[Xen-changelog] [xen-unstable] Introduce __must_be_array

[Xen patchbot-unstable <patchbot-unstable@xxxxxxxxxxxxxxxxxxx>]

# HG changeset patch
# User Keir Fraser <keir.fraser@xxxxxxxxxx>
# Date 1242829765 -3600
# Node ID 5369133d772c91512f0ae6e5a9381839d38de462
# Parent  23f9857f642f0b511553a2b7507a972465cc8930
Introduce __must_be_array

Cloning the similar construct from Linux, allowing to detect improper
uses of ARRAY_SIZE() at build time.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxxxx>
---
 xen/include/xen/compiler.h |    4 ++++
 xen/include/xen/config.h   |    2 +-
 xen/include/xen/lib.h      |    6 ++++++
 3 files changed, 11 insertions(+), 1 deletion(-)

diff -r 23f9857f642f -r 5369133d772c xen/include/xen/compiler.h
--- a/xen/include/xen/compiler.h        Wed May 20 15:27:30 2009 +0100
+++ b/xen/include/xen/compiler.h        Wed May 20 15:29:25 2009 +0100
@@ -35,6 +35,10 @@
 #define offsetof(a,b) ((unsigned long)&(((a *)0)->b))
 #endif
 
+/* &a[0] degrades to a pointer: a different type from an array */
+#define __must_be_array(a) \
+  BUILD_BUG_ON_ZERO(__builtin_types_compatible_p(typeof(a), typeof(&a[0])))
+
 #ifdef GCC_HAS_VISIBILITY_ATTRIBUTE
 /* Results in more efficient PIC code (no indirections through GOT or PLT). */
 #pragma GCC visibility push(hidden)
diff -r 23f9857f642f -r 5369133d772c xen/include/xen/config.h
--- a/xen/include/xen/config.h  Wed May 20 15:27:30 2009 +0100
+++ b/xen/include/xen/config.h  Wed May 20 15:29:25 2009 +0100
@@ -11,7 +11,7 @@
 
 #define EXPORT_SYMBOL(var)
 #define EXPORT_SYMBOL_GPL(var)
-#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
+#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]) + __must_be_array(x))
 
 /*
  * The following log levels are as follows:
diff -r 23f9857f642f -r 5369133d772c xen/include/xen/lib.h
--- a/xen/include/xen/lib.h     Wed May 20 15:27:30 2009 +0100
+++ b/xen/include/xen/lib.h     Wed May 20 15:29:25 2009 +0100
@@ -17,6 +17,12 @@ void __warn(char *file, int line);
 
 /* Force a compilation error if condition is true */
 #define BUILD_BUG_ON(condition) ((void)sizeof(struct { int:-!!(condition); }))
+
+/* Force a compilation error if condition is true, but also produce a
+   result (of value 0 and type size_t), so the expression can be used
+   e.g. in a structure initializer (or where-ever else comma expressions
+   aren't permitted). */
+#define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
 
 #ifndef assert_failed
 #define assert_failed(p)                                        \

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