https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109047
Bug ID: 109047 Summary: Harmonize __attribute__((target_clones)) requirement in function prototype Product: gcc Version: unknown Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: hbucher at gmail dot com Target Milestone: --- Let's say I have a library that I want to share with multiple targets ```c++ #include <array> #include <cstdio> using Vector = std::array<float, 2>; using Matrix = std::array<float, 4>; __attribute__((target_clones("default","arch=core2","arch=znver2"))) Vector multiply(const Matrix& m, const Vector& v) { Vector r; r[0] = v[0] * m[0] + v[1] * m[2]; r[1] = v[0] * m[1] + v[1] * m[3]; return r; } ``` and I want to use that as below ```c++ #include <array> #include <cstdio> using Vector = std::array<float, 2>; using Matrix = std::array<float, 4>; Vector multiply(const Matrix& m, const Vector& v); int main() { Matrix m{1,2,3,4}; Vector v{1,2}; Vector r = multiply(m,v); printf( "%f %f\n", r[0], r[1] ); } ``` Godbolt project: https://godbolt.org/z/3hd4MrzsG GCC will be happy to compile and link the above two files together but clang++ will complain about undefined references. ``` ld: CMakeFiles/example.dir/example.cpp.o: in function `main': example.cpp:(.text+0x2a): undefined reference to `multiply(std::array<float, 4ul> const&, std::array<float, 2ul> const&)' ``` To make this work, I have to add all the targets in the prototype as well as in ``` #include <array> #include <cstdio> using Vector = std::array<float, 2>; using Matrix = std::array<float, 4>; __attribute__((target_clones("default","arch=core2","arch=znver2"))) Vector multiply(const Matrix& m, const Vector& v); int main() { Matrix m{1,2,3,4}; Vector v{1,2}; Vector r = multiply(m,v); printf( "%f %f\n", r[0], r[1] ); } ``` Looking at the object files generated, it seeems that the main difference is that GCC generates an indirect link for the naked prototype while clang generate an indirect link to .ifunc and that requires knowledge of the target attributes. ``` $ diff nm.gcc nm.clang 3,4d2 < U _GLOBAL_OFFSET_TABLE_ < U __stack_chk_fail 6,13c4,11 < i multiply(std::array<float, 4ul> const&, std::array<float, 2ul> const&) < t multiply(std::array<float, 4ul> const&, std::array<float, 2ul> const&) [clone .arch_cascadelake.3] < t _Z8multiplyRKSt5arrayIfLm4EERKS_IfLm2EE.arch_core2.0 < t multiply(std::array<float, 4ul> const&, std::array<float, 2ul> const&) [clone .arch_haswell.2] < t multiply(std::array<float, 4ul> const&, std::array<float, 2ul> const&) [clone .arch_sandybridge.1] < t _Z8multiplyRKSt5arrayIfLm4EERKS_IfLm2EE.arch_znver1.4 < t _Z8multiplyRKSt5arrayIfLm4EERKS_IfLm2EE.arch_znver2.5 < t multiply(std::array<float, 4ul> const&, std::array<float, 2ul> const&) [clone .default.6] --- > T multiply(std::array<float, 4ul> const&, std::array<float, 2ul> const&) > [clone .arch_cascadelake.3] > T _Z8multiplyRKSt5arrayIfLm4EERKS_IfLm2EE.arch_core2.0 > T multiply(std::array<float, 4ul> const&, std::array<float, 2ul> const&) > [clone .arch_haswell.2] > T multiply(std::array<float, 4ul> const&, std::array<float, 2ul> const&) > [clone .arch_sandybridge.1] > T _Z8multiplyRKSt5arrayIfLm4EERKS_IfLm2EE.arch_znver1.4 > T _Z8multiplyRKSt5arrayIfLm4EERKS_IfLm2EE.arch_znver2.5 > T multiply(std::array<float, 4ul> const&, std::array<float, 2ul> const&) > [clone .default.6] > i multiply(std::array<float, 4ul> const&, std::array<float, 2ul> const&) > [clone .ifunc] 20d17 < r std::piecewise_construct ``` is this behavior by design?