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From: "rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/98848] [9/10/11 regression] vectorizer failed
 to reduce max pattern since r9-1590
Date: Fri, 29 Jan 2021 07:31:23 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98848

--- Comment #5 from rguenther at suse dot de <rguenther at suse dot de> ---
On Thu, 28 Jan 2021, jakub at gcc dot gnu.org wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98848
>=20
> --- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
> Alternatively, couldn't we support truncation in the reductions if
> SSA_NAME_RANGE_INFO suggests that the values are always in the narrower r=
ange?

Yes, we probably could.  But note that changes in reduction support
are quite fragile and we're currently just set up for sign changes
(via emitting V_C_E) but not required promotions/demotions so
there would be a lot of changes needed.  It's also not clear
promoting/demoting the reduction IV all the time is doing any good
(unless you suggest that we'd magically undo the pattern by promoting
the non-reduction OP instead - but that would require even more changes).

So I guess the better approach might be to somehow allow late "undoing"
of pattern recog (but it's a bit complicated because of how that=20
influences VF compute and also relevant/liveness compute).=