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From: "unlvsur at live dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
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 [thread overview]
Message-ID: <bug-99434-4-MTVJ0imGQx@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-99434-4@http.gcc.gnu.org/bugzilla/>
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99434
--- 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);
> }
>
> struct u64x2_t
> {
> #if __BYTE_ORDER__ == __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);
> }
>
> 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=static_cast<std::uint64_t>(res>>64);
return static_cast<std::uint64_t>(res);
}
prev parent reply other threads:[~2021-03-08 20:40 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-06 20:12 [Bug tree-optimization/99434] New: " unlvsur at live dot com
2021-03-06 21:25 ` [Bug target/99434] " pinskia at gcc dot gnu.org
2021-03-06 21:36 ` unlvsur at live dot com
2021-03-06 22:50 ` pinskia at gcc dot gnu.org
2021-03-08 16:49 ` jakub at gcc dot gnu.org
2021-03-08 16:50 ` jakub at gcc dot gnu.org
2021-03-08 20:40 ` unlvsur at live dot com [this message]
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