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From: "unlvsur at live dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/99434] std::bit_cast generates more instructions than
 __builtin_bit_cast and memcpy with -march=native
Date: Mon, 08 Mar 2021 20:40:32 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99434

--- Comment #6 from cqwrteur <unlvsur at live dot com> ---
(In reply to Jakub Jelinek from comment #5)
> Testcase without includes:
> template<typename _To, typename _From>
> constexpr _To
> bit_cast(const _From& __from) noexcept
> {
>   return __builtin_bit_cast(_To, __from);
> }
>=20
> struct u64x2_t
> {
> #if __BYTE_ORDER__ =3D=3D __ORDER_BIG_ENDIAN__
>   unsigned long long high,low;
> #else
>   unsigned long long low,high;
> #endif
> };
> u64x2_t umul5(unsigned long long a,unsigned long long b) noexcept
> {
>   return bit_cast<u64x2_t>(static_cast<__uint128_t>(a)*b);
> }
>=20
> u64x2_t umul_builtin(unsigned long long a,unsigned long long b) noexcept
> {
>   return __builtin_bit_cast(u64x2_t,static_cast<__uint128_t>(a)*b);
> }

did you fix the >>64 bug here? should I start another bug to report that?

The bit_cast/memcpy trick is to deal with the issue of. Probably can be
recognized early on than std::bit_cast?

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=3Dstatic_cast<std::uint64_t>(res>>64);
    return static_cast<std::uint64_t>(res);
}=