cpukit/score/cpu/riscv/include/rtems/score/cpu.h

@@ -383,6 +383,11 @@ void _CPU_Context_switch(
   Context_Control  *heir
 );
 
+RTEMS_NO_RETURN void _CPU_Context_switch_no_return(
+  Context_Control *executing,
+  Context_Control *heir
+);
+
 /*
  *  _CPU_Context_restore
  *

cpukit/score/cpu/riscv/riscv-context-switch.S

@@ -37,9 +37,11 @@
 	.align	2
 
 PUBLIC(_CPU_Context_switch)
+PUBLIC(_CPU_Context_switch_no_return)
 PUBLIC(_CPU_Context_restore)
 
 SYM(_CPU_Context_switch):
+SYM(_CPU_Context_switch_no_return):
 	GET_SELF_CPU_CONTROL	a2
 	lw	a3, PER_CPU_ISR_DISPATCH_DISABLE(a2)
 

cpukit/score/cpu/sparc/cpu_asm.S

@@ -57,7 +57,9 @@
 
         .align 4
         PUBLIC(_CPU_Context_switch)
+        PUBLIC(_CPU_Context_switch_no_return)
 SYM(_CPU_Context_switch):
+SYM(_CPU_Context_switch_no_return):
         st      %g5, [%o0 + G5_OFFSET]       ! save the global registers
 
         /*

cpukit/score/cpu/sparc/include/rtems/score/cpu.h

@@ -971,6 +971,11 @@ void _CPU_Context_switch(
   Context_Control  *heir
 );
 
+RTEMS_NO_RETURN void _CPU_Context_switch_no_return(
+  Context_Control *executing,
+  Context_Control *heir
+);
+
 /**
  * @brief SPARC specific context restore.
  *

cpukit/score/src/threadstartmultitasking.c

@@ -59,7 +59,7 @@ void _Thread_Start_multitasking( void )
      * executing to the currently selected heir thread.
      */
     _CPU_Context_Set_is_executing( &trash, true );
-    _CPU_Context_switch( &trash, &heir->Registers );
+    _CPU_Context_switch_no_return( &trash, &heir->Registers );
     RTEMS_UNREACHABLE();
   }
 #else