c-rbtree: align CRBTree with CRBNode

We sometimes store pointers to `CRBTree` in `CRBNode*` variables, so we
must make sure CRBTree has matching alignment guarantees. We already
check for that with static-assertions.

This commit aligns CRBTree with CRBNode for 2-byte aligned machines.
While at it, add a short comment explaining what the unions are for.

Signed-off-by: David Rheinsberg <david.rheinsberg@gmail.com>

https://github.com/c-util/c-rbtree/commit/c795b7657f4fbe7285b846f9aa63b2b9fc2b34b0

1 files changed, 7 insertions, 2 deletions

diff --git a/shared/c-rbtree/src/c-rbtree.h b/shared/c-rbtree/src/c-rbtree.h
index d1941185fb..d4d0fe45c0 100644
--- a/shared/c-rbtree/src/c-rbtree.h
+++ b/shared/c-rbtree/src/c-rbtree.h
@@ -61,7 +61,8 @@ typedef struct CRBTree CRBTree;
 struct CRBNode {
         union {
                 unsigned long __parent_and_flags;
-                alignas(4) char __dmmy_for_struct_alignment;
+                /* enforce >=4-byte alignment for @__parent_and_flags */
+                alignas(4) unsigned char __align_dummy;
         };
         CRBNode *left;
         CRBNode *right;
@@ -92,7 +93,11 @@ void c_rbnode_unlink_stale(CRBNode *n);
  * To initialize an RB-Tree, set it to NULL / all zero.
  */
 struct CRBTree {
-        CRBNode *root;
+        union {
+                CRBNode *root;
+                /* enforce >=4-byte alignment for @root */
+                alignas(4) unsigned char __align_dummy;
+        };
 };
 
 #define C_RBTREE_INIT {}