https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96738
Bug ID: 96738 Summary: GCC generates worse assembly than clang and It fails to vectorized code compared to clang Product: gcc Version: 11.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: rtl-optimization Assignee: unassigned at gcc dot gnu.org Reporter: euloanty at live dot com Target Milestone: --- https://godbolt.org/z/9K3369 #include<array> #include<cstdint> struct number { std::array<std::uint64_t,5> num; inline constexpr std::uint64_t& operator[](std::size_t position) noexcept { return num[position]; } inline constexpr std::uint64_t const& operator[](std::size_t position) const noexcept { return num[position]; } }; number add_reduce(number const& a,number const& b) noexcept { constexpr auto reduce_mask_51{(static_cast<std::uint64_t>(1) << 51) - 1}; number out; std::uint64_t c{(a[0] + b[0])>>51}; out[1] = a[1] + b[1] + c; c = (out[1] >> 51); out[1] &= reduce_mask_51; out[2] = a[2] + b[2] + c; c = (out[2] >> 51); out[2] &= reduce_mask_51; out[3] = a[3] + b[3] + c; c = (out[3] >> 51); out[3] &= reduce_mask_51; out[4] = a[4] + b[4] + c; c = (out[4] >> 51); out[4] &= reduce_mask_51; out[0] = c * 19; return out; } gcc: add_reduce(number const&, number const&): movq (%rdx), %rax addq (%rsi), %rax movq %rdi, %r8 movq %rdx, %rdi shrq $51, %rax movq 8(%rdx), %rdx addq 8(%rsi), %rdx movq %rsi, %rcx movabsq $2251799813685247, %rsi addq %rdx, %rax movq %rax, %rdx shrq $51, %rax andq %rsi, %rdx movq %rdx, 8(%r8) movq 16(%rdi), %rdx addq 16(%rcx), %rdx addq %rdx, %rax movq %rax, %rdx shrq $51, %rax andq %rsi, %rdx movq %rdx, 16(%r8) movq 24(%rdi), %rdx addq 24(%rcx), %rdx addq %rax, %rdx movq %rdx, %rax shrq $51, %rdx andq %rsi, %rax movq %rax, 24(%r8) movq 32(%rdi), %rax addq 32(%rcx), %rax addq %rdx, %rax andq %rax, %rsi shrq $51, %rax leaq (%rax,%rax,8), %rdx movq %rsi, 32(%r8) leaq (%rax,%rdx,2), %rax movq %rax, (%r8) movq %r8, %rax ret clang: add_reduce(number const&, number const&): # @add_reduce(number const&, number const&) movq %rdi, %rax movq (%rdx), %rcx movq 8(%rdx), %rdi addq (%rsi), %rcx shrq $51, %rcx addq 8(%rsi), %rdi addq %rcx, %rdi movq %rdi, %rcx shrq $51, %rcx movabsq $2251799813685247, %r8 # imm = 0x7FFFFFFFFFFFF andq %r8, %rdi movq %rdi, 8(%rax) movq 16(%rdx), %rdi addq 16(%rsi), %rdi addq %rcx, %rdi movq %rdi, %rcx shrq $51, %rcx andq %r8, %rdi movq %rdi, 16(%rax) movq 24(%rdx), %rdi addq 24(%rsi), %rdi addq %rcx, %rdi movq %rdi, %rcx andq %r8, %rdi movq %rdi, 24(%rax) movq 32(%rdx), %rdx addq 32(%rsi), %rdx shrq $51, %rcx addq %rcx, %rdx movq %rdx, %rcx shrq $51, %rcx andq %r8, %rdx movq %rdx, 32(%rax) leaq (%rcx,%rcx,8), %rdx leaq (%rcx,%rdx,2), %rcx movq %rcx, (%rax) retq clang with -march=native .LCPI0_0: .quad 2251799813685247 add_reduce(number const&, number const&): # @add_reduce(number const&, number const&) movq %rdi, %rax movq (%rdx), %rcx movq 8(%rdx), %rdi addq (%rsi), %rcx shrq $51, %rcx addq 8(%rsi), %rdi addq %rcx, %rdi vmovq %rdi, %xmm0 shrq $51, %rdi movq 16(%rdx), %rcx addq 16(%rsi), %rcx addq %rdi, %rcx vmovq %rcx, %xmm1 shrq $51, %rcx movq 24(%rdx), %rdi addq 24(%rsi), %rdi addq %rcx, %rdi vmovq %rdi, %xmm2 shrq $51, %rdi movq 32(%rdx), %rcx addq 32(%rsi), %rcx addq %rdi, %rcx vpunpcklqdq %xmm1, %xmm0, %xmm0 # xmm0 = xmm0[0],xmm1[0] vmovq %rcx, %xmm1 vpunpcklqdq %xmm1, %xmm2, %xmm1 # xmm1 = xmm2[0],xmm1[0] vinserti128 $1, %xmm1, %ymm0, %ymm0 vpandq .LCPI0_0(%rip){1to4}, %ymm0, %ymm0 shrq $51, %rcx vmovdqu %ymm0, 8(%rax) leaq (%rcx,%rcx,8), %rdx leaq (%rcx,%rdx,2), %rcx movq %rcx, (%rax) vzeroupper retq