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From: "qing.zhao at oracle dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/104550] bogus warning from -Wuninitialized +
 -ftrivial-auto-var-init=pattern
Date: Thu, 17 Feb 2022 16:47:13 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104550

--- Comment #14 from Qing Zhao <qing.zhao at oracle dot com> ---
> On Feb 17, 2022, at 1:54 AM, rguenther at suse dot de <gcc-bugzilla@gcc.g=
nu.org> wrote:
>>=20
>> If padding clearing is exposed too late till RTL expansion, some tree
>> optimization will not be able to be applied on the expanded stores?=20
>=20
> Doesn't the same argument apply to .DEFERRED_INIT itself?  Dependent
> on the .DEFERRED_INIT expansion strathegy the padding clearing might
> be unneccessary (for example when using memset())?

Yes, because we use memset to initialize auto-var when it=E2=80=99s in memo=
ry, we do
not insert a call to
__builtin_clear_padding to zero initialization.  We only insert
__builtin_clear_padding to pattern
Initialization to set the padding to zeroes. That=E2=80=99s why this bug on=
ly exposed
with pattern init.
>=20
>> the approach to mark the load "MEM" as not needing a warning might be be=
tter?
>=20
> It's probably a good thing anyway, the 'R' in the RMW cycle isn't really
> a use.
If it=E2=80=99s not a real use, should we exclude this case from emitting t=
he
uninitialized warning directly?=