Why does Perl think 1<<-1 is 9223372036854775808? - perl

Question

If I understand perldoc perlop correctly, this operation should something undefined:
The result of overflowing the range of the integers is undefined because it is undefined also in C. In other words, using 32-bit integers, 1 << 32 is undefined. Shifting by a negative number of bits is also undefined.
I'm noticing two things on my setup that I cannot explain:
1 << -1 returns 9223372036854775808 without bigint or integer pragmas (returns NaN when they are active).
> perl -le "print 1<<-1"
9223372036854775808
1 << -1 returns true when tested with defined, regardless of whether or not the bigint or integer pragmas are in effect
> perl -le "print 'yes' if defined(1<<-1)"
yes
> perl -le "use integer; print 'yes' if defined(1<<-1)"
yes
> perl -le "use bigint; print 'yes' if defined(1<<-1)"
yes
Details of my setup:
> perl -V
Set up gcc environment - gcc.exe (rubenvb-4.5.4) 4.5.4
Summary of my perl5 (revision 5 version 16 subversion 3) configuration:
Platform:
osname=MSWin32, osvers=5.2, archname=MSWin32-x64-multi-thread
uname=''
config_args='undef'
hint=recommended, useposix=true, d_sigaction=undef
useithreads=define, usemultiplicity=define
useperlio=define, d_sfio=undef, uselargefiles=define, usesocks=undef
use64bitint=define, use64bitall=undef, uselongdouble=undef
usemymalloc=n, bincompat5005=undef
Compiler:
cc='C:\Perl64\site\bin\gcc.exe', ccflags ='-DNDEBUG -DWIN32 -D_CONSOLE -DNO_STRICT -DWIN64 -DCONSERVATIVE -DPERL_TEXTMODE_SCRIPTS -DUSE_SITECUSTOMIZE -DPERL_IMPLICIT_CONTEXT -DPERL_IMPLICIT_SYS -DUSE_PERLIO -DHASATTRIBUTE -fno-strict-aliasing -mms-bitfields',
optimize='-O2',
cppflags='-DWIN32'
ccversion='', gccversion='gcc.exe (rubenvb-4.5.4) 4.5.4', gccosandvers=''
intsize=4, longsize=4, ptrsize=8, doublesize=8, byteorder=12345678
d_longlong=undef, longlongsize=8, d_longdbl=define, longdblsize=8
ivtype='__int64', ivsize=8, nvtype='double', nvsize=8, Off_t='__int64', lseeksize=8
alignbytes=8, prototype=define
Linker and Libraries:
ld='C:\Perl64\site\bin\g++.exe', ldflags ='-L"C:\Perl64\lib\CORE"'
libpth=\lib
libs=-lkernel32 -luser32 -lgdi32 -lwinspool -lcomdlg32 -ladvapi32 -lshell32 -lole32 -loleaut32 -lnetapi32 -luuid -lws2_32 -lmpr -lwinmm -lversion -lodbc32 -lodbccp32 -lcomctl32 -lmsvcrt
perllibs=-lkernel32 -luser32 -lgdi32 -lwinspool -lcomdlg32 -ladvapi32 -lshell32 -lole32 -loleaut32 -lnetapi32 -luuid -lws2_32 -lmpr -lwinmm -lversion -lodbc32 -lodbccp32 -lcomctl32 -lmsvcrt
libc=msvcrt.lib, so=dll, useshrplib=true, libperl=perl516.lib
gnulibc_version=''
Dynamic Linking:
dlsrc=dl_win32.xs, dlext=dll, d_dlsymun=undef, ccdlflags=' '
cccdlflags=' ', lddlflags='-mdll -L"C:\Perl64\lib\CORE"'
Characteristics of this binary (from libperl):
Compile-time options: HAS_TIMES HAVE_INTERP_INTERN MULTIPLICITY
PERLIO_LAYERS PERL_DONT_CREATE_GVSV
PERL_IMPLICIT_CONTEXT PERL_IMPLICIT_SYS
PERL_MALLOC_WRAP PERL_PRESERVE_IVUV PL_OP_SLAB_ALLOC
USE_64_BIT_INT USE_ITHREADS USE_LARGE_FILES
USE_LOCALE USE_LOCALE_COLLATE USE_LOCALE_CTYPE
USE_LOCALE_NUMERIC USE_PERLIO USE_PERL_ATOF
USE_SITECUSTOMIZE
Locally applied patches:
ActivePerl Build 1603 [296746]
Built under MSWin32
Compiled at Mar 13 2013 13:31:10
#INC:
C:/Perl64/site/lib
C:/Perl64/lib
.

Answer 1

When they say undefined, they are referring to undefined behaviour, not the value undef.
If the result of an operation is undefined behaviour, it means that anything could happen -- you could get a surprising result, a different result on different platforms, a different result each time you do the same thing, or 3-foot tall snails could come out of your computer and start speaking Swahili (though that last one is rather uncommon in practice).
In other words, because it's explicitly documented as undefined behaviour, anything perl does is technically correct -- which means you can't rely on it to do anything useful.

Answer 2

The number 9223372036854775808 is equal to 2^63. Given a 64-bit unsigned integer initialized to 1, rotating it by one bit to the right gives this result. This is probably what's happening: << -1 is being interpreted as a rotate right by one.
But since shifting by negative values is undefined, this is not something that can be depended on. The coincidence is worth noting, however.