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c-rbtree: reduce alignment constraints

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There are some Debian-supported architectures where `max_align_t` is
only aligned to 4-bytes. This is unfortunate and breaks our assumptions.
While glibc-malloc still guarantees 8 / 16 bytes alignment, this is not
necessarily guaranteed by the C standard (and alternative allocators
will deviate (see jemalloc, for instance)).

Fortunately, we only need 2 flags, so a 4-byte alignment is more than
enough.

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

https://github.com/c-util/c-rbtree/pull/4
(cherry picked from commit 1554936e33)
This commit is contained in:
David Rheinsberg 2020-10-07 15:54:15 +02:00 • committed by Thomas Haller
parent b569687f5f
commit 282fac4afb
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2 changed files with 11 additions and 11 deletions
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14
shared/c-rbtree/src/c-rbtree.c
View file

@ -31,15 +31,17 @@
#include "c-rbtree-private.h"
/*
* We use alignas(8) to enforce 64bit alignment of structure fields. This is
* according to ISO-C11, so we rely on the compiler to implement this. However,
* at the same time we don't want to exceed native malloc() alignment on target
* platforms. Hence, we also verify against max_align_t.
* We use the lower 2 bits of CRBNode pointers to store flags. Make sure
* CRBNode is 4-byte aligned, so the lower 2 bits are actually unused. We also
* sometimes store a pointer to the root-node, so make sure this one is also 4
* byte aligned.
* Note that there are actually some architectures where `max_align_t` is 4, so
* we do not have much wiggle-room to extend this flag-set.
*/
static_assert(alignof(CRBNode) <= alignof(max_align_t), "Invalid RBNode alignment");
static_assert(alignof(CRBNode) >= 8, "Invalid CRBNode alignment");
static_assert(alignof(CRBNode) >= 4, "Invalid CRBNode alignment");
static_assert(alignof(CRBTree) <= alignof(max_align_t), "Invalid RBTree alignment");
static_assert(alignof(CRBTree) >= 8, "Invalid CRBTree alignment");
static_assert(alignof(CRBTree) >= 4, "Invalid CRBTree alignment");
/**
* c_rbnode_leftmost() - return leftmost child

8
shared/c-rbtree/src/c-rbtree.h
View file

@ -27,7 +27,6 @@ extern "C" {
#endif
#include <assert.h>
#include <stdalign.h>
#include <stddef.h>
typedef struct CRBNode CRBNode;
@ -36,8 +35,7 @@ typedef struct CRBTree CRBTree;
/* implementation detail */
#define C_RBNODE_RED (0x1UL)
#define C_RBNODE_ROOT (0x2UL)
#define C_RBNODE_UNUSED3 (0x4UL)
#define C_RBNODE_FLAG_MASK (0x7UL)
#define C_RBNODE_FLAG_MASK (0x3UL)
/**
* struct CRBNode - Node of a Red-Black Tree
@ -60,7 +58,7 @@ typedef struct CRBTree CRBTree;
* C_RBNODE_INIT.
*/
struct CRBNode {
alignas(8) unsigned long __parent_and_flags;
unsigned long __parent_and_flags;
CRBNode *left;
CRBNode *right;
};
@ -90,7 +88,7 @@ void c_rbnode_unlink_stale(CRBNode *n);
* To initialize an RB-Tree, set it to NULL / all zero.
*/
struct CRBTree {
alignas(8) CRBNode *root;
CRBNode *root;
};
#define C_RBTREE_INIT {}