Use volatile for result and arg_list in GapElement to prevent optimization issues

Author: cxzhong

src/sage/libs/gap/element.pyx

@@ -984,7 +984,7 @@ cdef class GapElement(RingElement):
             GAPError: Error, no method found!
             Error, no 1st choice method found for `+' on 2 arguments
         """
-        cdef Obj result
+        cdef volatile Obj result
         try:
             sig_GAP_Enter()
             sig_on()
@@ -1012,7 +1012,7 @@ cdef class GapElement(RingElement):
             ...
             GAPError: Error, no method found! ...
         """
-        cdef Obj result
+        cdef volatile Obj result
         try:
             sig_GAP_Enter()
             sig_on()
@@ -1041,7 +1041,7 @@ cdef class GapElement(RingElement):
             GAPError: Error, no method found!
             Error, no 1st choice method found for `*' on 2 arguments
         """
-        cdef Obj result
+        cdef volatile Obj result
         try:
             sig_GAP_Enter()
             sig_on()
@@ -1075,7 +1075,7 @@ cdef class GapElement(RingElement):
             ...
             GAPError: Error, Rational operations: <divisor> must not be zero
         """
-        cdef Obj result
+        cdef volatile Obj result
         try:
             sig_GAP_Enter()
             sig_on()
@@ -1102,7 +1102,7 @@ cdef class GapElement(RingElement):
             GAPError: Error, no method found!
             Error, no 1st choice method found for `mod' on 2 arguments
         """
-        cdef Obj result
+        cdef volatile Obj result
         try:
             sig_GAP_Enter()
             sig_on()
@@ -1150,6 +1150,7 @@ cdef class GapElement(RingElement):
             GAPError: Error, no method found! Error, no 1st choice
             method found for `InverseMutable' on 1 arguments
         """
+        cdef volatile Obj result
         try:
             sig_GAP_Enter()
             sig_on()
@@ -2484,24 +2485,23 @@ cdef class GapElement_Function(GapElement):
             sage: libgap.Sum(rnd)
             6
             
-            sage: from sage.libs.gap.util import GAPError
             sage: for i in range(100):
             ....:     rnd = [ randint(-10,10) for i in range(randint(0,7)) ]
             ....:     # compute the sum in GAP
             ....:     _ = libgap.Sum(rnd)
             ....:     try:
             ....:         libgap.Sum(*rnd)
-            ....:         print('This should have triggered a GAPError')
+            ....:         print('This should have triggered a ValueError')
             ....:         print('because Sum needs a list as argument')
-            ....:     except GAPError:
+            ....:     except ValueError:
             ....:         pass
             
             sage: libgap_exec = libgap.eval("Exec")
             sage: libgap_exec('echo hello from the shell')
             hello from the shell
         """
-        cdef Obj result = NULL
-        cdef Obj arg_list
+        cdef volatile Obj result = NULL
+        cdef volatile Obj arg_list
         cdef int n = len(args)
         cdef volatile Obj v2