From: Junio C Hamano <gitster@pobox.com>
To: Jessica Clarke <jrtc27@jrtc27.com>
Cc: git@vger.kernel.org
Subject: Re: [PATCH v2] mem-pool: Don't assume uintmax_t is aligned enough for all types
Date: Sun, 23 Jan 2022 12:17:16 -0800 [thread overview]
Message-ID: <xmqqsfte8awj.fsf@gitster.g> (raw)
In-Reply-To: <20220123152458.72540-1-jrtc27@jrtc27.com> (Jessica Clarke's message of "Sun, 23 Jan 2022 15:24:58 +0000")
Jessica Clarke <jrtc27@jrtc27.com> writes:
> Currently 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
Lose "Currently", as the present tense describes the status quo, the
shape of the problematic code we have today that wants improvement
by the proposed patch.
> 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.
OK.
> 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
> needed.
Nicely described.
> +/*
> + * 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.
> + *
> + * 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)
> +
The original computed the alignment requirement for uintmax_t as
sizeof(uintmax_t), not as
offsetof(struct {
char unalign;
union { uintmax_t i; } aligned;
}, aligned)
because if you have an array of a type, each element of it must be
aligned appropriately already for that type, without the unalignment
the outer struct enforces. I wonder if your complex offsetof is
equivalent to sizeof(union { uintmax_t u; void *p; })?
IOW, in this struct:
struct max_alignment_helper {
char unalign;
union {
uintmax_t uintmax_t_unused;
void *pointer_unused;
} u[2];
} s;
both s.u[0] and s.u[1] must be properly aligned, so wouldn't the
alignment requirement for the union type, which can be used to hold
a single value of either uintmax_t or a poinhter, be the distance
between these two array elements, i.e. sizeof(s.u[0])?
To put it differently in yet another way, wouldn't it simplify down
to this?
union max_alignment_helper {
uintmax_t uintmax_t_unused;
void *pointer_unused;
};
#define GIT_MAX_ALIGNMENT sizeof(union max_alignment_helper);
I am not saying that the "a forcibly unaligned union in a struct" is
a bad/wrong way to express what you want to achieve. I just do not
know if there is a reason to choose it over a seemingly simpler
sizeof(that union) without the outer struct and unalign member.
Other than that, looks OK to me. Especially the parts that use the
macro look correctly converted.
Thanks.
> @@ -69,9 +85,9 @@ 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))
> + len += GIT_MAX_ALIGNMENT - (len & (GIT_MAX_ALIGNMENT - 1));
>
> if (pool->mp_block &&
> pool->mp_block->end - pool->mp_block->next_free >= len)
> /*
> * 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 +85,9 @@ 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))
> + len += GIT_MAX_ALIGNMENT - (len & (GIT_MAX_ALIGNMENT - 1));
>
> if (pool->mp_block &&
> pool->mp_block->end - pool->mp_block->next_free >= len)
next prev parent reply other threads:[~2022-01-23 20:17 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-01-05 13:23 [PATCH] Properly align memory allocations and temporary buffers Jessica Clarke
2022-01-06 21:46 ` Taylor Blau
2022-01-06 21:56 ` Jessica Clarke
2022-01-06 22:27 ` Junio C Hamano
2022-01-06 22:56 ` Jessica Clarke
2022-01-07 0:10 ` Junio C Hamano
2022-01-07 0:22 ` Jessica Clarke
2022-01-07 0:31 ` brian m. carlson
2022-01-07 0:39 ` Jessica Clarke
2022-01-07 1:43 ` brian m. carlson
2022-01-07 2:08 ` Jessica Clarke
2022-01-07 2:11 ` Jessica Clarke
2022-01-07 19:30 ` Junio C Hamano
2022-01-07 19:33 ` Jessica Clarke
2022-01-07 20:56 ` René Scharfe
2022-01-07 21:30 ` Junio C Hamano
2022-01-07 23:30 ` René Scharfe
2022-01-08 0:18 ` Elijah Newren
2022-01-06 23:22 ` brian m. carlson
2022-01-06 23:31 ` Jessica Clarke
2022-01-07 14:57 ` Philip Oakley
2022-01-07 16:08 ` René Scharfe
2022-01-07 16:21 ` Jessica Clarke
2022-01-12 13:58 ` Jessica Clarke
2022-01-12 15:47 ` René Scharfe
2022-01-12 15:49 ` Jessica Clarke
2022-01-23 15:24 ` [PATCH v2] mem-pool: Don't assume uintmax_t is aligned enough for all types Jessica Clarke
2022-01-23 20:17 ` Junio C Hamano [this message]
2022-01-23 20:23 ` Jessica Clarke
2022-01-23 20:28 ` Junio C Hamano
2022-01-23 20:33 ` [PATCH v3] " Jessica Clarke
2022-01-24 17:11 ` Junio C Hamano
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