From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugs-return-494551-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 40735 invoked by alias); 11 Aug 2015 09:12:15 -0000
Mailing-List: contact gcc-bugs-help@gcc.gnu.org; run by ezmlm
Precedence: bulk
List-Id: <gcc-bugs.gcc.gnu.org>
List-Archive: <http://gcc.gnu.org/ml/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-help@gcc.gnu.org>
Sender: gcc-bugs-owner@gcc.gnu.org
Received: (qmail 40676 invoked by uid 48); 11 Aug 2015 09:12:11 -0000
From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug fortran/67170] PRE can't hoist out a readonly argument
Date: Tue, 11 Aug 2015 09:12:00 -0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: fortran
X-Bugzilla-Version: 6.0
X-Bugzilla-Keywords:
X-Bugzilla-Severity: normal
X-Bugzilla-Who: rguenth at gcc dot gnu.org
X-Bugzilla-Status: NEW
X-Bugzilla-Resolution:
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags:
X-Bugzilla-Changed-Fields: bug_status cf_reconfirmed_on everconfirmed
Message-ID: <bug-67170-4-wJCeAVEZff@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-67170-4@http.gcc.gnu.org/bugzilla/>
References: <bug-67170-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 7bit
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
X-SW-Source: 2015-08/txt/msg00693.txt.bz2

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67170

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2015-08-11
     Ever confirmed|0                           |1

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
Ah.

  # USE = nonlocal escaped { D.3438 }
  # CLB = nonlocal escaped
  foo (&D.3438);

As it clobbers nonlocal it clobbers the incoming argument :/  Because of course
as the argument is pointing to possibly global memory it can access it via
other means than through the argument - and the function attribute in use
(".r") just means accesses through the first argument do not modify the
contents pointed to.

I think Fortran guarantees that things passed as argument are not modified
in other ways (or it makes that undefined).  Not sure about called functions
though.

Now.  In principle I think C says that with

void foo (int * __restrict p)
{
 ...
 bar ();
 ...
}

bar () can only access *p through the use of p or a derived pointer.  So


int x;
bar()
{
  x = 1;
}
main()
{
  foo (&x); 
}

would invoke undefined behavior.

Of course our restrict implementation isn't able to capture that (calls
don't have a clique/base which they would get from the "memory" passed
as reference arguments).

So - confirmed.  But no easy way out :(

Old idea about special-casing recursive calls also exists but without clear
idea of what would be legal special alias answers here.