AP_Scripting: stash and restore FPU register context in LUAI_TRY

this fixes an issue where a lua library function triggers an exception
after it does a math operation which changes the floating point
registers on M7 MCUs (such as STM32H7). An example is math.random()
which calls math_random(), which pre-calculates a double value before
checking if the arguments to the call are valid. When it then checks
and finds invalid values the exception longjmp does not restore the
floating point registers.

libraries/AP_Scripting/AP_Scripting.cpp

@@ -222,6 +222,15 @@ void AP_Scripting::repl_stop(void) {
     // can't do any more cleanup here, closing the open FD's is the REPL's responsibility
 }
 
+/*
+  avoid optimisation of the thread function. This avoids nasty traps
+  where setjmp/longjmp does not properly handle save/restore of
+  floating point registers on exceptions. This is an extra protection
+  over the top of the fix in luaD_rawrunprotected() for the same issue
+ */
+#pragma GCC push_options
+#pragma GCC optimize ("O0")
+
 void AP_Scripting::thread(void) {
     while (true) {
         // reset flags
@@ -289,6 +298,7 @@ void AP_Scripting::thread(void) {
         }
     }
 }
+#pragma GCC pop_options
 
 void AP_Scripting::handle_mission_command(const AP_Mission::Mission_Command& cmd_in)
 {

libraries/AP_Scripting/lua/src/ldo.c

@@ -133,21 +133,29 @@ l_noret luaD_throw (lua_State *L, int errcode) {
   }
 }
 
-
+// remove optimization
+#pragma GCC push_options
+#pragma GCC optimize ("O0")
 int luaD_rawrunprotected (lua_State *L, Pfunc f, void *ud) {
   unsigned short oldnCcalls = L->nCcalls;
   struct lua_longjmp lj;
   lj.status = LUA_OK;
   lj.previous = L->errorJmp;  /* chain new error handler */
   L->errorJmp = &lj;
   LUAI_TRY(L, &lj,
+#ifdef ARM_MATH_CM7
+    __asm__("vpush {s16-s31}");
+#endif
     (*f)(L, ud);
   );
+#ifdef ARM_MATH_CM7
+  __asm__("vpop {s16-s31}");
+#endif
   L->errorJmp = lj.previous;  /* restore old error handler */
   L->nCcalls = oldnCcalls;
   return lj.status;
 }
-
+#pragma GCC pop_options
 /* }====================================================== */