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From: "tnfchris at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/103417] [12 Regression] wrong code at -O1 and
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Date: Thu, 25 Nov 2021 02:44:59 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103417

--- Comment #5 from Tamar Christina <tnfchris at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #4)
> Created attachment 51870 [details]
> gcc12-pr103417.patch
>=20
> Untested fix.  Handling GE in that simplification is clearly bogus, we
> should just fold it to true elsewhere, not bother with it (it doesn't han=
dle
> LT either,
> which should also fold to false elsewhere).

Indeed, that one is wrong..

> Handling LE and GT there isn't wrong, but makes no sense.  Elsewhere we
> canonicalize x > 0U into x !=3D 0U and x <=3D 0U into x =3D=3D 0U and for=
 signed it
> was handling only EQ and NE already before.

Well, the intention is to simplify the bitmask. Most vector ISAs can create=
 the
simple bitmask much easier than the complex one. i.e. 0xFFFFFF00 is much ha=
rder
to create than 0xFF. for scalar yes it doesn't matter much.

but e.g.

     for (int i =3D 0; i < (n & -16); i++)
       x[i] =3D (x[i]&(~255)) <=3D 0U;

generates worse code when the mask 0xFFFFFF00 is to be used.  The patch is
mainly addressing vector code but we added scalar for uniformity.

So I would like LE and GT to stay, at the very least for vector where it ma=
kes
a difference.  It's not something we can fix in the backend because we can't
differentiate between signed and unsigned.=