From @sisyphus

Hi,

I'm running perl-5.27.10, but the same issue is evident in perl-5.20.0 and
perl-5.26.0.

For perls built with -Duselongdouble on my powerpc64 box the 'long double'
is a 'doubledouble'.
As an example of the problem​:

$ perl -le 'print 2 ** -1074;'
4.450147717014402766180465434665e-308

But this is the same as 2 ** -1021​:

$ perl -le 'print 2 ** -1021;'
4.450147717014402766180465434665e-308

Similarly for all other subnormals I tried, perl's print and printf( "%e",
..) functions output values that are off by a factor of 2**53.
Using printf's "%a" formatting produces the correct result.

Also, we see that the value being held by the NV is correct when we unpack
it​:
$ perl -le ' print scalar reverse unpack "h*", pack "F<", 2 ** -1074;'
00000000000000010000000000000000

This is clearly the pair of doubles DBL_DENORM_MIN and zero - which is the
smallest positive value that the doubledouble can hold.

Cheers,
Rob

$ perl -V
Summary of my perl5 (revision 5 version 27 subversion 10) configuration​:

  Platform​:
  osname=linux
  osvers=3.2.0-4-powerpc64
  archname=ppc64-linux-thread-multi-ld
  uname='linux debian-sis 3.2.0-4-powerpc64 #1 smp debian 3.2.78-1 ppc64
gnulinux '
  config_args='-des -Uversiononly -Dcc=gcc -m64 -Dprefix=/home/sisyphus-sis/perl-5.27.10-ld
-Dusedevel -Duselongdouble -Dusethreads -Duse64bitint'
  hint=recommended
  useposix=true
  d_sigaction=define
  useithreads=define
  usemultiplicity=define
  use64bitint=define
  use64bitall=define
  uselongdouble=define
  usemymalloc=n
  default_inc_excludes_dot=define
  bincompat5005=undef
  Compiler​:
  cc='gcc -m64'
  ccflags
='-D_REENTRANT -D_GNU_SOURCE -fwrapv -fno-strict-aliasing -pipe -fstack-protector
-I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -D_FORTIFY_SOURCE=2'
  optimize='-O1'
  cppflags='-D_REENTRANT -D_GNU_SOURCE -fwrapv -fno-strict-aliasing -pipe
-fstack-protector -I/usr/local/include'
  ccversion=''
  gccversion='4.6.3'
  gccosandvers=''
  intsize=4
  longsize=8
  ptrsize=8
  doublesize=8
  byteorder=87654321
  doublekind=4
  d_longlong=define
  longlongsize=8
  d_longdbl=define
  longdblsize=16
  longdblkind=6
  ivtype='long'
  ivsize=8
  nvtype='long double'
  nvsize=16
  Off_t='off_t'
  lseeksize=8
  alignbytes=16
  prototype=define
  Linker and Libraries​:
  ld='gcc -m64'
  ldflags =' -fstack-protector -L/usr/local/lib'
  libpth=/usr/local/lib /usr/lib/gcc/powerpc-linux-gnu/4.6/include-fixed
/usr/lib /lib/powerpc-linux-gnu /lib/../lib /usr/lib/powerpc-linux-gnu
/usr/lib/../lib /lib /lib64 /usr/lib64
  libs=-lpthread -lnsl -ldl -lm -lcrypt -lutil -lc
  perllibs=-lpthread -lnsl -ldl -lm -lcrypt -lutil -lc
  libc=libc-2.13.so
  so=so
  useshrplib=false
  libperl=libperl.a
  gnulibc_version='2.13'
  Dynamic Linking​:
  dlsrc=dl_dlopen.xs
  dlext=so
  d_dlsymun=undef
  ccdlflags='-Wl,-E'
  cccdlflags='-fPIC'
  lddlflags='-shared -O1 -L/usr/local/lib -fstack-protector'

Characteristics of this binary (from libperl)​:
  Compile-time options​:
  HAS_TIMES
  MULTIPLICITY
  PERLIO_LAYERS
  PERL_COPY_ON_WRITE
  PERL_DONT_CREATE_GVSV
  PERL_IMPLICIT_CONTEXT
  PERL_MALLOC_WRAP
  PERL_OP_PARENT
  PERL_PRESERVE_IVUV
  PERL_USE_DEVEL
  USE_64_BIT_ALL
  USE_64_BIT_INT
  USE_ITHREADS
  USE_LARGE_FILES
  USE_LOCALE
  USE_LOCALE_COLLATE
  USE_LOCALE_CTYPE
  USE_LOCALE_NUMERIC
  USE_LOCALE_TIME
  USE_LONG_DOUBLE
  USE_PERLIO
  USE_PERL_ATOF
  USE_REENTRANT_API
  Built under linux
  Compiled at Apr 3 2018 20​:52​:50
  @​INC​:
  /home/sisyphus-sis/perl-5.27.10-ld/lib/site_perl/5.27.10/ppc64-linux-thread-multi-ld
  /home/sisyphus-sis/perl-5.27.10-ld/lib/site_perl/5.27.10
  /home/sisyphus-sis/perl-5.27.10-ld/lib/5.27.10/ppc64-linux-thread-multi-ld
  /home/sisyphus-sis/perl-5.27.10-ld/lib/5.27.10

From @sisyphus

On Fri, 13 Apr 2018 22​:39​:50 -0700, sisyphus wrote​:

$ perl -le 'print 2 ** -1074;'
4.450147717014402766180465434665e-308

C's printf() produces the same incorrect output - so, either perl and C are making the same mistake, or perl is just passing on the value that C provides.

I suspect the latter - ie that this is NOT a bug in Perl.

I'm using gcc-4.6.3 and libc-2.13, which is fairly old stuff.

Cheers,
Rob

The RT System itself - Status changed from 'new' to 'open'