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From: "rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug rtl-optimization/53533] [10/11/12/13 regression] vectorization
 causes loop unrolling test slowdown as measured by Adobe's C++Benchmark
Date: Mon, 30 May 2022 09:14:44 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D53533

--- Comment #48 from rguenther at suse dot de <rguenther at suse dot de> ---
On Mon, 30 May 2022, crazylht at gmail dot com wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D53533
>=20
> --- Comment #47 from Hongtao.liu <crazylht at gmail dot com> ---
>=20
> >=20
> > The issue is that the re-association pass doesn't handle operations
> > with undefined overflow behavior, we do have duplicate bugreports
> > for this.
> >=20
>=20
> I saw below in match.pd
>=20
>  478/* Combine successive multiplications.  Similar to above, but handling
>  479   overflow is different.  */
>  480(simplify
>  481 (mult (mult @0 INTEGER_CST@1) INTEGER_CST@2)
>  482 (with {
>  483   wi::overflow_type overflow;
>  484   wide_int mul =3D wi::mul (wi::to_wide (@1), wi::to_wide (@2),
>  485                           TYPE_SIGN (type), &overflow);
>  486  }
>  487  /* Skip folding on overflow: the only special case is @1 * @2 =3D=3D
> -INT_MIN,
>  488     otherwise undefined overflow implies that @0 must be zero.  */
>  489  (if (!overflow || TYPE_OVERFLOW_WRAPS (type))
>  490   (mult @0 { wide_int_to_tree (type, mul); }))))
>=20
> Can it be extend to (mult (plus_minus (mult @0 INTEGER_CST@1) INTEGER_CST=
@3)
> INTEGER_CST@2), so at least we can handle it under -fwrapv?

With -fwrapv the reassoc pass might do this already (not sure with
mixing multiplication and addition, you'd have to try).  But sure,
we could add a pattern for the above (with appropriate single-use
handling).=