[perl #128909] printf %a mishandles exponent-crossing rounding withlong double

# New Ticket Created by  Zefram 
# Please include the string:  [perl #128909]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=128909 >



This is a bug report for perl from zefram@fysh.org,
generated with the help of perlbug 1.40 running under perl 5.24.0.


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

On an x86 long-double build (63 fractional significand bits):

$ perl -lwe 'printf "%a\n", 1.9999999999999999999'
0xf.fffffffffffffffp-3
$ perl -lwe 'printf "%.3a\n", 1.9999999999999999999'
0x10.000p-3

The latter output is wrong.  It is numerically correct, but (per the C
spec) it is not permitted to have more than one hex digit before the
radix point.  It is necessary for the output to be expressed with a
higher exponent, such as "0x1.000p+1".  FWIW, glibc uses the same digit
alignment for this long double type, so gives identical output for the
unrounded case, and gives "0x1.000p+1" for the rounded case.

[Please do not change anything below this line]
-----------------------------------------------------------------
---
Flags:
    category=core
    severity=low
---
Site configuration information for perl 5.24.0:

Configured by zefram at Mon May  9 19:42:55 BST 2016.

Summary of my perl5 (revision 5 version 24 subversion 0) configuration:
   
  Platform:
    osname=linux, osvers=3.16.0-4-amd64, archname=x86_64-linux-thread-multi-ld
    uname='linux barba.rous.org 3.16.0-4-amd64 #1 smp debian 3.16.7-ckt11-1+deb8u6 (2015-11-09) x86_64 gnulinux '
    config_args='-des -Dprefix=/home/zefram/usr/perl/perl_install/perl-5.24.0-i64-f63 -Duselargefiles -Dusethreads -Uafs -Ud_csh -Uusesfio -Uusenm -Duseshrplib -Dusedevel -Uversiononly -Ui_db -Duselongdouble'
    hint=recommended, useposix=true, d_sigaction=define
    useithreads=define, usemultiplicity=define
    use64bitint=define, use64bitall=define, uselongdouble=define
    usemymalloc=n, bincompat5005=undef
  Compiler:
    cc='cc', ccflags ='-D_REENTRANT -D_GNU_SOURCE -fwrapv -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -D_FORTIFY_SOURCE=2',
    optimize='-O2',
    cppflags='-D_REENTRANT -D_GNU_SOURCE -fwrapv -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include'
    ccversion='', gccversion='4.9.2', gccosandvers=''
    intsize=4, longsize=8, ptrsize=8, doublesize=8, byteorder=12345678, doublekind=3
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=16, longdblkind=3
    ivtype='long', ivsize=8, nvtype='long double', nvsize=16, Off_t='off_t', lseeksize=8
    alignbytes=16, prototype=define
  Linker and Libraries:
    ld='cc', ldflags =' -fstack-protector-strong -L/usr/local/lib'
    libpth=/usr/local/lib /usr/lib/gcc/x86_64-linux-gnu/4.9/include-fixed /usr/include/x86_64-linux-gnu /usr/lib /lib/x86_64-linux-gnu /lib/../lib /usr/lib/x86_64-linux-gnu /usr/lib/../lib /lib
    libs=-lpthread -lnsl -ldb -ldl -lm -lcrypt -lutil -lc
    perllibs=-lpthread -lnsl -ldl -lm -lcrypt -lutil -lc
    libc=libc-2.19.so, so=so, useshrplib=true, libperl=libperl.so
    gnulibc_version='2.19'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E -Wl,-rpath,/home/zefram/usr/perl/perl_install/perl-5.24.0-i64-f63/lib/5.24.0/x86_64-linux-thread-multi-ld/CORE'
    cccdlflags='-fPIC', lddlflags='-shared -O2 -L/usr/local/lib -fstack-protector-strong'


---
@INC for perl 5.24.0:
    /home/zefram/usr/perl/perl_install/perl-5.24.0-i64-f63/lib/site_perl/5.24.0/x86_64-linux-thread-multi-ld
    /home/zefram/usr/perl/perl_install/perl-5.24.0-i64-f63/lib/site_perl/5.24.0
    /home/zefram/usr/perl/perl_install/perl-5.24.0-i64-f63/lib/5.24.0/x86_64-linux-thread-multi-ld
    /home/zefram/usr/perl/perl_install/perl-5.24.0-i64-f63/lib/5.24.0
    .

---
Environment for perl 5.24.0:
    HOME=/home/zefram
    LANG (unset)
    LANGUAGE (unset)
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PATH=/home/zefram/usr/perl/perl_install/perl-5.24.0-i64-f63/bin:/home/zefram/usr/perl/util:/home/zefram/pub/x86_64-unknown-linux-gnu/bin:/home/zefram/pub/common/bin:/usr/bin:/bin:/usr/local/bin:/usr/games
    PERL_BADLANG (unset)
    SHELL=/usr/bin/zsh

• [perl #128909] printf %a mishandles exponent-crossing rounding withlong double by Jarkko Hietaniemi via RT
• [perl #128909] printf %a mishandles exponent-crossing rounding withlong double by Zefram