Use __alignof__ instead of sizeof for stringfield length storage.

Kevin P Fleming suggested that r343157 should use __alignof__ instead
of sizeof. For most systems this won't be an issue, but better fix it
now while it's still fresh.

Review: https://reviewboard.asterisk.org/r/1573
........

Merged revisions 344843 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 344845 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@344846 65c4cc65-6c06-0410-ace0-fbb531ad65f3

2 files changed, 47 insertions, 23 deletions

diff --git a/include/asterisk/stringfields.h b/include/asterisk/stringfields.h
index 4039bf482..5f2ac1a73 100644
--- a/include/asterisk/stringfields.h
+++ b/include/asterisk/stringfields.h
@@ -135,7 +135,7 @@ struct ast_string_field_pool {
 	size_t size;				/*!< the total size of the pool */
 	size_t used;				/*!< the space used in the pool */
 	size_t active;				/*!< the amount of space actively in use by fields */
-	char base[0] __attribute__((aligned(sizeof(ast_string_field_allocation)))); /*!< storage space for the fields */
+	char base[0] __attribute__((aligned(__alignof__(ast_string_field_allocation)))); /*!< storage space for the fields */
 };
 
 /*!
@@ -302,8 +302,11 @@ void __ast_string_field_release_active(struct ast_string_field_pool *pool_head,
 /*!
   \brief Macro to provide access to the allocation field that lives immediately in front of a string field
   \param x Pointer to the string field
+
+  Note that x must be a pointer to a byte-sized type -- normally (char *) -- or this calculation
+  would break horribly
 */
-#define AST_STRING_FIELD_ALLOCATION(x) *((ast_string_field_allocation *) (x - sizeof(ast_string_field_allocation)))
+#define AST_STRING_FIELD_ALLOCATION(x) *((ast_string_field_allocation *) (x - __alignof__(ast_string_field_allocation)))
 
 /*!
   \brief Set a field to a simple string value
diff --git a/include/asterisk/utils.h b/include/asterisk/utils.h
index 4d015f3ee..5ef1f96f9 100644
--- a/include/asterisk/utils.h
+++ b/include/asterisk/utils.h
@@ -741,36 +741,57 @@ static void force_inline _ast_assert(int condition, const char *condition_str,
 #include "asterisk/strings.h"
 
 /*!
- * \brief Add space and let result be a multiple of space.
- * \param initial A number to add space to.
- * \param space The space to add, this would typically be sizeof(sometype).
- * \return The sum of initial plus space plus at most space-1.
+ * \brief Return the number of bytes used in the alignment of type.
+ * \param type
+ * \return The number of bytes required for alignment.
  *
- * Many systems prefer integers to be stored on aligned on memory locations.
- * A use case for this is when prepending length fields of type int to a buffer.
- * If you keep the total used bytes a multiple of the size of the integer type,
- * a next block of length+buffer will have the length field automatically
- * aligned.
+ * This is really just __alignof__(), but tucked away in this header so we
+ * don't have to look at the nasty underscores in the source.
+ */
+#define ast_alignof(type) __alignof__(type)
+
+/*!
+ * \brief Increase offset so it is a multiple of the required alignment of type.
+ * \param offset The value that should be increased.
+ * \param type The data type that offset should be aligned to.
+ * \return The smallest multiple of alignof(type) larger than or equal to offset.
+ * \see ast_make_room_for()
  *
- * It looks kind of ugly, but the compiler will optimize this down to 4 or 5
- * inexpensive instructions (on x86_64).
+ * Many systems prefer integers to be stored on aligned on memory locations.
+ * This macro will increase an offset so a value of the supplied type can be
+ * safely be stored on such a memory location.
  *
  * Examples:
- * ast_add_and_make_multiple_of(0x18, sizeof(int64_t)) ==> 0x20
- * ast_add_and_make_multiple_of(0x19, sizeof(int64_t)) ==> 0x28
+ * ast_align_for(0x17, int64_t) ==> 0x18
+ * ast_align_for(0x18, int64_t) ==> 0x18
+ * ast_align_for(0x19, int64_t) ==> 0x20
+ *
+ * Don't mind the ugliness, the compiler will optimize it.
  */
-#define ast_add_and_make_multiple_of(initial, space) (((initial + (2 * space - 1)) / space) * space)
+#define ast_align_for(offset, type) (((offset + __alignof__(type) - 1) / __alignof__(type)) * __alignof__(type))
 
 /*!
- * \brief Add bytes so that result is a multiple of size.
- * \param initial A number to enlarge.
- * \param size The block size the number should be a multiple of.
- * \return The sum of initial plus at most size-1.
+ * \brief Increase offset by the required alignment of type and make sure it is
+ *        a multiple of said alignment.
+ * \param offset The value that should be increased.
+ * \param type The data type that room should be reserved for.
+ * \return The smallest multiple of alignof(type) larger than or equal to offset
+ *         plus alignof(type).
+ * \see ast_align_for()
+ *
+ * A use case for this is when prepending length fields of type int to a buffer.
+ * If you keep the offset a multiple of the alignment of the integer type,
+ * a next block of length+buffer will have the length field automatically
+ * aligned.
+ *
+ * Examples:
+ * ast_make_room_for(0x17, int64_t) ==> 0x20
+ * ast_make_room_for(0x18, int64_t) ==> 0x20
+ * ast_make_room_for(0x19, int64_t) ==> 0x28
  *
- * Similar ast_add_and_make_multiple_of, except that this doesn't add the room
- * for the length object, it only ensures that the total is aligned.
+ * Don't mind the ugliness, the compiler will optimize it.
  */
-#define ast_make_multiple_of(initial, size) (((initial + size - 1) / size) * size)
+#define ast_make_room_for(offset, type) (((offset + (2 * __alignof__(type) - 1)) / __alignof__(type)) * __alignof__(type))
 
 /*!
  * \brief An Entity ID is essentially a MAC address, brief and unique