score: Add _CPU_Get_current_per_CPU_control()

Add optional method _CPU_Get_current_per_CPU_control() to obtain the
per-CPU control of the current processor.

This is optional.  Not every CPU port needs this.  It is only an
optional optimization variant.  In case this macro is undefined, the
default implementation using the current processor index will be used.

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

@@ -1462,6 +1462,24 @@ CPU_Counter_ticks _CPU_Counter_difference(
   CPU_Counter_ticks first
 );
 
+/**
+ * @brief Special register pointing to the per-CPU control of the current
+ * processor.
+ *
+ * This is optional.  Not every CPU port needs this.  It is only an optional
+ * optimization variant.
+ */
+register struct Per_CPU_Control *_CPU_Per_CPU_current asm( "rX" );
+
+/**
+ * @brief Optional method to obtain the per-CPU control of the current processor.
+ *
+ * This is optional.  Not every CPU port needs this.  It is only an optional
+ * optimization variant.  In case this macro is undefined, the default
+ * implementation using the current processor index will be used.
+ */
+#define _CPU_Get_current_per_CPU_control() ( _CPU_Per_CPU_current )
+
 #ifdef RTEMS_SMP
   /**
    * @brief Performs CPU specific SMP initialization in the context of the boot

cpukit/score/include/rtems/score/percpu.h

@@ -235,7 +235,7 @@ typedef struct {
  *
  *  This structure is used to hold per core state information.
  */
-typedef struct {
+typedef struct Per_CPU_Control {
   /**
    * @brief CPU port specific control.
    */
@@ -437,8 +437,12 @@ extern Per_CPU_Control_envelope _Per_CPU_Information[] CPU_STRUCTURE_ALIGNMENT;
  * we can use _Per_CPU_Get_snapshot()).  All other places must use
  * _Per_CPU_Get() so that we can add checks for RTEMS_DEBUG.
  */
-#define _Per_CPU_Get_snapshot() \
-  ( &_Per_CPU_Information[ _SMP_Get_current_processor() ].per_cpu )
+#if defined( _CPU_Get_current_per_CPU_control )
+  #define _Per_CPU_Get_snapshot() _CPU_Get_current_per_CPU_control()
+#else
+  #define _Per_CPU_Get_snapshot() \
+    ( &_Per_CPU_Information[ _SMP_Get_current_processor() ].per_cpu )
+#endif
 
 #if defined( RTEMS_SMP )
 static inline Per_CPU_Control *_Per_CPU_Get( void )