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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/104665] Failure to recognize memcpy
Date: Wed, 23 Feb 2022 22:35:16 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104665

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2022-02-23
           Severity|normal                      |enhancement
     Ever confirmed|0                           |1

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
A couple of reasons.
First is store merging happens too late.
Second reason is store merging does not work in the loop case.

Take:
#if 1
enum b : unsigned char{};
#else
typedef unsigned char b;
#endif

void serialize_le(b* __restrict dst, const unsigned* __restrict src)
{
   // for (int i =3D 0; i < 128; ++i, ++src)
    {
        unsigned t =3D *src;
        *dst++ =3D static_cast<b>((t >>  0) & 0xff);
        *dst++ =3D static_cast<b>((t >>  8) & 0xff);
        *dst++ =3D static_cast<b>((t >> 16) & 0xff);
        *dst++ =3D static_cast<b>((t >> 24) & 0xff);
    }
}

This gets optimized to one load followed by one store. But once you add the
loop, and use -fno-tree-vectorize (because GCC's vectorizer gets kicked in
which causes other issues), the stores are not merged into one.

Also store merging happens way after loop distrubution happens so ...=