Re: c/8395: gcc 2.95.4 and 3.2 generate wrong code for double onintel

[Marco Bernardo <bernardo at sti dot uniurb dot it>]

The following reply was made to PR c/8395; it has been noted by GNATS.

From: Marco Bernardo <bernardo@sti.uniurb.it>
To: Bruce Allen <ballen@aei.mpg.de>
Cc: gcc-gnats@gcc.gnu.org, <gcc-prs@gcc.gnu.org>,
	<gcc-bugs@gcc.gnu.org>, <nobody@gcc.gnu.org>
Subject: Re: c/8395: gcc 2.95.4 and 3.2 generate wrong code for double on
 intel
Date: Thu, 31 Oct 2002 09:39:56 +0100 (CET)

 Dear Bruce,
 
 Thanks for your message.
 
 >There's nothing wrong here.  It's very reasonable for this code to
 >produce ~10^-16 for double.
 >The reason is tha the number 1.2 can not be exactly represented as an
 >IEEE754 floating point number.
 >The numbers 5 and -6 CAN be exactly represented.
 
 I can understand that the problem is related to the IEEE 754
 representation.
 
 However, let me point out some weaknesses in your argument:
 
 1. If -1.2 does not have an exact representation,
    why is the value of variable y (i.e. -1.2)
    correctly displayed through printf?
 
 2. Let us consider the following variant of the program
    I attached to my report:
       #include <stdio.h>
       int main(void)
       {
 	      double x, y, z, y_times_z;
 
 	      x = -6.0;
 	      y = -1.2;
 	      z = 5;
 	      y_times_z = y * z;
 	      printf("%g %g %g %g %g %g\n",
 	             x,
 	             y,
 	             z,
 	             x - y * z,
 	             y_times_z,
 	             x - y_times_z);
 	      return(0);
       }
    With respect to the previous version, I added
    variable y_times_z together with the visualization
    of y_times_z as well as x - y_times_z.
    Surprisingly enough, the output is
       -6 -1.2 5 -2.22045e-16 -6 0
    i.e. the correct values are somehow restored.
    Now, passing through an additional variable
    like y_times_z is how the target code produced
    by gcc should be organized, isn't it?
    If so, then both x - y * z and x - y_times_z
    should evaluate to 0.
    Since this is not the case, gcc contains a bug
    in the way it translates the arithmetical expressions
    when doubles are involved.
 
 In conclusion, I still believe that gcc contains a serious bug
 on the intel platform, which should hopefully be fixed asap.
 
 If the bug is fixed, or you or someone else reading this message
 is able to point out some flaw in my argument, then my colleagues,
 my students, and I will be glad to keep using gcc.
 
 Best regards,
 		Marco
 
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 Prof. Marco Bernardo
 
 Universita` di Urbino
 Centro per l'Applicazione delle Scienze e Tecnologie dell'Informazione
 Piazza della Repubblica 13, 61029 Urbino, Italy
 
 Phone: +39-0722-4475    -  E-mail: bernardo@sti.uniurb.it
 Fax:   +39-0722-4475    -  WWW:    http://www.sti.uniurb.it/bernardo/
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