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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/109088] GCC does not always vectorize
 conditional reduction
Date: Wed, 27 Sep 2023 07:15:42 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109088

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rdapp at gcc dot gnu.org

--- Comment #9 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to JuzheZhong from comment #8)
> It's because the order of the operations we are doing:
>=20
> For code as follows:
>=20
> result +=3D mask ? a[i] + x : 0;
>=20
> GCC:
> result_ssa_1 =3D PHI <result_ssa_2, 0>
> ...
> STMT 1. tmp =3D a[i] + x;
> STMT 2. tmp2 =3D tmp + result_ssa_1;
> STMT 3. result_ssa_2 =3D mask ? tmp2 : result_ssa_1;
>=20
> Here we can see both STMT 2 and STMT 3 are using 'result_ssa_1',
> we end up with 2 uses of the PHI result. Then, we failed to vectorize.
>=20
> Wheras LLVM:
>=20
> result_ssa_1 =3D PHI <result_ssa_2, 0>
> ...
> IR 1. tmp =3D a[i] + x;
> IR 2. tmp2 =3D mask ? tmp : 0;
> IR 3. result_ssa_2 =3D tmp2 + result_ssa_1.

For floating point these are not equivalent (adding zero isn't a no-op).

> LLVM only has 1 use.
>=20
> Is it reasonable to swap the order in match.pd ?

if-conversion could be teached to swap this (it's if-conversion creating
the IL for conditional reductions) when valid.  IIRC Robin Dapp also has
a patch to make if-conversion emit .COND_ADD instead which should make
it even better to vectorize.=