From: Jeff King <peff@peff•net>
To: Ezekiel Newren <ezekielnewren@gmail•com>
Cc: "Phillip Wood" <phillip.wood123@gmail•com>,
phillip.wood@dunelm•org.uk,
"Ezekiel Newren via GitGitGadget" <gitgitgadget@gmail•com>,
git@vger•kernel.org, "René Scharfe" <l.s.r@web•de>
Subject: Re: [PATCH 01/10] ivec: introduce the C side of ivec
Date: Mon, 19 Jan 2026 15:40:10 -0500 [thread overview]
Message-ID: <20260119204010.GA3148606@coredump.intra.peff.net> (raw)
In-Reply-To: <CAH=ZcbCXAB3vzRbyHkunQh09njyLk4WXvfLVxynXaswEkBv+DA@mail.gmail.com>
On Mon, Jan 19, 2026 at 01:21:04PM -0700, Ezekiel Newren wrote:
> Ok..., is there a way to pad a field to the largest size needed so
> that this also works on the harvard architecture? If C isn't even self
> consistent then how are these structs going to be passed between C and
> Rust (which is THE point of ivec)?
If you make a union of the pointers, it will require the largest size
and the strictest alignment requirement. So:
struct foo {
union {
void *v;
uint8_t *u8;
} ptr;
size_t len;
};
would be a single struct you could use to store a void pointer _or_ a u8
pointer. The one thing you shouldn't do there, though, is assign via one
union member and read from the other. So I don't know if that helps you
or not (I confess I have not followed this rust discussion at all, and
know nothing about rust/c ABI compatibility, and just got roped in on C
esoterica).
> Or do we just tell the arcane Harvard architecture "too bad" Git won't
> run on it anymore?
Minor nit: the Harvard architecture is one where function pointers are
not the same as data pointers. An int/char distinction can happen even
on more common (von Neumann) machines.
But I think we can rephrase your question as: are there real-world
machines we care about that will have different pointer sizes, or can we
ignore this issue for practical purposes?
I don't know the answer. I suspect it probably is OK for Git not to run
on the machines mentioned in that C faq. But:
1. Sometimes there are subtle implications of undefined behavior that
may cause a compiler (even for a sensible machine) to do unexpected
things. I don't know offhand if that is the case here.
2. There are some modern platforms in which pointers are a bit more
opaque than just numeric addresses. For example, we've had a few
patches dealing with questionable pointer usage to make things work
on CHERI Arm systems. I'm not sure if any of that would matter
here, though (IIRC, it was mostly that pointers were unexpectedly
large and had matching alignment requirements, but all of them
equally so).
-Peff
next prev parent reply other threads:[~2026-01-19 20:40 UTC|newest]
Thread overview: 124+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-01-02 18:52 [PATCH 00/10] Xdiff cleanup part 3 Ezekiel Newren via GitGitGadget
2026-01-02 18:52 ` [PATCH 01/10] ivec: introduce the C side of ivec Ezekiel Newren via GitGitGadget
2026-01-04 5:32 ` Junio C Hamano
2026-01-17 16:06 ` Ezekiel Newren
2026-01-08 14:34 ` Phillip Wood
2026-01-15 15:55 ` Ezekiel Newren
2026-01-16 10:39 ` Phillip Wood
2026-01-16 20:19 ` René Scharfe
2026-01-17 13:55 ` Phillip Wood
2026-01-17 16:04 ` Ezekiel Newren
2026-01-18 14:58 ` René Scharfe
2026-01-17 16:14 ` Ezekiel Newren
2026-01-17 16:16 ` Ezekiel Newren
2026-01-17 17:40 ` Phillip Wood
2026-01-19 5:59 ` Jeff King
2026-01-19 20:21 ` Ezekiel Newren
2026-01-19 20:40 ` Jeff King [this message]
2026-01-20 2:36 ` D. Ben Knoble
2026-01-21 21:00 ` Ezekiel Newren
2026-01-21 21:20 ` Jeff King
2026-01-21 21:31 ` Junio C Hamano
2026-01-21 21:45 ` Ezekiel Newren
2026-01-20 13:46 ` Phillip Wood
2026-01-20 14:06 ` Phillip Wood
2026-01-21 21:39 ` Ezekiel Newren
2026-01-28 11:15 ` Phillip Wood
2026-01-16 20:19 ` René Scharfe
2026-01-17 15:58 ` Ezekiel Newren
2026-01-18 14:55 ` René Scharfe
2026-01-02 18:52 ` [PATCH 02/10] xdiff: make classic diff explicit by creating xdl_do_classic_diff() Ezekiel Newren via GitGitGadget
2026-01-20 15:01 ` Phillip Wood
2026-01-21 21:05 ` Ezekiel Newren
2026-01-02 18:52 ` [PATCH 03/10] xdiff: don't waste time guessing the number of lines Ezekiel Newren via GitGitGadget
2026-01-20 15:02 ` Phillip Wood
2026-01-21 21:12 ` Ezekiel Newren
2026-01-22 10:16 ` Phillip Wood
2026-01-02 18:52 ` [PATCH 04/10] xdiff: let patience and histogram benefit from xdl_trim_ends() Ezekiel Newren via GitGitGadget
2026-01-20 15:02 ` Phillip Wood
2026-01-21 14:49 ` Phillip Wood
2026-01-02 18:52 ` [PATCH 05/10] xdiff: use xdfenv_t in xdl_trim_ends() and xdl_cleanup_records() Ezekiel Newren via GitGitGadget
2026-01-20 16:32 ` Phillip Wood
2026-01-02 18:52 ` [PATCH 06/10] xdiff: cleanup xdl_trim_ends() Ezekiel Newren via GitGitGadget
2026-01-20 16:32 ` Phillip Wood
2026-01-02 18:52 ` [PATCH 07/10] xdiff: replace xdfile_t.dstart with xdfenv_t.delta_start Ezekiel Newren via GitGitGadget
2026-01-20 16:32 ` Phillip Wood
2026-01-28 10:51 ` Phillip Wood
2026-01-02 18:52 ` [PATCH 08/10] xdiff: replace xdfile_t.dend with xdfenv_t.delta_end Ezekiel Newren via GitGitGadget
2026-01-02 18:52 ` [PATCH 09/10] xdiff: remove dependence on xdlclassifier from xdl_cleanup_records() Ezekiel Newren via GitGitGadget
2026-01-16 20:19 ` René Scharfe
2026-01-17 16:34 ` Ezekiel Newren
2026-01-18 18:23 ` René Scharfe
2026-01-21 15:01 ` Phillip Wood
2026-01-02 18:52 ` [PATCH 10/10] xdiff: move xdl_cleanup_records() from xprepare.c to xdiffi.c Ezekiel Newren via GitGitGadget
2026-01-21 15:01 ` Phillip Wood
2026-01-28 10:56 ` Phillip Wood
2026-01-04 2:44 ` [PATCH 00/10] Xdiff cleanup part 3 Junio C Hamano
2026-01-04 6:01 ` Yee Cheng Chin
2026-01-28 14:40 ` Phillip Wood
2026-03-06 23:03 ` Junio C Hamano
2026-03-09 19:06 ` Ezekiel Newren
2026-03-09 23:31 ` Junio C Hamano
2026-03-25 21:11 ` [PATCH v2 0/5] " Ezekiel Newren via GitGitGadget
2026-03-25 21:11 ` [PATCH v2 1/5] xdiff/xdl_cleanup_records: delete local recs pointer Ezekiel Newren via GitGitGadget
2026-03-25 21:11 ` [PATCH v2 2/5] xdiff/xdl_cleanup_records: make limits more clear Ezekiel Newren via GitGitGadget
2026-03-25 21:11 ` [PATCH v2 3/5] xdiff/xdl_cleanup_records: make setting action easier to follow Ezekiel Newren via GitGitGadget
2026-03-25 21:11 ` [PATCH v2 4/5] xdiff/xdl_cleanup_records: simplify INVESTIGATE handling for clarity Ezekiel Newren via GitGitGadget
2026-03-25 21:11 ` [PATCH v2 5/5] xdiff/xdl_cleanup_records: use unambiguous types Ezekiel Newren via GitGitGadget
2026-03-25 21:58 ` Junio C Hamano
2026-03-26 6:26 ` [PATCH v2 0/5] Xdiff cleanup part 3 SZEDER Gábor
2026-03-27 19:23 ` [PATCH v3 0/6] " Ezekiel Newren via GitGitGadget
2026-03-27 19:23 ` [PATCH v3 1/6] xdiff/xdl_cleanup_records: delete local recs pointer Ezekiel Newren via GitGitGadget
2026-03-27 19:23 ` [PATCH v3 2/6] xdiff: use unambiguous types in xdl_bogo_sqrt() Ezekiel Newren via GitGitGadget
2026-03-27 19:23 ` [PATCH v3 3/6] xdiff/xdl_cleanup_records: use unambiguous types Ezekiel Newren via GitGitGadget
2026-03-27 19:23 ` [PATCH v3 4/6] xdiff/xdl_cleanup_records: make limits more clear Ezekiel Newren via GitGitGadget
2026-03-27 21:09 ` Junio C Hamano
2026-03-27 23:01 ` Junio C Hamano
2026-03-30 16:00 ` Ezekiel Newren
2026-03-30 19:59 ` Junio C Hamano
2026-03-31 1:29 ` Ezekiel Newren
2026-03-27 19:23 ` [PATCH v3 5/6] xdiff/xdl_cleanup_records: make setting action easier to follow Ezekiel Newren via GitGitGadget
2026-03-27 19:23 ` [PATCH v3 6/6] xdiff/xdl_cleanup_records: simplify INVESTIGATE handling for clarity Ezekiel Newren via GitGitGadget
2026-03-30 16:59 ` [PATCH v4 0/6] Xdiff cleanup part 3 Ezekiel Newren via GitGitGadget
2026-03-30 16:59 ` [PATCH v4 1/6] xdiff/xdl_cleanup_records: delete local recs pointer Ezekiel Newren via GitGitGadget
2026-03-30 17:23 ` Ezekiel Newren
2026-03-30 22:53 ` Junio C Hamano
2026-03-30 16:59 ` [PATCH v4 2/6] xdiff: use unambiguous types in xdl_bogo_sqrt() Ezekiel Newren via GitGitGadget
2026-03-30 22:59 ` Junio C Hamano
2026-03-30 17:00 ` [PATCH v4 3/6] xdiff/xdl_cleanup_records: use unambiguous types Ezekiel Newren via GitGitGadget
2026-03-30 17:00 ` [PATCH v4 4/6] xdiff/xdl_cleanup_records: make limits more clear Ezekiel Newren via GitGitGadget
2026-03-31 9:44 ` Phillip Wood
2026-03-31 16:13 ` Junio C Hamano
2026-04-14 21:58 ` Ezekiel Newren
2026-04-14 22:15 ` Junio C Hamano
2026-04-15 13:54 ` Phillip Wood
2026-03-30 17:00 ` [PATCH v4 5/6] xdiff/xdl_cleanup_records: make setting action easier to follow Ezekiel Newren via GitGitGadget
2026-03-30 23:02 ` Junio C Hamano
2026-03-31 9:44 ` Phillip Wood
2026-03-30 17:00 ` [PATCH v4 6/6] xdiff/xdl_cleanup_records: simplify INVESTIGATE handling for clarity Ezekiel Newren via GitGitGadget
2026-03-31 9:43 ` Phillip Wood
2026-04-01 16:00 ` Phillip Wood
2026-03-30 23:04 ` [PATCH v4 0/6] Xdiff cleanup part 3 Junio C Hamano
2026-03-31 9:45 ` Phillip Wood
2026-04-08 20:26 ` [PATCH v5 " Ezekiel Newren via GitGitGadget
2026-04-08 20:26 ` [PATCH v5 1/6] xdiff/xdl_cleanup_records: delete local recs pointer Ezekiel Newren via GitGitGadget
2026-04-08 20:26 ` [PATCH v5 2/6] xdiff: use unambiguous types in xdl_bogo_sqrt() Ezekiel Newren via GitGitGadget
2026-04-08 20:26 ` [PATCH v5 3/6] xdiff/xdl_cleanup_records: use unambiguous types Ezekiel Newren via GitGitGadget
2026-04-08 20:26 ` [PATCH v5 4/6] xdiff/xdl_cleanup_records: make limits more clear Ezekiel Newren via GitGitGadget
2026-04-14 10:09 ` Phillip Wood
2026-04-08 20:26 ` [PATCH v5 5/6] xdiff/xdl_cleanup_records: make setting action easier to follow Ezekiel Newren via GitGitGadget
2026-04-08 20:26 ` [PATCH v5 6/6] xdiff/xdl_cleanup_records: put braces around the else clause Ezekiel Newren via GitGitGadget
2026-04-08 21:28 ` [PATCH v5 0/6] Xdiff cleanup part 3 Junio C Hamano
2026-04-09 14:01 ` Phillip Wood
2026-04-14 10:08 ` Phillip Wood
2026-04-14 17:06 ` Junio C Hamano
2026-04-29 22:08 ` [PATCH v6 " Ezekiel Newren via GitGitGadget
2026-04-29 22:08 ` [PATCH v6 1/6] xdiff/xdl_cleanup_records: delete local recs pointer Ezekiel Newren via GitGitGadget
2026-04-29 22:08 ` [PATCH v6 2/6] xdiff: use unambiguous types in xdl_bogo_sqrt() Ezekiel Newren via GitGitGadget
2026-04-29 22:08 ` [PATCH v6 3/6] xdiff/xdl_cleanup_records: use unambiguous types Ezekiel Newren via GitGitGadget
2026-04-29 22:08 ` [PATCH v6 4/6] xdiff/xdl_cleanup_records: make limits more clear Ezekiel Newren via GitGitGadget
2026-04-29 22:08 ` [PATCH v6 5/6] xdiff/xdl_cleanup_records: make setting action easier to follow Ezekiel Newren via GitGitGadget
2026-04-29 22:08 ` [PATCH v6 6/6] xdiff/xdl_cleanup_records: make execution of " Ezekiel Newren via GitGitGadget
2026-04-30 13:35 ` [PATCH v6 0/6] Xdiff cleanup part 3 Phillip Wood
2026-04-30 21:08 ` Ezekiel Newren
2026-05-04 0:59 ` Junio C Hamano
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