search

Perl / perl5

🐪 The Perl programming language
https://dev.perl.org/perl5/
Other
1.97k stars 555 forks source link

Documentation of binary | and & operators #15633

Open p5pRT opened 8 years ago

p5pRT commented 8 years ago

Migrated from rt.perl.org#129371 (status was 'open')

Searchable as RT129371$

p5pRT commented 8 years ago

From @epa

Created by @epa

The documentation for & in perlfunc states that

  Binary "&" returns its operands ANDed together bit by bit.

Implicit in this is that both operands are always evaluated\, even if the LHS has a bit pattern of all zeroes. The operator does not short cut like &&. I think the documentation should state this explicitly.

Further\, can perl guarantee that on all platforms

  0 & 0 == 0   0 & 1 == 0   1 & 0 == 0   1 & 1 == 1

  0 | 0 == 0   0 | 1 == 1   1 | 0 == 1   1 | 1 == 1

  0 ^ 0 == 0   0 ^ 1 == 1   1 ^ 0 == 1   1 ^ 1 == 0

You can imagine some funky processor where the integer 0 has a bit pattern which isn't all zeroes\, for example\, so the above equations wouldn't hold. But I don't think Perl runs on any such architecture in practice. If minimally it could be guaranteed that the bitwise ops behave as expected with the integer values 0 and 1\, they would be more useful in programming.

Perl Info ``` Flags: category=docs severity=low Site configuration information for perl 5.22.2: Configured by Red Hat, Inc. at Wed Aug 3 14:06:20 UTC 2016. Summary of my perl5 (revision 5 version 22 subversion 2) configuration: Platform: osname=linux, osvers=4.6.3-300.fc24.x86_64, archname=x86_64-linux-thread-multi uname='linux buildvm-08.phx2.fedoraproject.org 4.6.3-300.fc24.x86_64 #1 smp fri jun 24 20:52:41 utc 2016 x86_64 x86_64 x86_64 gnulinux ' config_args='-des -Doptimize=none -Dccflags=-O2 -g -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector-strong --param=ssp-buffer-size=4 -grecord-gcc-switches -m64 -mtune=generic -Dldflags=-Wl,-z,relro -Dccdlflags=-Wl,--enable-new-dtags -Wl,-z,relro -Dlddlflags=-shared -Wl,-z,relro -Dshrpdir=/usr/lib64 -DDEBUGGING=-g -Dversion=5.22.2 -Dmyhostname=localhost -Dperladmin=root@localhost -Dcc=gcc -Dcf_by=Red Hat, Inc. -Dprefix=/usr -Dvendorprefix=/usr -Dsiteprefix=/usr/local -Dsitelib=/usr/local/share/perl5 -Dsitearch=/usr/local/lib64/perl5 -Dprivlib=/usr/share/perl5 -Dvendorlib=/usr/share/perl5/vendor_perl -Darchlib=/usr/lib64/perl5 -Dvendorarch=/usr/lib64/perl5/vendor_perl -Darchname=x86_64-linux-thread-multi -Dlibpth=/usr/local/lib64 /lib64 /usr/lib64 -Duseshrplib -Dusethreads -Duseithreads -Dusedtrace=/usr/bin/dtrace -Duselargefiles -Dd_semctl_semun -Di_db -Ui_ndbm -Di_gdbm -Di_shadow -Di_syslog -Dman3ext=3pm -Duseperlio -Dinstallusrbinperl=n -Ubincompat5005 -Uversiononly -Dpager=/usr/bin/less -isr -Dd_gethostent_r_proto -Ud_endhostent_r_proto -Ud_sethostent_r_proto -Ud_endprotoent_r_proto -Ud_setprotoent_r_proto -Ud_endservent_r_proto -Ud_setservent_r_proto -Dscriptdir=/usr/bin -Dusesitecustomize' hint=recommended, useposix=true, d_sigaction=define useithreads=define, usemultiplicity=define use64bitint=define, use64bitall=define, uselongdouble=undef usemymalloc=n, bincompat5005=undef Compiler: cc='gcc', ccflags ='-D_REENTRANT -D_GNU_SOURCE -O2 -g -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector-strong --param=ssp-buffer-size=4 -grecord-gcc-switches -m64 -mtune=generic -fwrapv -fno-strict-aliasing -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64', optimize=' -g', cppflags='-D_REENTRANT -D_GNU_SOURCE -O2 -g -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector-strong --param=ssp-buffer-size=4 -grecord-gcc-switches -m64 -mtune=generic -fwrapv -fno-strict-aliasing -I/usr/local/include' ccversion='', gccversion='5.3.1 20160406 (Red Hat 5.3.1-6)', 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='gcc', ldflags ='-Wl,-z,relro -fstack-protector-strong -L/usr/local/lib' libpth=/usr/local/lib64 /lib64 /usr/lib64 /usr/local/lib /usr/lib /lib/../lib64 /usr/lib/../lib64 /lib libs=-lpthread -lresolv -lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc -lgdbm_compat perllibs=-lpthread -lresolv -lnsl -ldl -lm -lcrypt -lutil -lc libc=libc-2.22.so, so=so, useshrplib=true, libperl=libperl.so gnulibc_version='2.22' Dynamic Linking: dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,--enable-new-dtags -Wl,-z,relro ' cccdlflags='-fPIC', lddlflags='-shared -Wl,-z,relro -L/usr/local/lib -fstack-protector-strong' Locally applied patches: Fedora Patch1: Removes date check, Fedora/RHEL specific Fedora Patch3: support for libdir64 Fedora Patch4: use libresolv instead of libbind Fedora Patch5: USE_MM_LD_RUN_PATH Fedora Patch6: Skip hostname tests, due to builders not being network capable Fedora Patch7: Dont run one io test due to random builder failures Fedora Patch15: Define SONAME for libperl.so Fedora Patch16: Install libperl.so to -Dshrpdir value Fedora Patch22: Document Math::BigInt::CalcEmu requires Math::BigInt (CPAN RT#85015) Fedora Patch26: Make *DBM_File desctructors thread-safe (RT#61912) Fedora Patch27: Make PadlistNAMES() lvalue again (CPAN RT#101063) Fedora Patch28: Make magic vtable writable as a work-around for Coro (CPAN RT#101063) Fedora Patch29: Fix duplicating PerlIO::encoding when spawning threads (RT#31923) Fedora Patch30: Do not let XSLoader load relative paths (CVE-2016-6185) Fedora Patch31: Avoid loading optional modules from default . (CVE-2016-1238) Fedora Patch200: Link XS modules to libperl.so with EU::CBuilder on Linux Fedora Patch201: Link XS modules to libperl.so with EU::MM on Linux @INC for perl 5.22.2: /home/eda/lib64/perl5/ /usr/local/lib64/perl5 /usr/local/share/perl5 /usr/lib64/perl5/vendor_perl /usr/share/perl5/vendor_perl /usr/lib64/perl5 /usr/share/perl5 Environment for perl 5.22.2: HOME=/home/eda LANG=en_GB.UTF-8 LANGUAGE (unset) LC_COLLATE=C LC_CTYPE=en_GB.UTF-8 LC_MESSAGES=en_GB.UTF-8 LC_MONETARY=en_GB.UTF-8 LC_NUMERIC=en_GB.UTF-8 LC_TIME=en_GB.UTF-8 LD_LIBRARY_PATH (unset) LOGDIR (unset) PATH=/home/eda/bin:/home/eda/bin:/usr/local/bin:/usr/bin:/sbin:/usr/sbin:/sbin:/usr/sbin PERL5LIB=/home/eda/lib64/perl5/ PERL_BADLANG (unset) SHELL=/bin/bash ```
p5pRT commented 8 years ago

From zefram@fysh.org

Ed Avis wrote​:

                                     The operator does not short

cut like &&. I think the documentation should state this explicitly.

Should the documentation for infix * also clarify that 0*$a doesn't short-circuit? I'm not really opposed to that kind of clarification\, but we should be consistent about it.

Further\, can perl guarantee that on all platforms

Yes\, that is guaranteed.

You can imagine some funky processor where the integer 0 has a bit pattern which isn't all zeroes\,

The C language standard requires that the representations of unsigned and non-negative signed integers be pure binary place value. Thus the representation of zero is always all bits zero. The bitwise operators are defined to operate on the binary place value representations. Perl depends on this aspect of C all over the place\, and effectively passes that guarantee on to Perl programs using Perl's bitwise operators. Perl further requires that the negative signed representation be twos-complement\, and so guarantees that too to Perl programs.

It is conceivable for a platform to use a different numeric representation at a very low level\, but its C implementation must then include enough translation to successfully pretend that it's pure binary. Such translation is outside our area of concern.

-zefram

p5pRT commented 8 years ago

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