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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/105968] GCC vectorizes but reports that it did not vectorize
Date: Tue, 14 Jun 2022 10:22:07 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105968

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |WONTFIX

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
> ./cc1 -quiet t.c -O3 -mavx2 -fopt-info
t.c:11:25: optimized: loops interchanged in loop nest
> ./cc1 -quiet t.c -O2 -mavx2 -fopt-info
t.c:14:19: optimized: loop vectorized using 32 byte vectors

so we interchange the loop to

    for (i =3D 0; i < N; ++i)
      for (times =3D 0; times < NTIMES; times++)
        r[i] =3D (a[i] + b[i]) * c[i];

which is indeed good for memory locality (now, we should then eliminate
the inner loop completely but we have no such facility - only unrolling
and DSE/DCE would do this but nothing on the high-level loop form).

"Benchmark" issue.  The outer loop should have a memory clobber.

Oh, and we should in theory be able to vectorize the outer loop if
N is a multiple of the vector element count.  But:

t.c:11:25: note:   =3D=3D=3D vect_analyze_data_ref_accesses =3D=3D=3D
t.c:11:25: note:   zero step in inner loop of nest
t.c:11:25: missed:   not vectorized: complicated access pattern.
t.c:15:14: missed:   not vectorized: complicated access pattern.
t.c:11:25: missed:  bad data access.

so we don't handle this exact issue (maybe the offending check can
simply be elided - assuming dependence checking handles zero steps
correctly).

Putting

    __asm__ volatile ("" : : : "memory");

at the end of the outer loop vectorizes with -O3 as well (but doesn't
interchange).

Not a bug I think unless you want to make it a bug about not vectorizing
the outer loop after interchange.=