https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98226
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, unsigned 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 <bit> int main() { unsigned j; for (unsigned i=0; i<(1 << 30); i++) { j = std::countr_one(i); } } --- g++ -std=c++20 ./test1.cpp -o test1 test2.cpp: #include <bit> int main() { unsigned j; for (unsigned i=0; i<(1 << 30); i++) { j = std::countr_zero(~i); } } --- g++ -std=c++20 ./test2.cpp -o test2 test3.cpp: #include <bit> int main() { unsigned j; for (unsigned i=0; i<(1 << 30); i++) { j = __builtin_ctz(~i); } } --- g++ -std=c++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=g++ COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/10/lto-wrapper OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa:hsa OFFLOAD_TARGET_DEFAULT=1 Target: x86_64-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Ubuntu 10.2.0-5ubuntu1~20.04' --with-bugurl=file:///usr/share/doc/gcc-10/README.Bugs --enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,m2 --prefix=/usr --with-gcc-major-version-only --program-suffix=-10 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib --enable-libphobos-checking=release --with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none=/build/gcc-10-WJNXnb/gcc-10-10.2.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-10-WJNXnb/gcc-10-10.2.0/debian/tmp-gcn/usr,hsa --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu Thread model: posix Supported LTO compression algorithms: zlib zstd gcc version 10.2.0 (Ubuntu 10.2.0-5ubuntu1~20.04)