From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugzilla@gcc.gnu.org>
Received: by sourceware.org (Postfix, from userid 48)
 id D5C9638708FF; Tue,  2 Jun 2020 12:00:28 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org D5C9638708FF
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
 s=default; t=1591099228;
 bh=UlIpsh8b9DeNuSxGlWCTmQmLjvsvIArSkOIK3XcA/uA=;
 h=From:To:Subject:Date:In-Reply-To:References:From;
 b=PFrpthm6q1ha3XNvFouKrSF51swpoJHmQo/fO0fg9qp/tS0YD+lIwaZIGR69xyhCJ
 A1YFYnCT6a+itawA1oiF+ayvyXr4zAvJR7v8kMHUqd43e2+7oUMlC2qMAg005jPA3H
 pnOC9KvWMlDWc72pyu+1dnd87eMgizmQiQqypLG0=
From: "rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/95237] LOCAL_DECL_ALIGNMENT shrinks alignment, FAIL
 gcc.target/i386/pr69454-2.c
Date: Tue, 02 Jun 2020 12:00:28 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: target
X-Bugzilla-Version: 11.0
X-Bugzilla-Keywords: ice-on-valid-code, wrong-code
X-Bugzilla-Severity: normal
X-Bugzilla-Who: rguenther at suse dot de
X-Bugzilla-Status: NEW
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: skpgkp2 at gmail dot com
X-Bugzilla-Target-Milestone: 11.0
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: 
Message-ID: <bug-95237-4-mrwuLGn6G1@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-95237-4@http.gcc.gnu.org/bugzilla/>
References: <bug-95237-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
X-BeenThere: gcc-bugs@gcc.gnu.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Gcc-bugs mailing list <gcc-bugs.gcc.gnu.org>
List-Unsubscribe: <http://gcc.gnu.org/mailman/options/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=unsubscribe>
List-Archive: <https://gcc.gnu.org/pipermail/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-request@gcc.gnu.org?subject=help>
List-Subscribe: <http://gcc.gnu.org/mailman/listinfo/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=subscribe>
X-List-Received-Date: Tue, 02 Jun 2020 12:00:28 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95237

--- Comment #9 from rguenther at suse dot de <rguenther at suse dot de> ---
On Tue, 2 Jun 2020, hjl.tools at gmail dot com wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95237
>=20
> --- Comment #8 from H.J. Lu <hjl.tools at gmail dot com> ---
> (In reply to Kito Cheng from comment #6)
> > Created attachment 48658 [details]
> > i386-Implement-ROUND_TYPE_ALIGN-to-make-sure-alignme.patch
> >=20
> > Some optimization might made decision depend on the alignment, and alig=
nment
> > shrinking might made the decision become wrong.
>=20
> Nothing will happen on i386 if alignment of long long isn't 8 bytes.

GIMPLE passes see DECL_ALIGN and can for example elide runtime checks
like if(_Alignof(var) !=3D 8) which is bogus when the actual alignment
is not ensured.  For this reason we cannot permit a larger aligment
early and resort to a lower one later.

> > So I prefer keep the assertion checking and implement ROUND_TYPE_ALIGN =
for
> > x86, so that it will set the alignment properly from the beginning.
> >=20
> > PoC/untested  patch attached.
>=20
> The i386 psABI specifies 4 byte alignment for long long.  But we want to
> use 8 byte alignment if there is no ABI implication and no stack realignm=
ent
> is needed.  Will ROUND_TYPE_ALIGN always align long long to 8 bytes?

So what we want here is RTL expansion pad out stack slots if they know
to be aligned but not adjust DECL_ALIGN.  Not sure if there's already
a target hook to pad out variables, if not this may be the way to get
what you desire here.

I guess the actual reason is crossing of cache-lines?=