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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/113441] [14 Regression] Fail to fold the last
 element with multiple loop since g:2efe3a7de0107618397264017fb045f237764cc7
Date: Fri, 01 Mar 2024 09:44:41 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113441

--- Comment #32 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Richard Sandiford from comment #31)
> (In reply to Tamar Christina from comment #29)
> > This works fine for normal gather and scatters but doesn't work for wid=
ening
> > gathers and narrowing scatters which only the pattern seems to handle.
> I'm supposedly on holiday, so didn't see the IRC discussion, but: as I
> remember it, there is no narrowing or widening for IFN gathers or scatters
> as such, even for patterns.  One vector's worth of offsets corresponds to
> one vector's worth of data.  But the widths of the data elements and the
> offset elements can be different.  Any sign or zero extension of a loaded
> vector, or any operation to double or halve the number of vectors, is done
> separately.

Yep.  The emulated gather/scatter and builtin paths do this widening/shorte=
ning
of the offset operand to what we expect on-the-fly.  This support is missing
from the IFN path which relies on patterns doing this.

Having widening/shortening explicitly represented is of course better but
using patterns for this has the unfortunate all-or-nothing effect (right no=
w).

I do hope with SLP only, where it's easier to insert/remove "stmts", we can
delay "pattern recognition" in these cases eventually even up to
vectorizable_* which would "simply" insert a widening/shortening operation
into the SLP graph to make itself happy.

In the mean time I think making the IFN path work also the same way as
emuated/builtin would make sense.  It's already half-way there.=