From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugs-return-124209-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 23488 invoked by alias); 28 Dec 2004 15:29:10 -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 23475 invoked by uid 48); 28 Dec 2004 15:29:08 -0000
Date: Tue, 28 Dec 2004 15:29:00 -0000
From: "tob at idlehands dot net" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Message-ID: <20041228152907.19177.tob@idlehands.net>
Reply-To: gcc-bugzilla@gcc.gnu.org
Subject: [Bug c/19177] New: Floating-point error with simple subtraction.
X-Bugzilla-Reason: CC
X-SW-Source: 2004-12/txt/msg03742.txt.bz2
List-Id: <gcc-bugs.sourceware.org>

Here's hoping this doesn't get marked as a duplicate of 323, since the summary contains the keywords 
"floating-point" and "error."  :)

With the following trivial program, which simply does 20 subtractions, a logic error occurs during a 
comparison against the floating point value, believing that o < 0.05 is true when o == 0.05.

This is reproducible on multiple processors.  I've tried it with gcc 3.3 on Mac OS X 10.3 (powerppc g4), 
as well as gcc 3.3 on Redhat 9.0 (i686) and run into the same result.  (For kicks, I did try -ffloat-store, 
as suggested in the 323 thread, but this had no effect).  The problem occurs at all optimization levels I 
tried.

int main()
{
    float o = 1.0;
    while (1)
    {
        printf("o: %f\n", o);
        if (o < 0.05) break;

        o -= 0.05;
    }

    printf("final o: %f\n", o);
    return 0;
}

-- 
           Summary: Floating-point error with simple subtraction.
           Product: gcc
           Version: 3.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: tob at idlehands dot net
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19177