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From: "rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/113372] wrong code with _BitInt() arithmetics
 at -O1
Date: Mon, 15 Jan 2024 11:31:11 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113372

--- Comment #5 from rguenther at suse dot de <rguenther at suse dot de> ---
On Mon, 15 Jan 2024, jakub at gcc dot gnu.org wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113372
>=20
> Jakub Jelinek <jakub at gcc dot gnu.org> changed:
>=20
>            What    |Removed                     |Added
> -------------------------------------------------------------------------=
---
>                  CC|                            |rguenth at gcc dot gnu.o=
rg
>=20
> --- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
> Seems this is invalid variable sharing during RTL, I'm afraid we have ton=
s of
> open bugs against that, and I'm afraid I don't know anything what to do o=
n the
> bitint lowering side.
> Bitint lowering creates 3 large variables:
>   unsigned long bitint.8[75];
>   unsigned long bitint.7[75];
>   unsigned long bitint.6[100];
> where bitint.6 corresponds to
>   _3 =3D _2 % y_14(D);
> and later when _3 isn't needed anymore to
>   _8 =3D -y_14(D);
> bitint.7 corresponds to
>   _4 =3D -_3;
>   _5 =3D (unsigned _BitInt(4745)) _4;
> and bitint.8 corresponds to
>   _7 =3D _5 * _6;
> Reusing the same variable bitint.6 for 2 different _BitInt(6384) SSA_NAMEs
> ought to be fine, they aren't live at the same time.
> After lowering, we end up with:
>   .DIVMODBITINT (0B, 0, &bitint.6, 6384, &D.2796, -8, &y, -6384);
>   // Above fills in bitint.6 array
> ...
>   loop which uses bitint.6 to compute bitint.7
>   bitint.6 =3D{v} {CLOBBER(eos)};
> ...
>   .MULBITINT (&bitint.8, 4745, &bitint.7, 4745, &D.2795, -8);
>   // Above fills in bitint.8 array
> ...
>   loop to compute bitint.6
> ...
>   loop which uses bitint.6
> ...
>   _21 =3D MEM[(unsigned long *)&bitint.8];
>   _22 =3D (_BitInt(8)) _21;
>   _18 =3D _22;
>   return _18;
>   // I.e. use bitint.8 for the return value
>=20
> But unfortunately RTL expansion decides to use the same underlying memory=
 for
> bitint.6 and bitint.8 arrays, even when the second uses of bitint.6 happe=
ns
> when bitint.8 is live.
>=20
> Or am I using incorrect CLOBBER kind when an underlying variable is used =
for
> more than one SSA_NAME?  Like should that be just eob rather than eos?

I think it's OK, but likely the address-taken of bitint.8 somehow breaks
things.  Does RTL expansion properly handle internal-functions here?=