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From: "rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/106075] Wrong DSE with -fnon-call-exceptions
Date: Mon, 08 May 2023 13:49:50 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106075

--- Comment #12 from rguenther at suse dot de <rguenther at suse dot de> ---
On Mon, 8 May 2023, ebotcazou at gcc dot gnu.org wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106075
>=20
> --- Comment #11 from Eric Botcazou <ebotcazou at gcc dot gnu.org> ---
> > Eric, maybe you can help with RTL DSE here?
>=20
> The problem is that add_non_frame_wild_read is exclusive, i.e. the read is
> turned into a read from *only* non-frame memory, but can_throw_external
> is conservative and may return true for a read from frame memory:
>=20
> (insn 15 13 16 4 (set (reg:QI 87 [ _20 ])
>         (mem:QI (plus:DI (reg/f:DI 100)
>                 (reg:DI 92 [ ivtmp.23 ])) [8 MEM <character>
> [(interfaces__unsigned_32 *)&value.4 + ivtmp.23_39 * 1]+0 S1 A8]))
> "opt47.adb":22:13 91 {*movqi_internal}
>=20
> value.4 is a local variable but can_throw_external return true on the rea=
d.

Ah - interesting.  I wonder how the CALL handling is correct in
using add_non_frame_wild_read then for the case we pass the address
of a frame variable to it.

So I suppose add_wild_read () would do the trick.

> So you would need to implement an inclusive version of add_non_frame_wild=
_read
> but the change is going to pessimize Ada and Go if the condition to invok=
e it
> is not made more precise.

So better would be to include the original effects of the insn _plus_
the non-frame wild read effect.

I'm defering this for now, the problem went unnoticed for long, so ...=