From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 48) id B01203844046; Thu, 10 Dec 2020 16:51:20 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org B01203844046 From: "zaikin.icc at gmail dot com" To: gcc-bugs@gcc.gnu.org Subject: [Bug libstdc++/98226] New: Slow std::countr_one Date: Thu, 10 Dec 2020 16:51:20 +0000 X-Bugzilla-Reason: CC X-Bugzilla-Type: new X-Bugzilla-Watch-Reason: None X-Bugzilla-Product: gcc X-Bugzilla-Component: libstdc++ X-Bugzilla-Version: 10.2.0 X-Bugzilla-Keywords: X-Bugzilla-Severity: normal X-Bugzilla-Who: zaikin.icc at gmail dot com X-Bugzilla-Status: UNCONFIRMED X-Bugzilla-Resolution: X-Bugzilla-Priority: P3 X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org X-Bugzilla-Target-Milestone: --- X-Bugzilla-Flags: X-Bugzilla-Changed-Fields: bug_id short_desc product version bug_status bug_severity priority component assigned_to reporter target_milestone Message-ID: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/ Auto-Submitted: auto-generated MIME-Version: 1.0 X-BeenThere: gcc-bugs@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gcc-bugs mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 10 Dec 2020 16:51:20 -0000 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98226 Bug ID: 98226 Summary: Slow std::countr_one Product: gcc Version: 10.2.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: libstdc++ Assignee: unassigned at gcc dot gnu.org Reporter: zaikin.icc at gmail dot com Target Milestone: --- The function std::countr_one() from C++20 is slow. For a given x (say, unsi= gned int) it in fact calls std::countr_zero(~x) which in turn calls __builtin_ctz(~x). Calling __builtin_ctz(~x) directly from std::countr_one() would increase the performance. The test case contains three sources, each = of which finally does the same but with different performance. Test case: --- test1.cpp: #include int main() { unsigned j; for (unsigned i=3D0; i<(1 << 30); i++) { j =3D std::countr_one(i); } } --- g++ -std=3Dc++20 ./test1.cpp -o test1 test2.cpp: #include int main() { unsigned j; for (unsigned i=3D0; i<(1 << 30); i++) { j =3D std::countr_zero(~i); } } --- g++ -std=3Dc++20 ./test2.cpp -o test2 test3.cpp: #include int main() { unsigned j; for (unsigned i=3D0; i<(1 << 30); i++) { j =3D __builtin_ctz(~i); } } --- g++ -std=3Dc++20 ./test3.cpp -o test3 The user time is reported below: time ./test1 5.266s time ./test2 3.028s time ./test3 0.741s $ g++ -v Using built-in specs. COLLECT_GCC=3Dg++ COLLECT_LTO_WRAPPER=3D/usr/lib/gcc/x86_64-linux-gnu/10/lto-wrapper OFFLOAD_TARGET_NAMES=3Dnvptx-none:amdgcn-amdhsa:hsa OFFLOAD_TARGET_DEFAULT=3D1 Target: x86_64-linux-gnu Configured with: ../src/configure -v --with-pkgversion=3D'Ubuntu 10.2.0-5ubuntu1~20.04' --with-bugurl=3Dfile:///usr/share/doc/gcc-10/README.= Bugs --enable-languages=3Dc,ada,c++,go,brig,d,fortran,objc,obj-c++,m2 --prefix= =3D/usr --with-gcc-major-version-only --program-suffix=3D-10 --program-prefix=3Dx86_64-linux-gnu- --enable-shared --enable-linker-build-= id --libexecdir=3D/usr/lib --without-included-gettext --enable-threads=3Dposix --libdir=3D/usr/lib --enable-nls --enable-clocale=3Dgnu --enable-libstdcxx-= debug --enable-libstdcxx-time=3Dyes --with-default-libstdcxx-abi=3Dnew --enable-gnu-unique-object --disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib --enable-libphobos-checking=3Drelea= se --with-target-system-zlib=3Dauto --enable-objc-gc=3Dauto --enable-multiarch --disable-werror --with-arch-32=3Di686 --with-abi=3Dm64 --with-multilib-list=3Dm32,m64,mx32 --enable-multilib --with-tune=3Dgeneric --enable-offload-targets=3Dnvptx-none=3D/build/gcc-10-WJNXnb/gcc-10-10.2.0/= debian/tmp-nvptx/usr,amdgcn-amdhsa=3D/build/gcc-10-WJNXnb/gcc-10-10.2.0/deb= ian/tmp-gcn/usr,hsa --without-cuda-driver --enable-checking=3Drelease --build=3Dx86_64-linux-gnu --host=3Dx86_64-linux-gnu --target=3Dx86_64-linux-gnu Thread model: posix Supported LTO compression algorithms: zlib zstd gcc version 10.2.0 (Ubuntu 10.2.0-5ubuntu1~20.04)=