[Bug modula2/115164] New: floating-point output generates misleading values with WriteFloat when sigfigs and width are zero

[Bug modula2/115164] New: floating-point output generates misleading values with WriteFloat when sigfigs and width are zero

[gaius at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115164

            Bug ID: 115164
           Summary: floating-point output generates misleading values with
                    WriteFloat when sigfigs and width are zero
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: modula2
          Assignee: gaius at gcc dot gnu.org
          Reporter: gaius at gcc dot gnu.org
  Target Milestone: ---

[Forwarded from the gm2 mailing list:
https://lists.nongnu.org/archive/html/gm2/2024-05/msg00009.html ]

For example if we use WriteFloat (pi, 0, 0) then the libraries issue a long
sequence of zeros once the precision runs out.  Consider the test code and
output below:

cat testwritereal.mod
MODULE testwritereal ;

FROM STextIO IMPORT WriteString, WriteLn ;
FROM SRealIO IMPORT WriteFloat ;
FROM RealMath IMPORT pi ;

BEGIN
   WriteString ('value of pi = ') ; WriteFloat (pi, 0, 0) ; WriteLn
END testwritereal.

$ gm2 testwritereal.mod
$ ./a.out 
value of pi =
31415926535897931159979634685441851615905761718750000000000000000000000000000000000000000000000000000

[Bug modula2/115164] floating-point output generates misleading values with WriteFloat when sigfigs and width are zero

[gaius at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115164

Gaius Mulley <gaius at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2024-05-20
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |ASSIGNED

--- Comment #1 from Gaius Mulley <gaius at gcc dot gnu.org> ---
Confirmed.

Using the Matula and Goldberg formulas makes much more sense than the current
default output for width 0.  I'll add these formulas to LowReal, LowShortReal,
LowLongReal (there is already a constant 'places', which looks as if it is
incorrect).

gcc/m2/gm2-gcc/m2builtins.cc:597:doplaces: looks wrong.  A Matula value
for sigfigs should be adopted in SRealIO and friends (if the width and sigfigs
are 0).

[Bug modula2/115164] floating-point output generates misleading values with WriteFloat when sigfigs and width are zero

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115164

--- Comment #2 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Gaius Mulley <gaius@gcc.gnu.org>:

https://gcc.gnu.org/g:d642b66a298ece7394e786a6a2d14a4f0b561d9a

commit r15-700-gd642b66a298ece7394e786a6a2d14a4f0b561d9a
Author: Gaius Mulley <gaiusmod2@gmail.com>
Date:   Tue May 21 01:11:48 2024 +0100

    PR modula2/115164 initial test code highlighting the problem

    This patch includes some trivial testcode which highlights
    PR 115164.  Expect future test code to perform runtime checks
    for a series of trailing zeros.

    gcc/testsuite/ChangeLog:

            PR modula2/115164
            * gm2/isolib/run/pass/testlowread.mod: New test.
            * gm2/isolib/run/pass/testwritereal.mod: New test.

    Signed-off-by: Gaius Mulley <gaiusmod2@gmail.com>