[Pdx-pm] Hash question

[Bruce J Keeler]

On Fri, 2003-11-21 at 20:25, Randal L. Schwartz wrote:
>         
> I think it's even been shown that iteration is better:
> 
>     my @array = $db2->DataHash();
>     while (@array) {
>       $SignUpInfo{shift @array} = shift @array;
>     }
> 
Makes sense as it doesn't have to compute hash keys for %tmp.

> Of course, I'm cheating here, knowing that the left side
> is eval'ed before the right.  If you don't want that much magic:
> 
>     my %tmp = $db2->DataHash();
>     while (my ($k, $v) = each %tmp) {
>       $SignUpInfo{$k} = $v;
>     }

You're saying that's cheaper than

>     @SignUpInfo{keys %tmp} = values %tmp;

?

This I found hard to believe.  Why would Perl pessimize it so?  I
whipped up the following:

        #!/usr/bin/perl
        
        use Benchmark qw( cmpthese );
        
        push (@array, rand, rand) for (1..100);
        
        cmpthese ( -10, {
            iterated_hash => sub {
                my %dest;
        	my %tmp = @array;
                while (my ($k, $v) = each %tmp) {
        	    $dest{$k} = $v;
        	}
            },
            atonce => sub {
                my %dest;
        	my %tmp = @array;
        	@dest{keys %tmp} = values %tmp;
            },
            iterated_array => sub {
                my %dest;
        	my @tmp = @array;
        	while (@tmp) {
        	    $dest{shift @tmp} = shift @tmp;
        	}
            },
        } );

Results:

                 Rate iterated_array  iterated_hash         atonce
iterated_array 1198/s             --           -56%           -71%
iterated_hash  2709/s           126%             --           -34%
atonce         4098/s           242%            51%             --

It seems that the array method is worst of all.  Most interesting.

My perl is:

bruce at scrunge| /tmp % perl -V
Summary of my perl5 (revision 5.0 version 8 subversion 2) configuration:
  Platform:
    osname=linux, osvers=2.4.22-xfs+ti1211,
archname=i386-linux-thread-multi
    uname='linux kosh 2.4.22-xfs+ti1211 #1 sat oct 25 10:11:37 est 2003
i686 gnulinux '
    config_args='-Dusethreads -Duselargefiles -Dccflags=-DDEBIAN
-Dcccdlflags=-fPIC -Darchname=i386-linux -Dprefix=/usr
-Dprivlib=/usr/share/perl/5.8.2 -Darchlib=/usr/lib/perl/5.8.2
-Dvendorprefix=/usr -Dvendorlib=/usr/share/perl5
-Dvendorarch=/usr/lib/perl5 -Dsiteprefix=/usr/local
-Dsitelib=/usr/local/share/perl/5.8.2
-Dsitearch=/usr/local/lib/perl/5.8.2 -Dman1dir=/usr/share/man/man1
-Dman3dir=/usr/share/man/man3 -Dsiteman1dir=/usr/local/man/man1
-Dsiteman3dir=/usr/local/man/man3 -Dman1ext=1 -Dman3ext=3perl
-Dpager=/usr/bin/sensible-pager -Uafs -Ud_csh -Uusesfio -Uusenm
-Duseshrplib -Dlibperl=libperl.so.5.8.2 -Dd_dosuid -des'
    hint=recommended, useposix=true, d_sigaction=define
    usethreads=define use5005threads=undef useithreads=define
usemultiplicity=define
    useperlio=define d_sfio=undef uselargefiles=define usesocks=undef
    use64bitint=undef use64bitall=undef uselongdouble=undef
    usemymalloc=n, bincompat5005=undef
  Compiler:
    cc='cc', ccflags ='-D_REENTRANT -D_GNU_SOURCE -DTHREADS_HAVE_PIDS
-DDEBIAN -fno-strict-aliasing -I/usr/local/include -D_LARGEFILE_SOURCE
-D_FILE_OFFSET_BITS=64',
    optimize='-O3',
    cppflags='-D_REENTRANT -D_GNU_SOURCE -DTHREADS_HAVE_PIDS -DDEBIAN
-fno-strict-aliasing -I/usr/local/include'
    ccversion='', gccversion='3.3.2 (Debian)', gccosandvers=''
    intsize=4, longsize=4, ptrsize=4, doublesize=8, byteorder=1234
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=12
    ivtype='long', ivsize=4, nvtype='double', nvsize=8, Off_t='off_t',
lseeksize=8
    alignbytes=4, prototype=define
  Linker and Libraries:
    ld='cc', ldflags =' -L/usr/local/lib'
    libpth=/usr/local/lib /lib /usr/lib
    libs=-lgdbm -lgdbm_compat -ldb -ldl -lm -lpthread -lc -lcrypt
    perllibs=-ldl -lm -lpthread -lc -lcrypt
    libc=/lib/libc-2.3.2.so, so=so, useshrplib=true,
libperl=libperl.so.5.8.2
    gnulibc_version='2.3.2'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-rdynamic'
    cccdlflags='-fPIC', lddlflags='-shared -L/usr/local/lib'


Characteristics of this binary (from libperl): 
  Compile-time options: MULTIPLICITY USE_ITHREADS USE_LARGE_FILES
PERL_IMPLICIT_CONTEXT
  Built under linux
  Compiled at Nov 15 2003 17:52:08
  @INC:
    /etc/perl
    /usr/local/lib/perl/5.8.2
    /usr/local/share/perl/5.8.2
    /usr/lib/perl5
    /usr/share/perl5
    /usr/lib/perl/5.8.2
    /usr/share/perl/5.8.2
    /usr/local/lib/site_perl
    /usr/local/lib/perl/5.8.0
    /usr/local/share/perl/5.8.0
    .