[ID 20000417.001] goto and optimisation

This is a bug report for perl from zefram@fysh.org,
generated with the help of perlbug 1.26 running under perl 5.00503.


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[Please enter your report here]


perlsyn(1):

#       Goto
...
#                                      It also can't be used to go
#       into a construct that is optimized away.

Looks innocuous, doesn't it.  What it means:

$ perl -e 'if(0) {FOO:print"foo\n";exit} print "1\n"; goto FOO'
1
Can't find label FOO at -e line 1.
$ perl -e 'if(rand == -1) {FOO:print"foo\n";exit} print "1\n"; goto FOO'
1
foo
$

What constructs get optimized away?  The docs don't say.  The optimizer
is obviously foolable, so maybe it'll get cleverer in a future version,
changing the semantics of the above scripts.  I'm not supposed to have
to know what the optimizer does.  *And* the problem isn't diagnosed at
compile time, so it's likely not to be noticed if the goto is taken only
in unusual circumstances (as it is in the program I'm actually writing).

Unreachable code elimination is a good thing.  But if code has a label
in front of it then there's a fair chance that it's not unreachable.


[Please do not change anything below this line]
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Site configuration information for perl 5.00503:

Configured by torin at Mon Mar 27 07:50:39 PST 2000.

Summary of my perl5 (5.0 patchlevel 5 subversion 3) configuration:
  Platform:
    osname=linux, osvers=2.2.14, archname=i386-linux
    uname='linux perv 2.2.14 #1 sat jan 15 20:48:59 pst 2000 i686 unknown '
    hint=recommended, useposix=true, d_sigaction=define
    usethreads=undef useperlio=undef d_sfio=undef
  Compiler:
    cc='cc', optimize='-O2 ', gccversion=2.95.2 20000220 (Debian GNU/Linux)
    cppflags='-Dbool=char -DHAS_BOOL -D_REENTRANT -DDEBIAN -I/usr/local/include'
    ccflags ='-Dbool=char -DHAS_BOOL -D_REENTRANT -DDEBIAN -I/usr/local/include'
    stdchar='char', d_stdstdio=undef, usevfork=false
    intsize=4, longsize=4, ptrsize=4, doublesize=8
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=12
    alignbytes=4, usemymalloc=n, prototype=define
  Linker and Libraries:
    ld='cc', ldflags =' -L/usr/local/lib'
    libpth=/usr/local/lib /lib /usr/lib
    libs=-lnsl -lndbm -lgdbm -ldbm -ldb -ldl -lm -lc -lposix -lcrypt
    libc=, so=so, useshrplib=false, libperl=libperl.a
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-rdynamic'
    cccdlflags='-fPIC', lddlflags='-shared -L/usr/local/lib'

Locally applied patches:
    

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@INC for perl 5.00503:
    /usr/lib/perl5/5.005/i386-linux
    /usr/lib/perl5/5.005
    /usr/local/lib/site_perl/i386-linux
    /usr/local/lib/site_perl
    /usr/lib/perl5
    .

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Environment for perl 5.00503:
    HOME=/home/zefram
    LANG (unset)
    LANGUAGE (unset)
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PATH=/home/zefram/pub/i586-pc-linux-gnu/bin:/home/zefram/pub/common/bin:/usr/bin:/usr/X11/bin:/usr/X11R6/bin:/bin:/usr/local/bin:/usr/games:/opt/local/bin
    PERL_BADLANG (unset)
    SHELL=/usr/bin/zsh

• [ID 20000417.001] goto and optimisation by Zefram