xen: Swap order of actions in the FREE*() macros

Wherever possible, it is a good idea to NULL out the visible reference to an
object prior to freeing it.  The FREE*() macros already collect together both
parts, making it easy to adjust.

This has a marginal code generation improvement, as some of the calls to the
free() function can be tailcall optimised.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit c4f427ec87)

xen/include/xen/mm.h

@@ -80,8 +80,9 @@ bool scrub_free_pages(void);
 
 /* Free an allocation, and zero the pointer to it. */
 #define FREE_XENHEAP_PAGES(p, o) do { \
-    free_xenheap_pages(p, o);         \
+    void *_ptr_ = (p);                \
     (p) = NULL;                       \
+    free_xenheap_pages(_ptr_, o);     \
 } while ( false )
 #define FREE_XENHEAP_PAGE(p) FREE_XENHEAP_PAGES(p, 0)
 

xen/include/xen/xmalloc.h

@@ -66,9 +66,10 @@
 extern void xfree(void *);
 
 /* Free an allocation, and zero the pointer to it. */
-#define XFREE(p) do { \
-    xfree(p);         \
-    (p) = NULL;       \
+#define XFREE(p) do {                       \
+    void *_ptr_ = (p);                      \
+    (p) = NULL;                             \
+    xfree(_ptr_);                           \
 } while ( false )
 
 /* Underlying functions */