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[130.44.146.16]) by smtp.gmail.com with ESMTPSA id u10-20020a05622a198a00b0041cb8947ed2sm811916qtc.26.2023.10.20.12.10.24 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 20 Oct 2023 12:10:25 -0700 (PDT) Message-ID: Date: Fri, 20 Oct 2023 15:10:24 -0400 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] c-family: char8_t and aliasing in C vs C++ [PR111884] To: Marek Polacek , GCC Patches , Joseph Myers References: <20231020163121.25120-1-polacek@redhat.com> From: Jason Merrill In-Reply-To: <20231020163121.25120-1-polacek@redhat.com> 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=-12.4 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_H4,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 10/20/23 12:31, Marek Polacek wrote: > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk? OK. > -- >8 -- > In the PR, Joseph says that in C char8_t is not a distinct type. So > we should behave as if it can alias anything, like ordinary char. > In C, unsigned_char_type_node == char8_type_node, so with this patch > we return 0 instead of -1. And the following comment says: > > /* The C standard guarantees that any object may be accessed via an > lvalue that has narrow character type (except char8_t). */ > if (t == char_type_node > || t == signed_char_type_node > || t == unsigned_char_type_node) > return 0; > > Which appears to be wrong, so I'm adjusting that as well. > > PR c/111884 > > gcc/c-family/ChangeLog: > > * c-common.cc (c_common_get_alias_set): Return -1 for char8_t only > in C++. > > gcc/testsuite/ChangeLog: > > * c-c++-common/alias-1.c: New test. > --- > gcc/c-family/c-common.cc | 7 ++++--- > gcc/testsuite/c-c++-common/alias-1.c | 23 +++++++++++++++++++++++ > 2 files changed, 27 insertions(+), 3 deletions(-) > create mode 100644 gcc/testsuite/c-c++-common/alias-1.c > > diff --git a/gcc/c-family/c-common.cc b/gcc/c-family/c-common.cc > index f044db5b797..0efdc677217 100644 > --- a/gcc/c-family/c-common.cc > +++ b/gcc/c-family/c-common.cc > @@ -3828,12 +3828,13 @@ c_common_get_alias_set (tree t) > if (!TYPE_P (t)) > return -1; > > - /* Unlike char, char8_t doesn't alias. */ > - if (flag_char8_t && t == char8_type_node) > + /* Unlike char, char8_t doesn't alias in C++. (In C, char8_t is not > + a distinct type.) */ > + if (flag_char8_t && t == char8_type_node && c_dialect_cxx ()) > return -1; > > /* The C standard guarantees that any object may be accessed via an > - lvalue that has narrow character type (except char8_t). */ > + lvalue that has narrow character type. */ > if (t == char_type_node > || t == signed_char_type_node > || t == unsigned_char_type_node) > diff --git a/gcc/testsuite/c-c++-common/alias-1.c b/gcc/testsuite/c-c++-common/alias-1.c > new file mode 100644 > index 00000000000..d72fec47f76 > --- /dev/null > +++ b/gcc/testsuite/c-c++-common/alias-1.c > @@ -0,0 +1,23 @@ > +/* PR c/111884 */ > +/* { dg-do compile } */ > +/* { dg-options "-O2 -Wall" } */ > +/* { dg-additional-options "-std=c++20" { target c++ } } */ > +/* { dg-additional-options "-std=c2x" { target c } } */ > + > +int f(int i) > +{ > + int f = 1; > + return i[(unsigned char *)&f]; > +} > + > +int g(int i) > +{ > + int f = 1; > + return i[(signed char *)&f]; > +} > + > +int h(int i) > +{ > + int f = 1; > + return i[(char *)&f]; > +} > > base-commit: eb15fad3190a8b33e3e451b964ff1ecf08bbb113