move const_le{16, 23} to qemu/bswap.h, add comment

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1460441239-867-1-git-send-email-kraxel@redhat.com

include/hw/virtio/virtio-input.h

@@ -13,20 +13,6 @@ typedef struct virtio_input_absinfo virtio_input_absinfo;
 typedef struct virtio_input_config virtio_input_config;
 typedef struct virtio_input_event virtio_input_event;
 
-#if defined(HOST_WORDS_BIGENDIAN)
-# define const_le32(_x)                          \
-    ((((_x) & 0x000000ffU) << 24) |              \
-     (((_x) & 0x0000ff00U) <<  8) |              \
-     (((_x) & 0x00ff0000U) >>  8) |              \
-     (((_x) & 0xff000000U) >> 24))
-# define const_le16(_x)                          \
-    ((((_x) & 0x00ff) << 8) |                    \
-     (((_x) & 0xff00) >> 8))
-#else
-# define const_le32(_x) (_x)
-# define const_le16(_x) (_x)
-#endif
-
 /* ----------------------------------------------------------------- */
 /* qemu internals                                                    */
 

include/qemu/bswap.h

@@ -125,6 +125,25 @@ static inline uint32_t qemu_bswap_len(uint32_t value, int len)
     return bswap32(value) >> (32 - 8 * len);
 }
 
+/*
+ * Same as cpu_to_le{16,23}, except that gcc will figure the result is
+ * a compile-time constant if you pass in a constant.  So this can be
+ * used to initialize static variables.
+ */
+#if defined(HOST_WORDS_BIGENDIAN)
+# define const_le32(_x)                          \
+    ((((_x) & 0x000000ffU) << 24) |              \
+     (((_x) & 0x0000ff00U) <<  8) |              \
+     (((_x) & 0x00ff0000U) >>  8) |              \
+     (((_x) & 0xff000000U) >> 24))
+# define const_le16(_x)                          \
+    ((((_x) & 0x00ff) << 8) |                    \
+     (((_x) & 0xff00) >> 8))
+#else
+# define const_le32(_x) (_x)
+# define const_le16(_x) (_x)
+#endif
+
 /* Unions for reinterpreting between floats and integers.  */
 
 typedef union {