From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugs-return-154880-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 15490 invoked by alias); 18 Aug 2005 23:51:05 -0000
Mailing-List: contact gcc-bugs-help@gcc.gnu.org; run by ezmlm
Precedence: bulk
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 15358 invoked by uid 48); 18 Aug 2005 23:50:40 -0000
Date: Fri, 19 Aug 2005 01:03:00 -0000
From: "pinskia at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Message-ID: <20050818235036.23471.pinskia@gcc.gnu.org>
Reply-To: gcc-bugzilla@gcc.gnu.org
Subject: [Bug tree-optimization/23471] New: a*a (for signed ints with -fno-wrapv) is always postive
X-Bugzilla-Reason: CC
X-SW-Source: 2005-08/txt/msg02159.txt.bz2
List-Id: <gcc-bugs.sourceware.org>

Hmm, this is both a VRP bug and a folding bug.  Even though a is VARYING, we know the that a*a will 
always be postive (this with -fno-wrapv which is default for C, C++ and fortran).
void link_error(void);
void f(int a)
{
  int b = a;
  b*=a;
  if (b < 0)
   link_error();
}

---
There should be no references to link_error().

There is another way of fixing this via the fold:
void link_error(void);
void f(int a)
{
  if (a*a < 0)
   link_error();
}

But that will miss:
void link_error(void);
void f(int a)
{
  if (a*a*a*a*a*a < 0)
   link_error();
}

---
Yes that is most likely to overflow but that is the whole point of -fno-wrapv.

Also ICC does not do this optimization at all.

-- 
           Summary: a*a (for signed ints with -fno-wrapv) is always postive
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P2
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: pinskia at gcc dot gnu dot org
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23471