[devhelp] DhLink: use a union inside the struct to use less memory

[Sébastien Wilmet <swilmet src gnome org>]

commit 51e0f156b8d41cf73225f4c9184d9be9b24e96cc
Author: Sébastien Wilmet <swilmet gnome org>
Date:   Thu Dec 7 20:27:07 2017 +0100

    DhLink: use a union inside the struct to use less memory
    
    At commit 326d80ff551cc29cde3380e6de217d27217184de, the DhLink struct
    size was 56 bytes on x86_64. Now it is 40 bytes, because two pointers
    have been removed.
    
    With all the gtk-doc docs installed on my system, the memory used for
    that struct is reduced from 3.8MB to 2.7MB (67899 DhLink's).
    
    It is probably not transcendental, but it's anyway nice to fix that
    FIXME. Also, the union has the benefit that it self-documents the fact
    that some fields are available only for book links and vice versa.

 src/dh-link.c |   43 ++++++++++++++++++++++---------------------
 1 files changed, 22 insertions(+), 21 deletions(-)
---
diff --git a/src/dh-link.c b/src/dh-link.c
index 19a0d44..c841450 100644
--- a/src/dh-link.c
+++ b/src/dh-link.c
@@ -44,22 +44,23 @@ typedef struct {
 } BookData;
 
 struct _DhLink {
-        /* FIXME: it is possible to optimize this struct to use less memory, by
-         * having only one pointer for the book, for example with an union.
-         */
-
         /* To avoid some memory padding inside the struct, to use less memory,
          * the fields are placed in this order:
          * 1. All the pointers.
          * 2. Other types.
          * 3. Bit fields.
+         *
+         * Also, a union is used to use less memory. This struct is allocated a
+         * lot, so it is worth optimizing it.
          */
 
-        /* @book_data is set only for links of @type DH_LINK_TYPE_BOOK. */
-        BookData *book_data;
+        union {
+                /* @book.data is set only for links of @type DH_LINK_TYPE_BOOK. */
+                BookData *data;
 
-        /* @book_link is set only for links of @type != DH_LINK_TYPE_BOOK. */
-        DhLink *book_link;
+                /* @book.link is set only for links of @type != DH_LINK_TYPE_BOOK. */
+                DhLink *link;
+        } book;
 
         gchar *name;
         gchar *name_collation_key;
@@ -105,10 +106,10 @@ book_data_free (BookData *data)
 static void
 link_free (DhLink *link)
 {
-        book_data_free (link->book_data);
-
-        if (link->book_link != NULL)
-                dh_link_unref (link->book_link);
+        if (link->type == DH_LINK_TYPE_BOOK)
+                book_data_free (link->book.data);
+        else
+                dh_link_unref (link->book.link);
 
         g_free (link->name);
         g_free (link->name_collation_key);
@@ -159,7 +160,7 @@ dh_link_new_book (const gchar *base_path,
 
         link = dh_link_new_common (DH_LINK_TYPE_BOOK, name, relative_url);
 
-        link->book_data = book_data_new (base_path, book_id);
+        link->book.data = book_data_new (base_path, book_id);
 
         return link;
 }
@@ -189,7 +190,7 @@ dh_link_new (DhLinkType   type,
 
         link = dh_link_new_common (type, name, relative_url);
 
-        link->book_link = dh_link_ref (book);
+        link->book.link = dh_link_ref (book);
 
         return link;
 }
@@ -401,9 +402,9 @@ dh_link_get_uri (DhLink *link)
         g_return_val_if_fail (link != NULL, NULL);
 
         if (link->type == DH_LINK_TYPE_BOOK)
-                base_path = link->book_data->base_path;
+                base_path = link->book.data->base_path;
         else
-                base_path = link->book_link->book_data->base_path;
+                base_path = link->book.link->book.data->base_path;
 
         filename = g_build_filename (base_path, link->relative_url, NULL);
 
@@ -445,8 +446,8 @@ dh_link_get_book_name (DhLink *link)
         if (link->type == DH_LINK_TYPE_BOOK)
                 return link->name;
 
-        if (link->book_link != NULL)
-                return link->book_link->name;
+        if (link->book.link != NULL)
+                return link->book.link->name;
 
         return "";
 }
@@ -463,10 +464,10 @@ dh_link_get_book_id (DhLink *link)
         g_return_val_if_fail (link != NULL, NULL);
 
         if (link->type == DH_LINK_TYPE_BOOK)
-                return link->book_data->book_id;
+                return link->book.data->book_id;
 
-        if (link->book_link != NULL)
-                return link->book_link->book_data->book_id;
+        if (link->book.link != NULL)
+                return link->book.link->book.data->book_id;
 
         return "";
 }