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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/99394] s254 benchmark of TSVC is vectorized by clang
 and not by gcc
Date: Fri, 05 Mar 2021 08:20:39 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99394

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
This is a loop-carried data dependence which we can't handle (we avoid crea=
ting
those from PRE but here it appears in the source itself).  I wonder how
LLVM handles this (pre/post vectorization IL).

Specifically 'carry around variable' is something we don't handle.

Can you somehow extract a compilable testcase (with just this kernel)?

Looking at the source peeling a single iteration (to get rid of the initial
value) and then undoing the PRE, vectorizing

        for (int i =3D 1; i < LEN_1D; i++) {
            a[i] =3D (b[i] + b[i-1]) * (real_t).5;
        }

would likely result in optimal code.  The assembly from clang doesn't look
optimal to me - llvm likely materializes 'x' as temporary array, vectorizing

  x[0] =3D b[LEN_1D-1];
        for (int i =3D 0; i < LEN_1D; i++) {
            a[i] =3D (b[i] + x[i]) * (real_t).5;
            x[i+1] =3D b[i];
        }

and then somehow (like we handle OMP simd lane arrays?) uses two vectors
as a sliding window over x[].  At least the standard strathegy for
these kind of dependences is to get "rid" of them by making them data
dependences and then hope for the best.=