Add unitialized_var() macro for suppressing gcc warnings

Introduce a macro for suppressing gcc from generating a warning about a
probable uninitialized state of a variable.

Example:

-	spinlock_t *ptl;
+	spinlock_t *uninitialized_var(ptl);

Not a happy solution, but those warnings are obnoxious.

- Using the usual pointlessly-set-it-to-zero approach wastes several
  bytes of text.

- Using a macro means we can (hopefully) do something else if gcc changes
  cause the `x = x' hack to stop working

- Using a macro means that people who are worried about hiding true bugs
  can easily turn it off.
Signed-off-by: Borislav Petkov <bbpetkov@yahoo.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/compiler-gcc3.h

@@ -13,4 +13,10 @@
 #define __must_check		__attribute__((warn_unused_result))
 #endif
 
+/*
+ * A trick to suppress uninitialized variable warning without generating any
+ * code
+ */
+#define uninitialized_var(x) x = x
+
 #define __always_inline		inline __attribute__((always_inline))

include/linux/compiler-gcc4.h

@@ -16,3 +16,9 @@
 #define __must_check 		__attribute__((warn_unused_result))
 #define __compiler_offsetof(a,b) __builtin_offsetof(a,b)
 #define __always_inline		inline __attribute__((always_inline))
+
+/*
+ * A trick to suppress uninitialized variable warning without generating any
+ * code
+ */
+#define uninitialized_var(x) x = x

include/linux/compiler-intel.h

@@ -22,3 +22,5 @@
     (typeof(ptr)) (__ptr + (off)); })
 
 #endif
+
+#define uninitialized_var(x) x

mm/memory.c

@@ -1455,7 +1455,7 @@ static int apply_to_pte_range(struct mm_struct *mm, pmd_t *pmd,
 	pte_t *pte;
 	int err;
 	struct page *pmd_page;
-	spinlock_t *ptl = ptl;		/* Suppress gcc warning */
+	spinlock_t *uninitialized_var(ptl);
 
 	pte = (mm == &init_mm) ?
 		pte_alloc_kernel(pmd, addr) :