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From: "unlvsur at live dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/103565] New: GCC emits more assembly than
 clang for carry flag
Date: Sun, 05 Dec 2021 15:41:42 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103565

            Bug ID: 103565
           Summary: GCC emits more assembly than clang for carry flag
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: unlvsur at live dot com
  Target Milestone: ---

I tried both examples. One emulation with the pattern. Another is with the =
x86
intrinsic. GCC emits more instructions than clang.

https://godbolt.org/z/d15WEY85T
https://godbolt.org/z/cob36P8nz

Also can GCC be able to understand the pattern of add_carry_pattern just li=
ke
it understands std::rotl?=20
https://github.com/gcc-mirror/gcc/blob/8d4ef2299cbf9517877dab60d48f34835758=
a6ee/libstdc%2B%2B-v3/include/std/bit#L135

template<typename T>
inline constexpr bool add_carry_pattern(bool carry,T a,T b,T& out) noexcept
{
        T temp{carry+a};
        out=3Dtemp+b;
        return (out < b) | (temp < a);
}

So we do not need that intrinsic anymore and the whole thing can be optimiz=
ed
at SSA level, rather than RTL level??=