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[130.44.159.43]) by smtp.gmail.com with ESMTPSA id h2-20020ac85042000000b003a606428a59sm451879qtm.91.2022.11.15.14.57.27 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 15 Nov 2022 14:57:27 -0800 (PST) Message-ID: Date: Tue, 15 Nov 2022 17:57:26 -0500 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.4.2 Subject: Re: [PATCH] c++: Alignment changes to layout compatibility/common initial sequence - DR2583 To: Jakub Jelinek Cc: gcc-patches@gcc.gnu.org References: From: Jason Merrill In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Spam-Status: No, score=-6.2 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,KAM_NUMSUBJECT,NICE_REPLY_A,RCVD_IN_DNSWL_NONE,RCVD_IN_MSPIKE_H2,SPF_HELO_NONE,SPF_NONE,TXREP autolearn=no autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: On 11/14/22 01:28, Jakub Jelinek wrote: > Hi! > > Working virtually out of Baker Island. > > When trying to figure out what to do about alignment, > layout_compatible_type_p returns false if TYPE_ALIGN on > ENUMERAL_TYPE/CLASS_TYPE_P (but not scalar types?) differ, or if members > don't have the same positions. > > What is in DR2583 doesn't say anything like that though, on the other side > it says that if the corresponding entities don't have the same alignment > requirements, they aren't part of the common initial sequence. > > So, my understanding of this is we shouldn't check TYPE_ALIGN in > layout_compatible_type_p, but instead DECL_ALIGN in > next_common_initial_seqence. Agreed. > Lightly tested (on is-layout*/is-corresponding*/dr2583.C only) so far, > ok if it passes full bootstrap/regtest? > Or do we need different rules? > > 2022-11-14 Jakub Jelinek > > * typeck.cc (next_common_initial_seqence): Return false members have > different DECL_ALIGN. > (layout_compatible_type_p): Don't test TYPE_ALIGN of ENUMERAL_TYPE > or CLASS_TYPE_P. > > * g++.dg/cpp2a/is-layout-compatible3.C: Expect enums with different > alignas to be layout compatible, while classes with different > alignas on members layout incompatible. > * g++.dg/DRs/dr2583.C: New test. > > --- gcc/cp/typeck.cc.jj 2022-11-13 04:53:46.010682269 -1200 > +++ gcc/cp/typeck.cc 2022-11-13 23:14:41.355180354 -1200 > @@ -1833,6 +1833,8 @@ next_common_initial_seqence (tree &memb1 > if ((!lookup_attribute ("no_unique_address", DECL_ATTRIBUTES (memb1))) > != !lookup_attribute ("no_unique_address", DECL_ATTRIBUTES (memb2))) > return false; > + if (DECL_ALIGN (memb1) != DECL_ALIGN (memb2)) > + return false; > if (!tree_int_cst_equal (bit_position (memb1), bit_position (memb2))) > return false; > return true; > @@ -1854,15 +1856,13 @@ layout_compatible_type_p (tree type1, tr > type2 = cp_build_qualified_type (type2, TYPE_UNQUALIFIED); > > if (TREE_CODE (type1) == ENUMERAL_TYPE) > - return (TYPE_ALIGN (type1) == TYPE_ALIGN (type2) > - && tree_int_cst_equal (TYPE_SIZE (type1), TYPE_SIZE (type2)) > + return (tree_int_cst_equal (TYPE_SIZE (type1), TYPE_SIZE (type2)) > && same_type_p (finish_underlying_type (type1), > finish_underlying_type (type2))); > > if (CLASS_TYPE_P (type1) > && std_layout_type_p (type1) > && std_layout_type_p (type2) > - && TYPE_ALIGN (type1) == TYPE_ALIGN (type2) > && tree_int_cst_equal (TYPE_SIZE (type1), TYPE_SIZE (type2))) > { > tree field1 = TYPE_FIELDS (type1); > --- gcc/testsuite/g++.dg/cpp2a/is-layout-compatible3.C.jj 2021-08-18 21:42:27.414421719 -1200 > +++ gcc/testsuite/g++.dg/cpp2a/is-layout-compatible3.C 2022-11-13 23:20:05.008776825 -1200 > @@ -55,10 +55,10 @@ static_assert (!std::is_layout_compatibl > static_assert (!std::is_layout_compatible_v); > static_assert (!std::is_layout_compatible_v); > static_assert (!std::is_layout_compatible_v); > -static_assert (!std::is_layout_compatible_v); > +static_assert (std::is_layout_compatible_v); > static_assert (!std::is_layout_compatible_v); > static_assert (!std::is_layout_compatible_v); > static_assert (!std::is_layout_compatible_v); > -static_assert (std::is_layout_compatible_v); > +static_assert (!std::is_layout_compatible_v); > static_assert (std::is_layout_compatible_v); > static_assert (std::is_layout_compatible_v); > --- gcc/testsuite/g++.dg/DRs/dr2583.C.jj 2022-11-13 22:58:11.977640606 -1200 > +++ gcc/testsuite/g++.dg/DRs/dr2583.C 2022-11-13 23:18:04.630414835 -1200 > @@ -0,0 +1,31 @@ > +// DR 2583 - Common initial sequence should consider over-alignment. > +// { dg-do compile { target c++11 } } > + > +#include > + > +struct A { > + int i; > + char c; > +}; > + > +struct B { > + int i; > + alignas(8) char c; > +}; > + > +struct S0 { > + alignas(16) char x[128]; > + int i; > +}; > + > +struct alignas(16) S1 { > + char x[128]; > + int i; > +}; > + > +#if __cpp_lib_is_layout_compatible >= 201907L > +static_assert (std::is_corresponding_member (&A::i, &B::i), ""); > +static_assert (alignof (char) == 8 || !std::is_corresponding_member (&A::c, &B::c), ""); Maybe std_is_corresponding_member (&A::c, &B::c) == (alignof (char) == 8) ? Could also use an alignas(alignof(type)) case to verify that it matches a member with no alignas. > +static_assert (alignof (char) == 16 || !std::is_corresponding_member (&S0::x, &S1::x), ""); > +static_assert (alignof (char) == 16 || !std::is_corresponding_member (&S0::i, &S1::i), ""); > +#endif > > Jakub >