From: Richard Biener <rguenther@suse.de>
To: Jason Merrill <jason@redhat.com>
Cc: Bernd Edlinger <bernd.edlinger@hotmail.de>,
Jakub Jelinek <jakub@redhat.com>,
GCC Patches <gcc-patches@gcc.gnu.org>,
Marc Glisse <marc.glisse@inria.fr>,
Jonathan Wakely <jwakely@redhat.com>,
Jeff Law <law@redhat.com>
Subject: Re: [PATCH] Add a new type attribute always_alias (PR79671)
Date: Thu, 06 Apr 2017 07:23:00 -0000 [thread overview]
Message-ID: <alpine.LSU.2.20.1704060920240.30715@zhemvz.fhfr.qr> (raw)
In-Reply-To: <CADzB+2kGSL=yLDARcrXW2cAafEZC2Q84kTd0tjyALrLiRpqTRg@mail.gmail.com>
On Wed, 5 Apr 2017, Jason Merrill wrote:
> On Wed, Apr 5, 2017 at 1:41 PM, Bernd Edlinger
> <bernd.edlinger@hotmail.de> wrote:
> > On 04/05/17 17:20, Richard Biener wrote:
> >>> Also, wonder if you need to mark all types containing such arrays,
> >>> if you couldn't just set that flag in C++ on unsigned char/std::byte
> >>> arrays (and on anything with that attribute), have that imply alias set
> >>> 0 on it and then let the rest of alias machinery handle aggregate types
> >>> containing such fields.
> >>
> >> Yes, I expected it to work like this (didn't look at the patch yet).
>
> My impression is that this is how GCC 6 worked, but GCC 7 decides to
> ignore alias set 0 members. Is that right?
Yes, in GCC 6 an access via an aggregate type that had an alias-set zero
member was using alias-set zero.
So we'd re-instantiate that behavior for aggregates containing
a type marked with the proposed new flag.
> > I want to allow *only* what the C++ standard requires or what Jason says
> > of course :), and not a single bit more, because it suppresses otherwise
> > correct optimizations.
> >
> > So a struct with a std::byte member is not alias_set 0,
> > only the std::byte itself is, but an array of std::byte
> > is itself typeless_storage, and makes the whole structure
> > also typeless_storage
>
> Well, only the array member, not the whole structure, but it may make
> sense for GCC to treat the whole structure as such internally.
Yes, it's the easiest way to implement the required behavior. But
I wouldn't document it that way but clearly state that only the
marked member behaves so but that for the purpose of accesses via
a container type the standards access rules apply as if the member
is accessed with a character type.
Richard.
next prev parent reply other threads:[~2017-04-06 7:23 UTC|newest]
Thread overview: 65+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-04-05 9:46 Bernd Edlinger
2017-04-05 13:28 ` Jakub Jelinek
2017-04-05 15:20 ` Richard Biener
2017-04-05 17:41 ` Bernd Edlinger
2017-04-05 20:18 ` Jason Merrill
2017-04-05 20:46 ` Bernd Edlinger
2017-04-05 22:54 ` Pedro Alves
2017-04-06 10:08 ` Jonathan Wakely
2017-04-06 7:23 ` Richard Biener [this message]
2017-04-05 15:27 ` Bernd Edlinger
2017-04-05 15:29 ` Jakub Jelinek
2017-04-05 14:50 ` Florian Weimer
2017-04-05 15:23 ` Richard Biener
2017-04-05 15:38 ` Bernd Edlinger
2017-04-05 16:03 ` Jonathan Wakely
2017-04-05 16:08 ` Jakub Jelinek
2017-04-05 17:23 ` Bernd Edlinger
2017-04-05 21:02 ` Bernd Edlinger
2017-04-05 23:17 ` Sandra Loosemore
2017-04-06 5:40 ` Jakub Jelinek
2017-04-06 7:47 ` Richard Biener
2017-04-06 7:51 ` Jakub Jelinek
2017-04-06 7:55 ` Richard Biener
2017-04-06 14:11 ` Bernd Edlinger
2017-04-06 14:17 ` Florian Weimer
2017-04-06 14:23 ` Richard Biener
2017-04-06 14:43 ` Jonathan Wakely
2017-04-06 14:51 ` Florian Weimer
2017-04-06 15:05 ` Jakub Jelinek
2017-04-06 15:10 ` Florian Weimer
2017-04-06 19:13 ` Richard Biener
2017-04-11 10:43 ` Florian Weimer
2017-04-11 10:48 ` Richard Biener
2017-04-06 17:39 ` Bernd Edlinger
2017-04-06 17:48 ` Florian Weimer
2017-04-06 18:12 ` Bernd Edlinger
2017-04-06 18:19 ` Florian Weimer
2017-04-06 18:49 ` Bernd Edlinger
2017-04-06 19:05 ` Florian Weimer
2017-04-06 19:20 ` Bernd Edlinger
2017-04-07 6:47 ` Richard Biener
2017-04-07 12:58 ` Bernd Edlinger
2017-04-06 19:16 ` Richard Biener
2017-04-07 6:56 ` Florian Weimer
2017-04-07 8:01 ` Richard Biener
2017-04-06 19:14 ` Richard Biener
2017-04-06 19:51 ` Bernd Edlinger
2017-04-06 14:22 ` Richard Biener
2017-04-06 21:00 ` Bernd Edlinger
2017-04-07 6:54 ` Richard Biener
2017-04-07 13:37 ` Bernd Edlinger
2017-04-07 15:10 ` Richard Biener
2017-04-07 15:33 ` Bernd Edlinger
2017-04-07 20:22 ` Jason Merrill
2017-04-10 12:50 ` Richard Biener
2017-04-10 14:41 ` Jason Merrill
2017-04-10 15:31 ` Richard Biener
2017-04-10 16:35 ` Jason Merrill
2017-04-11 10:32 ` Richard Biener
2017-04-11 11:53 ` Richard Biener
2017-04-11 13:35 ` Richard Biener
2017-04-11 18:47 ` Jason Merrill
2017-04-10 21:40 ` Pedro Alves
2017-04-11 7:37 ` Richard Biener
2017-04-06 20:20 ` Bernd Edlinger
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