From: Joseph Myers <joseph@codesourcery.com>
To: Alexander Monakov <amonakov@ispras.ru>
Cc: <gcc-patches@gcc.gnu.org>, Richard Biener <rguenther@suse.de>
Subject: Re: [PATCH] doc: clarify the situation with pointer arithmetic
Date: Wed, 22 Jan 2020 01:29:00 -0000 [thread overview]
Message-ID: <alpine.DEB.2.21.2001220111280.21353@digraph.polyomino.org.uk> (raw)
In-Reply-To: <alpine.LNX.2.20.13.2001211527540.32566@monopod.intra.ispras.ru>
On Tue, 21 Jan 2020, Alexander Monakov wrote:
> My intent was more basic. I observed that the paragraph can be interpreted as
> saying that if you have a cast 'I1 = (intptr_t) P1', then perform some
> computations on I1 that do not in any way depend on values of other pointers,
> then casting the result back can not point to a different object than P1.
Yes. I don't think the wording in the existing standard (where anything
other than casting the same integer value back to pointer type - and,
furthermore, the result is only specified to compare equal to the original
pointer, not to be otherwise usable in place of it) can distinguish that
version (PVI) from PNVI.
> But that is not very helpful, because this is the case where the user
> might have used normal pointer arithmetic in the first place.
I think a typical use case of intptr_t might be e.g. masking off low bits
of a pointer value for alignment while remaining within the same object.
Another one might be to compare pointers to different objects in
implementing memmove-like functions, where ordered pointer comparison
would not be defined in ISO C.
> I think that the case discussed in the proposed patch is already not ambiguous
> and does not need further clarifications to the standard.
I don't think such a requirement for PNVI can be deduced from the existing
text.
--
Joseph S. Myers
joseph@codesourcery.com
prev parent reply other threads:[~2020-01-22 1:15 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-20 15:50 Alexander Monakov
2020-01-20 21:53 ` Sandra Loosemore
2020-01-20 23:11 ` Alexander Monakov
2020-01-21 0:44 ` Joseph Myers
2020-01-21 13:57 ` Alexander Monakov
2020-01-21 14:59 ` Richard Biener
2020-01-21 15:08 ` Alexander Monakov
2020-01-22 8:04 ` Richard Biener
2020-01-22 12:09 ` Martin Sebor
2020-01-23 13:47 ` Richard Biener
2020-01-24 0:06 ` Uecker, Martin
2020-01-27 15:14 ` Richard Biener
2020-01-28 4:02 ` Uecker, Martin
2020-01-28 8:28 ` Alexander Monakov
2020-01-28 10:16 ` Richard Biener
2020-01-28 13:28 ` Uecker, Martin
2020-01-29 9:01 ` Richard Biener
2020-01-29 14:28 ` Uecker, Martin
2020-01-30 9:47 ` Richard Biener
2020-01-30 14:42 ` Uecker, Martin
2020-01-30 16:59 ` Michael Matz
2020-01-30 17:27 ` Michael Matz
2020-01-30 17:29 ` Uecker, Martin
2020-01-31 9:31 ` Richard Biener
2020-01-31 12:26 ` Uecker, Martin
2020-01-31 13:22 ` Richard Biener
2020-01-28 13:08 ` Uecker, Martin
2020-01-28 18:04 ` Alexander Monakov
2020-01-22 1:37 ` Joseph Myers
2020-01-22 8:15 ` Richard Biener
2020-01-22 1:29 ` Joseph Myers [this message]
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