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From: "amker at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/101145] niter analysis fails for until-wrap
 condition
Date: Sat, 26 Jun 2021 02:53:25 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101145

--- Comment #7 from bin cheng <amker at gcc dot gnu.org> ---
(In reply to Jiu Fu Guo from comment #5)
> (In reply to bin cheng from comment #4)
> > (In reply to Jiu Fu Guo from comment #3)
> > > Yes, while the code in adjust_cond_for_loop_until_wrap seems somehow =
tricky:
> > >=20
> > >   /* Only support simple cases for the moment.  */
> > >   if (TREE_CODE (iv0->base) !=3D INTEGER_CST
> > >       || TREE_CODE (iv1->base) !=3D INTEGER_CST)
> > >     return false;
> > >=20
> > > This code requires both sides are constant.
> > Actually it requires an IV with constant base.
>=20
> I also feel that the intention of this function may only require one side
> constant for IV0 CODE IV1.
> As tests, for below loop, adjust_cond_for_loop_until_wrap return false:
>=20
> foo (int *__restrict__ a, int *__restrict__ b, unsigned i)
> {
>   while (++i > 100)
>     *a++ =3D *b++ + 1;
> }
>=20
> For below code, adjust_cond_for_loop_until_wrap returns true:
>   i =3D UINT_MAX - 200;
>   while (++i > 100)
>     *a++ =3D *b++ + 1;

Oh sorry for being misleading.  When I mentioned it requires something(...)=
, I
was describing the current behavior, not that the conditions are necessary.=
=20
Feel free to improve such cases.  Looking into niter analysis, these
cases(trade-offs) are not rare.

Thanks=