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From: "qing.zhao at oracle dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/105539] -ftrivial-auto-var-init=zero happening too
 late?
Date: Wed, 11 May 2022 14:09:17 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105539

--- Comment #9 from Qing Zhao <qing.zhao at oracle dot com> ---
> It's a conditional uninit use which we do not warn on early by design
> (for the fear of too many false positives, that is).
Okay.=20
>=20
>> 2. the ccp optimization deletes the if (z) statement completely:
>>=20
>> [opc@qinzhao-aarch64-ol8 105539]$ cat t.c.034t.ccp1
>>=20
>> ;; Function x (x, funcdef_no=3D0, decl_uid=3D3591, cgraph_uid=3D1, symbo=
l_order=3D0)
>>=20
>> Removing basic block 3
>> Merging blocks 2 and 4
>> int x (int z)
>> {
>>  int y;
>>=20
>>  <bb 2> :
>>  return 10;
>>=20
>> }
>=20
> That's expected from optimistic lattice propagation which merges TOP with
> the constant.

If y is initialized to 0 at declaration site, the =E2=80=9Cif z=E2=80=9D st=
atement is NOT
deleted by CCP.=20

However, per design of -ftrivial-auto-var-init, we intent to not treat

y =3D .DEFERRED_INIT (4, 2, &"y"[0]);
Same as
y=3D 0=20

In order to keep -Wuninitialized warning analysis working as expected.

So, yes, I agree that this issue is unavoided based on the current design.=
=20

>=20
>> are the above two bugs?
>=20
> Well, it works as designed.  But sure, that we fail to diagnose the uninit
> use is unfortunate and there exist bugs for this specific issue already.=