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mem-pool: don't assume uintmax_t is aligned enough for all types

mem_pool_alloc uses sizeof(uintmax_t) as a proxy for what should be
_Alignof(max_align_t) in C11. On most architectures this is sufficient
(though on m68k it is in fact overly strict, since the de-facto ABI,
which differs from the specified System V ABI, has the maximum alignment
of all types as 2 bytes), but on CHERI, and thus Arm's Morello
prototype, it is insufficient for any type that stores a pointer, which
must be aligned to 128 bits (on 64-bit architectures extended with
CHERI), whilst uintmax_t is a 64-bit integer.

Fix this by introducing our own approximation for max_align_t and a
means to compute _Alignof it without relying on C11. Currently this
union only contains uintmax_t and void *, but more types can be added as

Signed-off-by: Jessica Clarke <>
Signed-off-by: Junio C Hamano <>
Jessica Clarke 7 months ago committed by Junio C Hamano
  1. 26


@ -7,6 +7,26 @@
#define BLOCK_GROWTH_SIZE (1024 * 1024 - sizeof(struct mp_block))
* The inner union is an approximation for C11's max_align_t, and the
* struct + offsetof computes _Alignof. This can all just be replaced
* with _Alignof(max_align_t) if/when C11 is part of the baseline.
* Note that _Alignof(X) need not be the same as sizeof(X); it's only
* required to be a (possibly trivial) factor. They are the same for
* most architectures, but m68k for example has only 2-byte alignment
* for its 4-byte and 8-byte types, so using sizeof would waste space.
* Add more types to the union if the current set is insufficient.
struct git_max_alignment {
char unalign;
union {
uintmax_t max_align_uintmax;
void *max_align_pointer;
} aligned;
#define GIT_MAX_ALIGNMENT offsetof(struct git_max_alignment, aligned)
* Allocate a new mp_block and insert it after the block specified in
* `insert_after`. If `insert_after` is NULL, then insert block at the
@ -69,9 +89,9 @@ void *mem_pool_alloc(struct mem_pool *pool, size_t len)
struct mp_block *p = NULL;
void *r;
/* round up to a 'uintmax_t' alignment */
if (len & (sizeof(uintmax_t) - 1))
len += sizeof(uintmax_t) - (len & (sizeof(uintmax_t) - 1));
/* round up to a 'GIT_MAX_ALIGNMENT' alignment */
if (len & (GIT_MAX_ALIGNMENT - 1))
if (pool->mp_block &&
pool->mp_block->end - pool->mp_block->next_free >= len)