[Bug middle-end/36578] New: cast to long double not taken into account when result stored to a double

[Bug middle-end/36578] New: cast to long double not taken into account when result stored to a double

[vincent at vinc17 dot org]

The following program shows that the casts to long double are not taken into
account when the result to stored to a variable of type double. This bug occurs
with gcc from 3.4 to 4.3.1, but didn't occur with gcc 3.3.

The bug can be reproduced on a Linux/x86_64 machine (i.e. where the "double"
type corresponds to the IEEE-754 double precision and the "long double" type
corresponds to the traditional x86 extended precision) with the arguments:
  4294967219 4294967429

(these arguments allow the double rounding effect to be visible).

#include <stdio.h>
#include <stdlib.h>

int main (int argc, char **argv)
{
  double a, b, c, d, e;
  long double al, bl, dl, el;

  if (argc != 3)
    exit (1);

  a = atof (argv[1]);
  b = atof (argv[2]);
  al = a;
  bl = b;

  c = a * b;
  d = (long double) a * (long double) b;
  e = al * bl;
  dl = (long double) a * (long double) b;
  el = al * bl;

  printf ("a  =  %.0f\n", a);
  printf ("b  =  %.0f\n", b);
  printf ("c  =  %.0f\n", c);
  printf ("d  =  %.0f\n", d);
  printf ("e  =  %.0f\n", e);
  printf ("dl =  %.0Lf\n", dl);
  printf ("el =  %.0Lf\n", el);

  return 0;
}

Incorrect result (with gcc 3.4 to gcc 4.3.1):
a  =  4294967219
b  =  4294967429
c  =  18446744314227707904
d  =  18446744314227707904
e  =  18446744314227712000
dl =  18446744314227709952
el =  18446744314227709952

Correct result (as given by gcc 3.3) is the same except:
d  =  18446744314227712000

Note: I compiled with the options -std=c99 -Wall -pedantic.


-- 
           Summary: cast to long double not taken into account when result
                    stored to a double
           Product: gcc
           Version: 4.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: vincent at vinc17 dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36578

[Bug middle-end/36578] cast to long double not taken into account when result stored to a double

[vincent at vinc17 dot org]

------- Comment #1 from vincent at vinc17 dot org  2008-06-19 14:37 -------
To make things clear, perhaps I should have added:

#if __STDC__ == 1 && __STDC_VERSION__ >= 199901 && defined(__STDC_IEC_559__)
#pragma STDC FP_CONTRACT OFF
  printf ("__STDC_IEC_559__ defined:\n"
          "The implementation shall conform to the IEEE-754 standard.\n"
          "FLT_EVAL_METHOD is %d (see ISO/IEC 9899, 5.2.4.2.2#7).\n\n",
          (int) FLT_EVAL_METHOD);
#endif

which outputs:

__STDC_IEC_559__ defined:
The implementation shall conform to the IEEE-754 standard.
FLT_EVAL_METHOD is 0 (see ISO/IEC 9899, 5.2.4.2.2#7).

So, one can't even say that the value of d is correct because it is the same as
the exact result rounded to double precision. The fact that the double rounding
doesn't occur is a bug here.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36578

[Bug middle-end/36578] cast to long double not taken into account when result stored to a double

[rguenth at gcc dot gnu dot org]

------- Comment #2 from rguenth at gcc dot gnu dot org  2008-06-21 16:41 -------
Testcase

double foo (double x, double y)
{
  return (long double)x * (long double)y;
}

where we fold the multiplication to x * y.  This is only ok with
-funsafe-math-optimizations (like any other conversions removing
FP operations).  It is of course also ok if we for some reason
know that x * y is exactly representable in both double and
long double.

It is convert_to_real doing this optimization.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rguenth at gcc dot gnu dot
                   |                            |org
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
           Keywords|                            |wrong-code
   Last reconfirmed|0000-00-00 00:00:00         |2008-06-21 16:41:36
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36578

[Bug middle-end/36578] cast to long double not taken into account when result stored to a double

[joseph at codesourcery dot com]

------- Comment #3 from joseph at codesourcery dot com  2008-06-21 16:58 -------
Subject: Re:  cast to long double not taken into account
 when result stored to a double

On Sat, 21 Jun 2008, rguenth at gcc dot gnu dot org wrote:

> It is convert_to_real doing this optimization.

convert_to_real is also responsible for the problematic conversions in 
bug 35202.  I didn't try to argue the double rounding issues for those 
conversions, only the errno issue, since I don't have an example for the 
double rounding affecting those function call conversions (and right now 
implementations of many of the functions will not typically be correctly 
rounding - but that could change if C1x follows the new IEEE 754 
revision's recommendation of many more correctly rounded functions).  But 
it is potentially an issue for the conversions of function calls as well 
as the plain arithmetic example of this bug.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36578

[Bug middle-end/36578] cast to long double not taken into account when result stored to a double

[jsm28 at gcc dot gnu dot org]

------- Comment #4 from jsm28 at gcc dot gnu dot org  2008-10-29 17:06 -------
Subject: Bug 36578

Author: jsm28
Date: Wed Oct 29 17:05:42 2008
New Revision: 141432

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=141432
Log:
        PR middle-end/36578
        * convert.c (convert_to_real): Do not optimize conversions of
        binary arithmetic operations between binary and decimal
        floating-point types.  Consider mode of target type in determining
        decimal type for arithmetic.  Unless
        flag_unsafe_math_optimizations, do not optimize binary conversions
        where this may change rounding behavior.
        * real.c (real_can_shorten_arithmetic): New.
        * real.h (real_can_shorten_arithmetic): Declare.

testsuite:
        * gcc.dg/dfp/convert-bfp-13.c, gcc.dg/dfp/convert-bfp-14.c,
        gcc.dg/dfp/convert-dfp-fold-2.c, gcc.target/i386/pr36578-1.c,
        gcc.target/i386/pr36578-2.c: New tests.

Added:
    trunk/gcc/testsuite/gcc.dg/dfp/convert-bfp-13.c
    trunk/gcc/testsuite/gcc.dg/dfp/convert-bfp-14.c
    trunk/gcc/testsuite/gcc.dg/dfp/convert-dfp-fold-2.c
    trunk/gcc/testsuite/gcc.target/i386/pr36578-1.c
    trunk/gcc/testsuite/gcc.target/i386/pr36578-2.c
Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/convert.c
    trunk/gcc/real.c
    trunk/gcc/real.h
    trunk/gcc/testsuite/ChangeLog


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36578

[Bug middle-end/36578] cast to long double not taken into account when result stored to a double

[jsm28 at gcc dot gnu dot org]

------- Comment #5 from jsm28 at gcc dot gnu dot org  2008-10-29 17:12 -------
Fixed for 4.4.


-- 

jsm28 at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED
   Target Milestone|---                         |4.4.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36578

[Bug middle-end/36578] cast to long double not taken into account when result stored to a double

[janis at gcc dot gnu dot org]

------- Comment #6 from janis at gcc dot gnu dot org  2009-02-25 22:09 -------
Subject: Bug 36578

Author: janis
Date: Wed Feb 25 22:08:55 2009
New Revision: 144436

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=144436
Log:
2009-02-25  Janis Johnson  <janis187@us.ibm.com>

gcc/
        Backport from mainline:
        2008-10-29  Joseph Myers  <joseph@codesourcery.com>

        PR middle-end/36578
        * convert.c (convert_to_real): Do not optimize conversions of
        binary arithmetic operations between binary and decimal
        floating-point types.  Consider mode of target type in determining
        decimal type for arithmetic.  Unless
        flag_unsafe_math_optimizations, do not optimize binary conversions
        where this may change rounding behavior.
        * real.c (real_can_shorten_arithmetic): New.
        * real.h (real_can_shorten_arithmetic): Declare.

gcc/testsuite/
        Backport from mainline:
        2008-10-29  Joseph Myers  <joseph@codesourcery.com>

        PR middle-end/36578
        * gcc.dg/dfp/convert-bfp-13.c, gcc.dg/dfp/convert-bfp-14.c,
        gcc.dg/dfp/convert-dfp-fold-2.c, gcc.target/i386/pr36578-1.c,
        gcc.target/i386/pr36578-2.c: New tests.

Added:
    branches/gcc-4_3-branch/gcc/testsuite/gcc.dg/dfp/convert-bfp-13.c
    branches/gcc-4_3-branch/gcc/testsuite/gcc.dg/dfp/convert-bfp-14.c
    branches/gcc-4_3-branch/gcc/testsuite/gcc.dg/dfp/convert-dfp-fold-2.c
    branches/gcc-4_3-branch/gcc/testsuite/gcc.dg/dfp/pr39034.c
    branches/gcc-4_3-branch/gcc/testsuite/gcc.target/i386/pr36578-1.c
    branches/gcc-4_3-branch/gcc/testsuite/gcc.target/i386/pr36578-2.c
Modified:
    branches/gcc-4_3-branch/gcc/ChangeLog
    branches/gcc-4_3-branch/gcc/convert.c
    branches/gcc-4_3-branch/gcc/real.c
    branches/gcc-4_3-branch/gcc/real.h
    branches/gcc-4_3-branch/gcc/testsuite/ChangeLog


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36578