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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/99409] s252 benchmark of TSVC is vectorized
 by clang and not by gcc
Date: Mon, 08 Mar 2021 08:23:21 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99409

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
             Blocks|                            |53947
          Component|middle-end                  |tree-optimization

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Yes, we can't do 'scalar expansion'.  We'd need some pre-pass to turn PHIs
into data accesses.  Here we want

        t[0] =3D (real_t) 0.;
        for (int i =3D 0; i < LEN_1D; i++) {
            s =3D b[i] * c[i];
            a[i] =3D s + t[i];
            t[i+1] =3D s;
        }

and then of course the trick is to elide the actual array and instead do
clever shuffling of vector registers instead.

IIRC one of the other TSVC examples was similar.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D53947
[Bug 53947] [meta-bug] vectorizer missed-optimizations=