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From: Jason Merrill <jason@redhat.com>
To: Jakub Jelinek <jakub@redhat.com>
Cc: gcc-patches List <gcc-patches@gcc.gnu.org>
Subject: Re: C++ PATCH for testsuite failures with -std=c++17
Date: Thu, 25 May 2017 21:57:00 -0000	[thread overview]
Message-ID: <CADzB+2nU1RZp8oDa7+JhkAP6Gm1+WSV970G-HJ-RvRxBe6+1bg@mail.gmail.com> (raw)
In-Reply-To: <20170525090813.GZ8499@tucnak>

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On Thu, May 25, 2017 at 5:08 AM, Jakub Jelinek <jakub@redhat.com> wrote:
> On Thu, May 25, 2017 at 10:51:56AM +0200, Jakub Jelinek wrote:
>> On Tue, May 09, 2017 at 04:37:16PM -0400, Jason Merrill wrote:
>> > For C++17 aggregate bases, we have started adding base fields for
>> > empty bases.  The code for calculating whether a class is standard
>> > layout needs to ignore these.
>> >
>> > The C++17 mode diagnostic for direct-enum-init1.C was incorrect.
>> >
>> > Tested x86_64-pc-linux-gnu, applying to trunk.
>>
>> > commit 9a612cc30d4b3ef905ce45304545d8b99a3cf5b9
>> > Author: Jason Merrill <jason@redhat.com>
>> > Date:   Tue May 9 14:15:38 2017 -0400
>> >
>> >             * class.c (check_bases): Ignore empty bases.
>>
>> This should have referenced PR c++/80605 (and is also a 7 regression).
>>
>> > diff --git a/gcc/cp/class.c b/gcc/cp/class.c
>> > index fc71766..085dbc3 100644
>> > --- a/gcc/cp/class.c
>> > +++ b/gcc/cp/class.c
>> > @@ -1860,7 +1860,9 @@ check_bases (tree t,
>> >            members */
>> >         for (basefield = TYPE_FIELDS (basetype); basefield;
>> >              basefield = DECL_CHAIN (basefield))
>> > -         if (TREE_CODE (basefield) == FIELD_DECL)
>> > +         if (TREE_CODE (basefield) == FIELD_DECL
>> > +             && DECL_SIZE (basefield)
>> > +             && !integer_zerop (DECL_SIZE (basefield)))
>>
>> Is that what we really want?  I mean, shouldn't we at least also
>> check that the basefield we want to ignore is DECL_ARTIFICIAL,
>> or that it doesn't have DECL_NAME or something similar, to avoid
>> considering user fields with zero size the same?
>> I believe your change changes e.g.:
>> struct S { int a[0]; };
>> struct T : public S { int b[0]; int c; };
>> bool q = __is_standard_layout (T);
>> which previously e.g. with -std=gnu++14 emitted q = false, but
>> now emits q = true.
>
> We even have DECL_FIELD_IS_BASE macro, so can't the above be
>   if (TREE_CODE (basefield) == FIELD_DECL
>       && !DECL_FIELD_IS_BASE (basefield))
> or
>   if (TREE_CODE (basefield) == FIELD_DECL
>       && (!DECL_FIELD_IS_BASE (basefield)
>           || (DECL_SIZE (basefield)
>               && !integer_zerop (DECL_SIZE (basefield)))))
> or something similar?

Indeed, thanks.

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commit 49096cb5bc6c629c619ac9b5e08b971867dd1fc1
Author: Jason Merrill <jason@redhat.com>
Date:   Thu May 25 15:34:13 2017 -0400

            PR c++/80605 - __is_standard_layout and zero-length array
    
            * class.c (check_bases): Use DECL_FIELD_IS_BASE.

diff --git a/gcc/cp/class.c b/gcc/cp/class.c
index 984fb09..eddc118 100644
--- a/gcc/cp/class.c
+++ b/gcc/cp/class.c
@@ -1842,8 +1842,8 @@ check_bases (tree t,
 	    for (basefield = TYPE_FIELDS (basetype); basefield;
 		 basefield = DECL_CHAIN (basefield))
 	      if (TREE_CODE (basefield) == FIELD_DECL
-		  && DECL_SIZE (basefield)
-		  && !integer_zerop (DECL_SIZE (basefield)))
+		  && !(DECL_FIELD_IS_BASE (basefield)
+		       && integer_zerop (DECL_SIZE (basefield))))
 		{
 		  if (field)
 		    CLASSTYPE_NON_STD_LAYOUT (t) = 1;
diff --git a/gcc/testsuite/g++.dg/ext/is_std_layout2.C b/gcc/testsuite/g++.dg/ext/is_std_layout2.C
new file mode 100644
index 0000000..02dc4f7
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/is_std_layout2.C
@@ -0,0 +1,6 @@
+// { dg-do compile { target c++11 } }
+// { dg-options "" }
+
+struct S { int a[0]; };
+struct T : public S { int b[0]; int c; };
+static_assert(!__is_standard_layout (T), "");

      reply	other threads:[~2017-05-25 21:21 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-05-09 20:42 Jason Merrill
2017-05-25  9:08 ` Jakub Jelinek
2017-05-25  9:23   ` Jakub Jelinek
2017-05-25 21:57     ` Jason Merrill [this message]

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