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Wed, 27 Sep 2023 07:13:46 -0700 (PDT) X-Google-Smtp-Source: AGHT+IEjh4d/WLcxlGj0If7kx4cI6EUK8xpcA4OSaS04PxVgA9RdXoMdZcV0/KWZR0y4hu2I1ckHw3jwUGkTTXVIczE= X-Received: by 2002:a2e:b358:0:b0:2c0:af3:27db with SMTP id q24-20020a2eb358000000b002c00af327dbmr2234539lja.22.1695824026268; Wed, 27 Sep 2023 07:13:46 -0700 (PDT) MIME-Version: 1.0 References: <053faf76-f918-7527-4a41-755a18d0018a@redhat.com> In-Reply-To: From: Jonathan Wakely Date: Wed, 27 Sep 2023 15:13:35 +0100 Message-ID: Subject: Re: [PATCH] libstdc++: Ensure active union member is correctly set To: Nathaniel Shead Cc: Jonathan Wakely , Jason Merrill , "libstdc++" , gcc-patches X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset="UTF-8" X-Spam-Status: No, score=-12.2 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,GIT_PATCH_0,RCVD_IN_DNSWL_NONE,RCVD_IN_MSPIKE_H3,RCVD_IN_MSPIKE_WL,SPF_HELO_NONE,SPF_NONE,TXREP autolearn=ham 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 Sat, 23 Sept 2023 at 08:30, Nathaniel Shead via Libstdc++ wrote: > > On Sat, Sep 23, 2023 at 07:40:48AM +0100, Jonathan Wakely wrote: > > On Sat, 23 Sept 2023, 01:39 Nathaniel Shead via Libstdc++, < > > libstdc++@gcc.gnu.org> wrote: > > > > > Now that bootstrap has finished, I have gotten regressions in the > > > following libstdc++ tests: > > > > > > Running libstdc++:libstdc++-dg/conformance.exp ... > > > FAIL: 20_util/bitset/access/constexpr.cc -std=gnu++23 (test for excess > > > errors) > > > FAIL: 20_util/bitset/access/constexpr.cc -std=gnu++26 (test for excess > > > errors) > > > FAIL: 20_util/variant/constexpr.cc -std=gnu++20 (test for excess errors) > > > FAIL: 20_util/variant/constexpr.cc -std=gnu++26 (test for excess errors) > > > FAIL: 21_strings/basic_string/cons/char/constexpr.cc -std=gnu++20 (test > > > for excess errors) > > > FAIL: 21_strings/basic_string/cons/char/constexpr.cc -std=gnu++26 (test > > > for excess errors) > > > FAIL: 21_strings/basic_string/cons/wchar_t/constexpr.cc -std=gnu++20 (test > > > for excess errors) > > > FAIL: 21_strings/basic_string/cons/wchar_t/constexpr.cc -std=gnu++26 (test > > > for excess errors) > > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr-wchar_t.cc > > > -std=gnu++20 (test for excess errors) > > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr-wchar_t.cc > > > -std=gnu++26 (test for excess errors) > > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr.cc -std=gnu++20 > > > (test for excess errors) > > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr.cc -std=gnu++26 > > > (test for excess errors) > > > FAIL: std/ranges/adaptors/join_with/1.cc -std=gnu++23 (test for excess > > > errors) > > > UNRESOLVED: std/ranges/adaptors/join_with/1.cc -std=gnu++23 compilation > > > failed to produce executable > > > FAIL: std/ranges/adaptors/join_with/1.cc -std=gnu++26 (test for excess > > > errors) > > > UNRESOLVED: std/ranges/adaptors/join_with/1.cc -std=gnu++26 compilation > > > failed to produce executable > > > > > > On investigation though it looks like the issue might be with libstdc++ > > > rather than the patch itself; running the failing tests using clang with > > > libstdc++ also produces similar errors, and my reading of the code > > > suggests that this is correct. > > > > > > What's the way forward here? Should I look at creating a patch to fix > > > the libstdc++ issues before resubmitting this patch for the C++ > > > frontend? Or should I submit a version of this patch without the > > > `std::construct_at` changes and wait till libstdc++ gets fixed for that? > > > > > > > I think we should fix libstdc++. There are probably only a few places that > > need a fix, which cause all those failures. > > > > I can help with those fixes. I'll look into it after the weekend. > > > > Thanks. I did end up getting a chance to look at it earlier today, and > with the following patch I had no regressions when applying the frontend > changes. Bootstrapped and regtested on x86_64-pc-linux-gnu. > > -- >8 -- > > This patch ensures that the union members for std::string and > std::variant are always properly set when a change occurs. > > libstdc++-v3/ChangeLog: > > * include/bits/basic_string.h: (basic_string(basic_string&&)): > Activate _M_local_buf when needed. > (basic_string(basic_string&&, const _Alloc&)): Likewise. > * include/bits/basic_string.tcc: (basic_string::swap): Likewise. > * include/std/variant: (__detail::__variant::__construct_n): New. > (__detail::_variant::__emplace): Use __construct_n. > > Signed-off-by: Nathaniel Shead > --- > libstdc++-v3/include/bits/basic_string.h | 7 +++-- > libstdc++-v3/include/bits/basic_string.tcc | 8 +++--- > libstdc++-v3/include/std/variant | 32 ++++++++++++++++++++-- > 3 files changed, 38 insertions(+), 9 deletions(-) > > diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h > index 09fd62afa66..7c342879827 100644 > --- a/libstdc++-v3/include/bits/basic_string.h > +++ b/libstdc++-v3/include/bits/basic_string.h > @@ -678,7 +678,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11 > { > if (__str._M_is_local()) > { > - traits_type::copy(_M_local_buf, __str._M_local_buf, > + traits_type::copy(_M_use_local_data(), __str._M_local_buf, > __str.length() + 1); > } > else > @@ -691,7 +691,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11 > // basic_stringbuf relies on writing into unallocated capacity so > // we mess up the contents if we put a '\0' in the string. > _M_length(__str.length()); > - __str._M_data(__str._M_local_data()); > + __str._M_data(__str._M_use_local_data()); > __str._M_set_length(0); > } > > @@ -717,6 +717,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11 > { > if (__str._M_is_local()) > { > + _M_use_local_data(); Lets add a cast to void to make it clear we're intentionally discarding the return value here, and only calling it for its constexpr "side effects": (void) _M_use_local_data(); > traits_type::copy(_M_local_buf, __str._M_local_buf, > __str.length() + 1); > _M_length(__str.length()); > @@ -728,7 +729,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11 > _M_data(__str._M_data()); > _M_length(__str.length()); > _M_capacity(__str._M_allocated_capacity); > - __str._M_data(__str._M_local_buf); > + __str._M_data(__str._M_use_local_data()); > __str._M_set_length(0); > } > else > diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc > index 104a517f794..ee6e57da555 100644 > --- a/libstdc++-v3/include/bits/basic_string.tcc > +++ b/libstdc++-v3/include/bits/basic_string.tcc > @@ -79,7 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION > } > else if (__s.length()) > { > - traits_type::copy(_M_local_buf, __s._M_local_buf, > + traits_type::copy(_M_use_local_data(), __s._M_local_buf, I think we should call _M_use_local_data() before calling traits_type::copy, as you did for basic_string(basic_string&& __str, const _Alloc& __a) above. The problem is that _M_use_local_data() returns the allocator's pointer type, but traits_type::copy expects char_type*, and those might not be the same type. So: @@ -79,6 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION } else if (__s.length()) { + (void) _M_use_local_data(); traits_type::copy(_M_local_buf, __s._M_local_buf, __s.length() + 1); _M_length(__s.length()); > __s.length() + 1); > _M_length(__s.length()); > __s._M_set_length(0); > @@ -87,7 +87,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION > } > else if (length()) > { > - traits_type::copy(__s._M_local_buf, _M_local_buf, > + traits_type::copy(__s._M_use_local_data(), _M_local_buf, Same here, for __s._M_use_local_data() > length() + 1); > __s._M_length(length()); > _M_set_length(0); > @@ -97,7 +97,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION > else > { > const size_type __tmp_capacity = __s._M_allocated_capacity; > - traits_type::copy(__s._M_local_buf, _M_local_buf, > + traits_type::copy(__s._M_use_local_data(), _M_local_buf, And again. > length() + 1); > _M_data(__s._M_data()); > __s._M_data(__s._M_local_buf); > @@ -108,7 +108,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION > const size_type __tmp_capacity = _M_allocated_capacity; > if (__s._M_is_local()) > { > - traits_type::copy(_M_local_buf, __s._M_local_buf, > + traits_type::copy(_M_use_local_data(), __s._M_local_buf, And again. > __s.length() + 1); > __s._M_data(_M_data()); > _M_data(_M_local_buf); > diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant > index c0e41740dcf..7f24e760bb1 100644 > --- a/libstdc++-v3/include/std/variant > +++ b/libstdc++-v3/include/std/variant > @@ -320,6 +320,33 @@ namespace __variant > __get(_Variant&& __v) noexcept > { return __variant::__get_n<_Np>(std::forward<_Variant>(__v)._M_u); } > > + // Gets the _Uninitialized to construct into for __u. > + template > + constexpr decltype(auto) > + __construct_n(_Union& __u) noexcept > + { > + if constexpr (_Np == 0) > + return &__u._M_first; > + else if constexpr (_Np == 1) > + { > + std::_Construct(&__u._M_rest); > + return &__u._M_rest._M_first; > + } > + else if constexpr (_Np == 2) > + { > + std::_Construct(&__u._M_rest); > + std::_Construct(&__u._M_rest._M_rest); > + return &__u._M_rest._M_rest._M_first; > + } > + else > + { > + std::_Construct(&__u._M_rest); > + std::_Construct(&__u._M_rest._M_rest); > + std::_Construct(&__u._M_rest._M_rest._M_rest); > + return __variant::__construct_n<_Np - 3>(__u._M_rest._M_rest._M_rest); > + } > + } This is nice, thanks for optimizing it to reduce the recursion depth. > + > template > struct _Traits > { > @@ -536,8 +563,9 @@ namespace __variant > __emplace(_Variant_storage<_Triv, _Types...>& __v, _Args&&... __args) > { > __v._M_reset(); > - auto* __addr = std::__addressof(__variant::__get_n<_Np>(__v._M_u)); > - std::_Construct(__addr, std::forward<_Args>(__args)...); > + auto* __addr = __variant::__construct_n<_Np>(__v._M_u); > + std::_Construct(__addr, in_place_index<0>, > + std::forward<_Args>(__args)...); > // Construction didn't throw, so can set the new index now: > __v._M_index = _Np; > } > -- > 2.41.0 >