From: Jason Merrill <jason@redhat.com>
To: Richard Biener <rguenther@suse.de>
Cc: Bernd Edlinger <bernd.edlinger@hotmail.de>,
Jakub Jelinek <jakub@redhat.com>,
Jonathan Wakely <jwakely@redhat.com>,
Florian Weimer <fweimer@redhat.com>,
GCC Patches <gcc-patches@gcc.gnu.org>, Jeff Law <law@redhat.com>,
Jan Hubicka <jh@suse.de>
Subject: Re: [PATCH] Add a new type attribute always_alias (PR79671)
Date: Tue, 11 Apr 2017 18:47:00 -0000 [thread overview]
Message-ID: <CADzB+2nJk_WW7ns49zFVhBW0uEdzai4JzqN7jMZdCRG4UKopug@mail.gmail.com> (raw)
In-Reply-To: <alpine.LSU.2.20.1704111531390.30715@zhemvz.fhfr.qr>
On Tue, Apr 11, 2017 at 9:35 AM, Richard Biener <rguenther@suse.de> wrote:
> On Tue, 11 Apr 2017, Richard Biener wrote:
>
>> On Tue, 11 Apr 2017, Richard Biener wrote:
>>
>> > On Mon, 10 Apr 2017, Jason Merrill wrote:
>> >
>> > > On Mon, Apr 10, 2017 at 11:30 AM, Richard Biener <rguenther@suse.de> wrote:
>> > > > On Mon, 10 Apr 2017, Jason Merrill wrote:
>> > > >> On Mon, Apr 10, 2017 at 8:50 AM, Richard Biener <rguenther@suse.de> wrote:
>> > > >> > * tree.c (build_cplus_array_type): Set TYPE_TYPELESS_STORAGE
>> > > >> > for arrays of unsigned char or std::byte.
>> > > >>
>> > > >> I think it would be good to have a flag to select whether these
>> > > >> semantics apply to any char variant and std::byte, only unsigned char
>> > > >> and std::byte, or only std::byte.
>> > > >
>> > > > Any suggestion? Not sure we really need it (I'm hesitant to write
>> > > > all the testcases to verify it actually works).
>> > >
>> > > Well, there's existing code that uses plain char (e.g. boost) that I
>> > > want to support. If there's a significant optimization penalty for
>> > > allowing that, we probably also want to be able to limit the impact.
>> > > If there isn't much penalty, let's just support all char variants.
>> >
>> > I haven't assessed the penalty involved but it can't be worse than
>> > the situation we had in GCC 6. So I think it's reasonable to support
>> > all char variants for now. One could add some instrumenting to
>> > alias_set_subset_of / alias_sets_conflict_p but it would only yield
>> > an upper bound on the number of failed queries (TBAA is a quite early
>> > out before PTA info is used for example).
>> >
>> > The following variant -- added missing
>> >
>> > Index: gcc/cp/tree.c
>> > ===================================================================
>> > --- gcc/cp/tree.c (revision 246832)
>> > +++ gcc/cp/tree.c (working copy)
>> > @@ -972,6 +979,7 @@ build_cplus_array_type (tree elt_type, t
>> > as it will overwrite alignment etc. of all variants. */
>> > TYPE_SIZE (t) = TYPE_SIZE (m);
>> > TYPE_SIZE_UNIT (t) = TYPE_SIZE_UNIT (m);
>> > + TYPE_TYPELESS_STORAGE (t) = TYPE_TYPELESS_STORAGE (m);
>> > }
>> >
>> > TYPE_MAIN_VARIANT (t) = m;
>> >
>> > that caused LTO bootstrap to fail and removed the tree-ssa-structalias.c
>> > change (committed separately) [LTO] bootstrapped and tested ok on
>> > x86_64-unknown-linux-gnu.
>> >
>> > I've tested some template examples and they seem to work fine.
>> >
>> > Ok for trunk?
>> >
>> > Disclaimer: there might still be an issue with cross-language LTO
>> > support, but it might at most result in TYPE_TYPELESS_STORAGE
>> > getting lost. Trying to craft a testcase to verify my theory.
>>
>> Not too difficult in the end (see below). A fix (below) is to
>> not treat arrays with differing TYPE_TYPELESS_STORAGE as
>> compatible for the purpose of computing TYPE_CANONICAL (and
>> thus recursively structures containing them). While they'd
>> still alias each other (because currently a TYPE_TYPELESS_STORAGE
>> member makes the whole thing effectively alias anything) this
>> results in warnings in case such a type is used in the interface
>> between C and C++ (it's also considered a ODR type).
>>
>> t.C:17:17: warning: type of ‘bar’ does not match original declaration
>> [-Wlto-type-mismatch]
>> extern "C" void bar (X *);
>> ^
>> t2.c:5:6: note: ‘bar’ was previously declared here
>> void bar (struct X *p) {}
>> ^
>>
>> if you add a struct X * parameter to bar().
>>
>> So it's not the optimal solution here. Another fix would be to
>> somehow make TYPE_TYPELESS_STORAGE "sticky" when merging canonical
>> types but there's not precedent in doing this kind of thing and
>> I'm not sure we'll get everything merged before computing alias
>> sets.
>>
>> CCing Honza.
>
> So after discussion and some more thinking we don't really benefit
> (and really can't) from having different aggregates with such
> members distignuishable via their alias set. So the following
> simplifies the approach and makes the above LTO bits trivial
> by more following Bernds approach of assigning the types alias
> set zero and instead of in the alias machinery propagate the
> flag on the types.
>
> It should also make reference_alias_ptr_type correct and it
> does the flag propagation in type layout.
>
> [LTO] bootstrap and regtest running on x86_64-unknown-linux-gnu.
>
> The C++ bits are unchanged.
The C++ bits are OK.
Jason
next prev parent reply other threads:[~2017-04-11 18:47 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
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 [this message]
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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