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c-rbtree: fix struct alignment of CRBTree on m68k architectures

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On m68k, 32bit integer are aligned to only 2 bytes. This breaks
assumptions and a static assertion of c-rbtree.

Explicitly require that the first field is aligned to at least 4 bytes.
This fixes the build and ensures that all valid pointers to a CRBTree have
the lowest two bits unset (so they can be used for storing 2 additional flags).

Use a union instead of aligning __parent_and_flags itself. That is
because alignas() cannot lower the natural alignment, so if we would
want to align __parent_and_flags, we could only do

  alignas(sizeof(unsigned long) > 4 ? sizeof(unsigned long) : 4)

That would not be correct if "long" is 8 bytes long but had a natural
alignment of only 4. The union allows us to specify an alignment
of at least 4, but otherwise follow natural alignment.

10d973a9e6
This commit is contained in:
Thomas Haller 2020-10-13 09:34:04 +02:00
parent b6d6a16b2a
commit 143130066b
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GPG key ID: 29C2366E4DFC5728
1 changed files with 5 additions and 1 deletions
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6
shared/c-rbtree/src/c-rbtree.h
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@ -27,6 +27,7 @@ extern "C" {
#endif
#include <assert.h>
#include <stdalign.h>
#include <stddef.h>
typedef struct CRBNode CRBNode;
@ -58,7 +59,10 @@ typedef struct CRBTree CRBTree;
* C_RBNODE_INIT.
*/
struct CRBNode {
unsigned long __parent_and_flags;
union {
unsigned long __parent_and_flags;
alignas(4) char __dmmy_for_struct_alignment;
};
CRBNode *left;
CRBNode *right;
};