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From: "tnfchris at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/98877] [AArch64] Inefficient code generated for tbl NEON
 intrinsics
Date: Thu, 29 Feb 2024 07:26:53 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98877

--- Comment #11 from Tamar Christina <tnfchris at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #10)
> (In reply to Tamar Christina from comment #9)
> > While RA should be able to deal with this,
> > shouldn't we also just lower TBLs in gimple?
> >=20
> > This no reason why this can't be a VEC_PERM_EXPR which would also get t=
he
> > copies
> > removed at the gimple level and allows us to optimize this to something=
 else
> > depending on the index.
>=20
> Yes there is a reason, `out of range` values for VEC_PERM is undefined wh=
ile
> tbl is well defined  ( If an index is out of range for the table, the res=
ult
> for that lookup is 0.).
>=20

I don't think that's not a good reason. The out of range values can be made
implementation defined. i.e. mid-end shouldn't care about them.

not lowering this in gimple means we lose a heck of a lot of optimizations =
that
are impossible to cover in RTL.=