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[perl #120426] Strange and unwarranted underflow in string-to-number

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From:
perlbug-followup
Date:
October 31, 2013 18:08
Subject:
[perl #120426] Strange and unwarranted underflow in string-to-number
Message ID:
rt-4.0.18-27918-1383231577-1588.120426-75-0@perl.org
# New Ticket Created by   
# Please include the string:  [perl #120426]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=120426 >



This is a bug report for perl from mwelinder@gmail.com,
generated with the help of perlbug 1.39 running under perl 5.16.0.


-----------------------------------------------------------------
[Please describe your issue here]

Compare the following two commands.  The first one thinks the
number is zero, the second one works fine.  The whole difference
is an extra "0" in the mantissa.

# echo 0.1530e-305 | perl -e '$v = <STDIN>; print "v=", $v + 0, "\n";'
v=0
# echo 0.153e-305 | perl -e '$v = <STDIN>; print "v=", $v + 0, "\n";'
v=1.53e-306

The above is not supposed to underflow; there are still a couple of orders
of magnitude left:
# echo 0.1e-307 | perl -e '$v = <STDIN>; print "v=", $v + 0, "\n";'
v=9.99999999999999e-309

Observed with 5.16.0
Not observed with 5.6.1
Not observed with 5.12.4

Observed with OpenSuSE build (5.16.0).



[Please do not change anything below this line]
-----------------------------------------------------------------
---
Flags:
    category=core
    severity=medium
---
This perlbug was built using Perl 5.16.0 - Mon Mar 11 10:57:50 UTC 2013
It is being executed now by  Perl 5.16.0 - Mon Mar 11 10:54:42 UTC 2013.

Site configuration information for perl 5.16.0:

Configured by abuild at Mon Mar 11 10:54:42 UTC 2013.

Summary of my perl5 (revision 5 version 16 subversion 0) configuration:
   
  Platform:
    osname=linux, osvers=3.4.6-2.10-default, archname=x86_64-linux-thread-multi
    uname='linux build30 3.4.6-2.10-default #1 smp thu jul 26 09:36:26 utc 2012 (641c197) x86_64 x86_64 x86_64 gnulinux '
    config_args='-ds -e -Dprefix=/usr -Dvendorprefix=/usr -Dinstallusrbinperl -Dusethreads -Di_db -Di_dbm -Di_ndbm -Di_gdbm -Dd_dbm_open -Duseshrplib=true -Doptimize=-fmessage-length=0 -O2 -Wall -D_FORTIFY_SOURCE=2 -fstack-protector -funwind-tables -fasynchronous-unwind-tables -g -Wall -pipe -Accflags=-DPERL_USE_SAFE_PUTENV -Dotherlibdirs=/usr/lib/perl5/site_perl'
    hint=recommended, useposix=true, d_sigaction=define
    useithreads=define, usemultiplicity=define
    useperlio=define, d_sfio=undef, uselargefiles=define, usesocks=undef
    use64bitint=define, use64bitall=define, uselongdouble=undef
    usemymalloc=n, bincompat5005=undef
  Compiler:
    cc='cc', ccflags ='-D_REENTRANT -D_GNU_SOURCE -DPERL_USE_SAFE_PUTENV -fno-strict-aliasing -pipe -fstack-protector -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64',
    optimize='-fmessage-length=0 -O2 -Wall -D_FORTIFY_SOURCE=2 -fstack-protector -funwind-tables -fasynchronous-unwind-tables -g -Wall -pipe',
    cppflags='-D_REENTRANT -D_GNU_SOURCE -DPERL_USE_SAFE_PUTENV -fno-strict-aliasing -pipe -fstack-protector'
    ccversion='', gccversion='4.7.1 20120723 [gcc-4_7-branch revision 189773]', gccosandvers=''
    intsize=4, longsize=8, ptrsize=8, doublesize=8, byteorder=12345678
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=16
    ivtype='long', ivsize=8, nvtype='double', nvsize=8, Off_t='off_t', lseeksize=8
    alignbytes=8, prototype=define
  Linker and Libraries:
    ld='cc', ldflags =' -L/usr/local/lib64 -fstack-protector'
    libpth=/lib64 /usr/lib64 /usr/local/lib64
    libs=-lm -ldl -lcrypt -lpthread
    perllibs=-lm -ldl -lcrypt -lpthread
    libc=/lib64/libc-2.15.so, so=so, useshrplib=true, libperl=libperl.so
    gnulibc_version='2.15'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E -Wl,-rpath,/usr/lib/perl5/5.16.0/x86_64-linux-thread-multi/CORE'
    cccdlflags='-fPIC', lddlflags='-shared -L/usr/local/lib64 -fstack-protector'

Locally applied patches:
    

---
@INC for perl 5.16.0:
    /usr/lib/perl5/site_perl/5.16.0/x86_64-linux-thread-multi
    /usr/lib/perl5/site_perl/5.16.0
    /usr/lib/perl5/vendor_perl/5.16.0/x86_64-linux-thread-multi
    /usr/lib/perl5/vendor_perl/5.16.0
    /usr/lib/perl5/5.16.0/x86_64-linux-thread-multi
    /usr/lib/perl5/5.16.0
    /usr/lib/perl5/site_perl/5.16.0/x86_64-linux-thread-multi
    /usr/lib/perl5/site_perl/5.16.0
    /usr/lib/perl5/site_perl
    .

---
Environment for perl 5.16.0:
    HOME=/home/welinder
    LANG=en_US.UTF-8
    LANGUAGE (unset)
    LD_LIBRARY_PATH=/usr/lib64/mpi/gcc/openmpi/lib64
    LOGDIR (unset)
    PATH=/home/welinder/bin/shell:/home/welinder/bin/shell:/usr/lib64/mpi/gcc/openmpi/bin:/home/welinder/bin:/usr/local/bin:/usr/bin:/bin:/usr/bin/X11:/usr/X11R6/bin:/usr/games:/opt/kde3/bin:/usr/lib/mit/bin:/usr/lib/mit/sbin
    PERL_BADLANG (unset)
    SHELL=/bin/bash


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