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[24.228.8.148]) by smtp.gmail.com with ESMTPSA id n7-20020ac85a07000000b002f1421dac8csm3190786qta.80.2022.04.15.11.42.55 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 15 Apr 2022 11:42:55 -0700 (PDT) From: Patrick Palka X-Google-Original-From: Patrick Palka Date: Fri, 15 Apr 2022 14:42:54 -0400 (EDT) To: Jonathan Wakely cc: Patrick Palka , gcc Patches , libstdc++ Subject: Re: [PATCH] libstdc++: Avoid double-deref of __first in ranges::minmax [PR104858] In-Reply-To: Message-ID: References: <20220414152111.229967-1-ppalka@redhat.com> MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=US-ASCII X-Spam-Status: No, score=-14.5 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_LOW, RCVD_IN_MSPIKE_H5, RCVD_IN_MSPIKE_WL, SPF_HELO_NONE, SPF_NONE, TXREP, T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.4 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on server2.sourceware.org X-BeenThere: libstdc++@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libstdc++ mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 15 Apr 2022 18:42:59 -0000 On Thu, 14 Apr 2022, Jonathan Wakely wrote: > On Thu, 14 Apr 2022 at 16:21, Patrick Palka via Libstdc++ > wrote: > > > > Tested on x86_64-pc-linux-gnu, does this look OK for trunk and 11/10 > > once the branch is unfrozen? > > > > PR libstdc++/104858 > > > > libstdc++-v3/ChangeLog: > > > > * include/bits/ranges_algo.h (__minmax_fn): Avoid dereferencing > > __first twice at the start. > > * testsuite/25_algorithms/minmax/constrained.cc (test06): New test. > > --- > > libstdc++-v3/include/bits/ranges_algo.h | 2 +- > > .../25_algorithms/minmax/constrained.cc | 23 +++++++++++++++++++ > > 2 files changed, 24 insertions(+), 1 deletion(-) > > > > diff --git a/libstdc++-v3/include/bits/ranges_algo.h b/libstdc++-v3/include/bits/ranges_algo.h > > index 62dc605080a..3d30fb1428c 100644 > > --- a/libstdc++-v3/include/bits/ranges_algo.h > > +++ b/libstdc++-v3/include/bits/ranges_algo.h > > @@ -3084,7 +3084,7 @@ namespace ranges > > auto __last = ranges::end(__r); > > __glibcxx_assert(__first != __last); > > auto __comp_proj = __detail::__make_comp_proj(__comp, __proj); > > - minmax_result> __result = {*__first, *__first}; > > + minmax_result> __result = {*__first, __result.min}; > > Clever ... I'm surprised this even works. I would have expected it to > evaluate both initializers before actually initializing the members. > TIL. Indeed, it seems to do the right thing, practically speaking at least :) FWIW the alternative approach - minmax_result> __result = {*__first, *__first}; + minmax_result> __result; + __result.max = __result.min = *__first; wouldn't be right because the value type is not necessarily default constructible. I beefed up the new testcase to verify we don't demand default constructibility here.