[Bug tree-optimization/99434] New: std::bit_cast generates more instructions than __builtin_bit_cast and memcpy with -march=native

["unlvsur at live dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99434

            Bug ID: 99434
           Summary: std::bit_cast generates more instructions than
                    __builtin_bit_cast and memcpy with -march=native
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: unlvsur at live dot com
  Target Milestone: ---

https://godbolt.org/z/5KWM8Y
struct u64x2_t
{
#if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
    std::uint64_t high,low;
#else
    std::uint64_t low,high;
#endif
};
u64x2_t umul5(std::uint64_t a,std::uint64_t b) noexcept
{
    return std::bit_cast<u64x2_t>(static_cast<__uint128_t>(a)*b);
}

u64x2_t umul_builtin(std::uint64_t a,std::uint64_t b) noexcept
{
    return __builtin_bit_cast(u64x2_t,static_cast<__uint128_t>(a)*b);
}

assembly:
umul5(unsigned long, unsigned long):
        movq    %rdi, %rdx
        mulx    %rsi, %rdx, %rcx
        movq    %rdx, %rax
        movq    %rcx, %rdx
        ret
umul_builtin(unsigned long, unsigned long):
        movq    %rdi, %rdx
        mulx    %rsi, %rax, %rdx
        ret

There is another issue:

std::uint64_t umul128(std::uint64_t a,std::uint64_t b,std::uint64_t& high)
noexcept
{
    __uint128_t res{static_cast<__uint128_t>(a)*b};
    high=static_cast<std::uint64_t>(res>>64);
    return static_cast<std::uint64_t>(res);
}
I cannot do this since this generates more instructions than using memcpy to
pun types.

clang does not have this issue and all cases are dealt with correctly.