Deprecate uses_fp: always enable FPU state save on all threads

GCC on x86-64 may emit SSE instructions (e.g. for struct zeroing)
even in functions that don't explicitly use floating point. On RTAI,
threads created with uses_fp=0 skip FPU/SSE state save/restore,
silently corrupting XMM registers of other Linux processes and
causing heap corruption and system crashes.

Fix by always enabling FPU state save regardless of the uses_fp
parameter:
- rtai_rtapi.c: always pass 1 to rt_task_init_cpuid
- hal_lib.c: override uses_fp=0 to 1 in hal_create_thread
- hal_lib.c: remove addf FP compatibility check (now meaningless)

Emit deprecation warnings when uses_fp=0 is requested.

Ref: #3895

Author: Luca Toniolo

src/hal/hal_lib.c

@@ -1917,6 +1917,12 @@ int hal_create_thread(const char *name, unsigned long period_nsec, int uses_fp)
     long prev_period, curr_period;
     char buf[HAL_NAME_LEN + 1];
 
+    if (!uses_fp) {
+	rtapi_print_msg(RTAPI_MSG_WARN,
+	    "HAL: WARNING: thread '%s' created with uses_fp=0, "
+	    "overriding to uses_fp=1 (uses_fp is deprecated).\n", name);
+	uses_fp = 1;
+    }
     rtapi_print_msg(RTAPI_MSG_DBG,
 	"HAL: creating thread %s, %ld nsec\n", name, period_nsec);
     if (hal_data == 0) {
@@ -2202,13 +2208,8 @@ int hal_add_funct_to_thread(const char *funct_name, const char *thread_name, int
 	    "HAL: ERROR: thread '%s' not found\n", thread_name);
 	return -EINVAL;
     }
-    /* ok, we have thread and function, are they compatible? */
-    if ((funct->uses_fp) && (!thread->uses_fp)) {
-	rtapi_mutex_give(&(hal_data->mutex));
-	rtapi_print_msg(RTAPI_MSG_ERR,
-	    "HAL: ERROR: function '%s' needs FP\n", funct_name);
-	return -EINVAL;
-    }
+    /* All threads now save FPU state (uses_fp is deprecated),
+       so the FP compatibility check is no longer needed. */
     /* find insertion point */
     list_root = &(thread->funct_list);
     list_entry = list_root;

src/rtapi/rtai_rtapi.c

@@ -685,9 +685,18 @@ int rtapi_task_new(void (*taskcode) (void *), void *arg,
     }
     task->taskcode = taskcode;
     task->arg = arg;
-    /* call OS to initialize the task - use predetermined CPU */
+    if (!uses_fp) {
+	rtapi_print_msg(RTAPI_MSG_WARN,
+	    "RTAPI: WARNING: task created with uses_fp=0, "
+	    "overriding to uses_fp=1. All threads now save FPU state "
+	    "(uses_fp is deprecated).\n");
+    }
+    /* call OS to initialize the task - use predetermined CPU
+       Always enable FPU state save/restore: modern compilers may emit
+       SSE instructions even in code that doesn't explicitly use FP,
+       causing silent XMM register corruption without FPU save. */
     retval = rt_task_init_cpuid(ostask_array[task_id], wrapper, task_id,
-	 stacksize, prio, uses_fp, 0 /* signal */, rtapi_data->rt_cpu );
+	 stacksize, prio, 1 /* always save FPU */, 0 /* signal */, rtapi_data->rt_cpu );
     if (retval != 0) {
 	/* couldn't create task, free task data memory */
 	kfree(ostask_array[task_id]);