polyroots fail on simple 3rd degree polynomial

[pipatron]

The simple polynomial 2x^3+x^2+1 breaks polyroots in PDL v2.099 under perl 5.036 on Debian stable:

pdl> pdl(2, 1, 0, 1)->r2C->polyroots
Error in polyroots:cpoly: noshft returned NaN at lib/PDL/Math.pd line 371.
	PDL::polyroots(PDL=SCALAR(0x5585832985a0)) called at (eval 370) line 4
	main::__ANON__() called at /home/pipe/perl5/bin/perldl line 729
	eval {...} called at /home/pipe/perl5/bin/perldl line 727
	main::eval_and_report("pdl(2, 1, 0, 1)->r2C->polyroots\x{a}") called at /home/pipe/perl5/bin/perldl line 663
	main::process_input() called at /home/pipe/perl5/bin/perldl line 683
	eval {...} called at /home/pipe/perl5/bin/perldl line 683

As you can see, there's nothing special about it, there's one real and two complex roots:

Output from perldl -V

perlDL shell v1.357
 PDL comes with ABSOLUTELY NO WARRANTY. For details, see the file
 'COPYING' in the PDL distribution. This is free software and you
 are welcome to redistribute it under certain conditions, see
 the same file for details.

Summary of my PDL configuration

VERSION: PDL v2.099 (supports bad values)

Summary of my perl5 (revision 5 version 36 subversion 0) configuration:
   
  Platform:
    osname=linux
    osvers=4.19.0
    archname=x86_64-linux-gnu-thread-multi
    uname='linux localhost 4.19.0 #1 smp debian 4.19.0 x86_64 gnulinux '
    config_args='-Dmksymlinks -Dusethreads -Duselargefiles -Dcc=x86_64-linux-gnu-gcc -Dcpp=x86_64-linux-gnu-cpp -Dld=x86_64-linux-gnu-gcc -Dccflags=-DDEBIAN -Wdate-time -D_FORTIFY_SOURCE=2 -g -O2 -ffile-prefix-map=/dummy/build/dir=. -fstack-protector-strong -Wformat -Werror=format-security -Dldflags= -Wl,-z,relro -Dlddlflags=-shared -Wl,-z,relro -Dcccdlflags=-fPIC -Darchname=x86_64-linux-gnu -Dprefix=/usr -Dprivlib=/usr/share/perl/5.36 -Darchlib=/usr/lib/x86_64-linux-gnu/perl/5.36 -Dvendorprefix=/usr -Dvendorlib=/usr/share/perl5 -Dvendorarch=/usr/lib/x86_64-linux-gnu/perl5/5.36 -Dsiteprefix=/usr/local -Dsitelib=/usr/local/share/perl/5.36.0 -Dsitearch=/usr/local/lib/x86_64-linux-gnu/perl/5.36.0 -Dman1dir=/usr/share/man/man1 -Dman3dir=/usr/share/man/man3 -Dsiteman1dir=/usr/local/man/man1 -Dsiteman3dir=/usr/local/man/man3 -Duse64bitint -Dman1ext=1 -Dman3ext=3perl -Dpager=/usr/bin/sensible-pager -Uafs -Ud_csh -Ud_ualarm -Uusesfio -Uusenm -Ui_libutil -Ui_xlocale -Uversiononly -DDEBUGGING=-g -Doptimize=-O2 -dEs -Duseshrplib -Dlibperl=libperl.so.5.36.0'
    hint=recommended
    useposix=true
    d_sigaction=define
    useithreads=define
    usemultiplicity=define
    use64bitint=define
    use64bitall=define
    uselongdouble=undef
    usemymalloc=n
    default_inc_excludes_dot=define
  Compiler:
    cc='x86_64-linux-gnu-gcc'
    ccflags ='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fwrapv -fno-strict-aliasing -pipe -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64'
    optimize='-O2 -g'
    cppflags='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fwrapv -fno-strict-aliasing -pipe -I/usr/local/include'
    ccversion=''
    gccversion='12.2.0'
    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='double'
    nvsize=8
    Off_t='off_t'
    lseeksize=8
    alignbytes=8
    prototype=define
  Linker and Libraries:
    ld='x86_64-linux-gnu-gcc'
    ldflags =' -fstack-protector-strong -L/usr/local/lib'
    libpth=/usr/local/lib /usr/lib/x86_64-linux-gnu /usr/lib /lib/x86_64-linux-gnu /lib
    libs=-lgdbm -lgdbm_compat -ldb -ldl -lm -lpthread -lc -lcrypt
    perllibs=-ldl -lm -lpthread -lc -lcrypt
    libc=/lib/x86_64-linux-gnu/libc.so.6
    so=so
    useshrplib=true
    libperl=libperl.so.5.36
    gnulibc_version='2.36'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs
    dlext=so
    d_dlsymun=undef
    ccdlflags='-Wl,-E'
    cccdlflags='-fPIC'
    lddlflags='-shared -L/usr/local/lib -fstack-protector-strong'

[mohawk2]

Thank you for the report! This was caused by an off-by-one error missed during simplifying the indexing to make cpoly.c more C-like and less Fortran-like.