https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114966
Bug ID: 114966 Summary: fails to optimize avx2 in-register permute written with std::experimental::simd Product: gcc Version: 14.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: tree-optimization Assignee: unassigned at gcc dot gnu.org Reporter: lee.imple at gmail dot com Target Milestone: --- This is actually another attempt to permute a simd register with std::experimental::simd, like PR114908, but written differently. Following is the same function written in both std::experimental::simd and GNU vector extension versions (available online at https://godbolt.org/z/n3WvqcePo ). The purpose is to permute the register from [w, x, y, z] into [0, w, x, y]. ```c++ #include <experimental/simd> #include <cstdint> namespace stdx = std::experimental; using data_t = std::uint64_t; constexpr std::size_t data_size = 4; template <std::size_t N> using simd_of = std::experimental::simd<data_t, std::experimental::simd_abi::deduce_t<data_t, N>>; using simd_t = simd_of<data_size>; // stdx version simd_t permute_simd(simd_t data) { return simd_t([=](auto i) -> data_t { constexpr size_t index = i - 1; if constexpr (index < data_size) { return data[index]; } else { return 0; } }); } typedef data_t vector_t [[gnu::vector_size(data_size * sizeof(data_t))]]; // gnu vector extension version vector_t permute_vector(vector_t data) { return __builtin_shufflevector(data, vector_t{0}, 4, 0, 1, 2); } ``` The code is compiled with the options `-O3 -march=x86-64-v3 -std=c++20`. Although they should have the same functionality, generated assembly (by GCC) is so different. ```asm permute_simd(std::experimental::parallelism_v2::simd<unsigned long, std::experimental::parallelism_v2::simd_abi::_VecBuiltin<32> >): vmovq %xmm0, %rax vpsrldq $8, %xmm0, %xmm1 vextracti128 $0x1, %ymm0, %xmm0 vpunpcklqdq %xmm0, %xmm1, %xmm1 vpxor %xmm0, %xmm0, %xmm0 vpinsrq $1, %rax, %xmm0, %xmm0 vinserti128 $0x1, %xmm1, %ymm0, %ymm0 ret permute_vector(unsigned long __vector(4)): vpxor %xmm1, %xmm1, %xmm1 vpermq $144, %ymm0, %ymm0 vpblendd $3, %ymm1, %ymm0, %ymm0 ret ``` However, Clang can optimize `permute_simd` into the same assembly as `permute_vector`, so I think, instead of a bug in the std::experimental::simd, it is a missed optimization in GCC. ```asm permute_simd(std::experimental::parallelism_v2::simd<unsigned long, std::experimental::parallelism_v2::simd_abi::_VecBuiltin<32> >): # @permute_simd(std::experimental::parallelism_v2::simd<unsigned long, std::experimental::parallelism_v2::simd_abi::_VecBuiltin<32> >) vpermpd $144, %ymm0, %ymm0 # ymm0 = ymm0[0,0,1,2] vxorps %xmm1, %xmm1, %xmm1 vblendps $3, %ymm1, %ymm0, %ymm0 # ymm0 = ymm1[0,1],ymm0[2,3,4,5,6,7] retq permute_vector(unsigned long __vector(4)): # @permute_vector(unsigned long __vector(4)) vpermpd $144, %ymm0, %ymm0 # ymm0 = ymm0[0,0,1,2] vxorps %xmm1, %xmm1, %xmm1 vblendps $3, %ymm1, %ymm0, %ymm0 # ymm0 = ymm1[0,1],ymm0[2,3,4,5,6,7] retq ```