From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugzilla@gcc.gnu.org>
Received: by sourceware.org (Postfix, from userid 48)
 id 8D49A385802E; Sat,  6 Mar 2021 20:12:59 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 8D49A385802E
From: "unlvsur at live dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/99434] New: std::bit_cast generates more
 instructions than __builtin_bit_cast and memcpy with -march=native
Date: Sat, 06 Mar 2021 20:12:59 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 11.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: unlvsur at live dot com
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: bug_id short_desc product version bug_status
 bug_severity priority component assigned_to reporter target_milestone
Message-ID: <bug-99434-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
X-BeenThere: gcc-bugs@gcc.gnu.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Gcc-bugs mailing list <gcc-bugs.gcc.gnu.org>
List-Unsubscribe: <https://gcc.gnu.org/mailman/options/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=unsubscribe>
List-Archive: <https://gcc.gnu.org/pipermail/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-request@gcc.gnu.org?subject=help>
List-Subscribe: <https://gcc.gnu.org/mailman/listinfo/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=subscribe>
X-List-Received-Date: Sat, 06 Mar 2021 20:12:59 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99434

            Bug ID: 99434
           Summary: std::bit_cast generates more instructions than
                    __builtin_bit_cast and memcpy with -march=3Dnative
           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__ =3D=3D __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=3Dstatic_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.=