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... while a hash is purported to be O(1).

Don't blame the hash lookup performance here.

The thing that slows the dispatch table is simply that subroutines in Perl are slow relative to inline blocks.

If you make it so that the ifelse chain also has to do hash lookups, and the dispatch table doesn't call the subroutines, it (unsurprisingly) makes the dispatch table fastest even for very low numbers of choices:

C:\test>587072 -N=10 -KEYS=a,b dispatch => # line 1 "dispatch.pl" my $x += ( $dispatch{ $_ } ||= $dispatch{ default } ) for @da +ta; ifelse => # line 1 "ifelse.pl" for( @data ) { if( $_ eq "a" ) { my $x = $dispatch{ a } } elsif( $_ eq "b" ) { my $x = $dispatch{ b } } else{ my $x = $dispatch{ default } } } Rate ifelse dispatch ifelse 39306/s -- -29% dispatch 55351/s 41% --

Equally, if we have both call the subroutines, the dispatch table wins:

C:\test>587072 -N=10 -KEYS=a,b dispatch => # line 1 "dispatch.pl" my $x += ( $dispatch{ $_ } ||= $dispatch{ default } )->() for + @data; ifelse => # line 1 "ifelse.pl" for( @data ) { if( $_ eq "a" ) { my $x = $dispatch{ a }->() } elsif( $_ eq "b" ) { my $x = $dispatch{ b }->() } else{ my $x = $dispatch{ default }->() } } Rate ifelse dispatch ifelse 21966/s -- -15% dispatch 25880/s 18% -- C:\test>587072 -N=10 -KEYS=a,b,c dispatch => # line 1 "dispatch.pl" my $x += ( $dispatch{ $_ } ||= $dispatch{ default } )->() for + @data; ifelse => # line 1 "ifelse.pl" for( @data ) { if( $_ eq "a" ) { my $x = $dispatch{ a }->() } elsif( $_ eq "b" ) { my $x = $dispatch{ b }->() } elsif( $_ eq "c" ) { my $x = $dispatch{ c }->() } else{ my $x = $dispatch{ default }->() } } Rate ifelse dispatch ifelse 16250/s -- -22% dispatch 20798/s 28% --

But if you leave the lookups in both, but only have the dispatch table call the subs, the ifelse chain wins upto similar numbers to those you report:

C:\test>587072 -N=10 -KEYS=a,b,c,d,e,f,g dispatch => # line 1 "dispatch.pl" my $x += ( $dispatch{ $_ } ||= $dispatch{ default } )->() for + @data; ifelse => # line 1 "ifelse.pl" for( @data ) { if( $_ eq "a" ) { my $x = $dispatch{ a } } elsif( $_ eq "b" ) { my $x = $dispatch{ b } } elsif( $_ eq "c" ) { my $x = $dispatch{ c } } elsif( $_ eq "d" ) { my $x = $dispatch{ d } } elsif( $_ eq "e" ) { my $x = $dispatch{ e } } elsif( $_ eq "f" ) { my $x = $dispatch{ f } } elsif( $_ eq "g" ) { my $x = $dispatch{ g } } else{ my $x = $dispatch{ default } } } Rate dispatch ifelse dispatch 10511/s -- -8% ifelse 11428/s 9% -- C:\test>587072 -N=10 -KEYS=a,b,c,d,e,f,g,h dispatch => # line 1 "dispatch.pl" my $x += ( $dispatch{ $_ } ||= $dispatch{ default } )->() for + @data; ifelse => # line 1 "ifelse.pl" for( @data ) { if( $_ eq "a" ) { my $x = $dispatch{ a } } elsif( $_ eq "b" ) { my $x = $dispatch{ b } } elsif( $_ eq "c" ) { my $x = $dispatch{ c } } elsif( $_ eq "d" ) { my $x = $dispatch{ d } } elsif( $_ eq "e" ) { my $x = $dispatch{ e } } elsif( $_ eq "f" ) { my $x = $dispatch{ f } } elsif( $_ eq "g" ) { my $x = $dispatch{ g } } elsif( $_ eq "h" ) { my $x = $dispatch{ h } } else{ my $x = $dispatch{ default } } } Rate dispatch ifelse dispatch 9558/s -- -1% ifelse 9635/s 1% -- C:\test>587072 -N=10 -KEYS=a,b,c,d,e,f,g,h,i dispatch => # line 1 "dispatch.pl" my $x += ( $dispatch{ $_ } ||= $dispatch{ default } )->() for + @data; ifelse => # line 1 "ifelse.pl" for( @data ) { if( $_ eq "a" ) { my $x = $dispatch{ a } } elsif( $_ eq "b" ) { my $x = $dispatch{ b } } elsif( $_ eq "c" ) { my $x = $dispatch{ c } } elsif( $_ eq "d" ) { my $x = $dispatch{ d } } elsif( $_ eq "e" ) { my $x = $dispatch{ e } } elsif( $_ eq "f" ) { my $x = $dispatch{ f } } elsif( $_ eq "g" ) { my $x = $dispatch{ g } } elsif( $_ eq "h" ) { my $x = $dispatch{ h } } elsif( $_ eq "i" ) { my $x = $dispatch{ i } } else{ my $x = $dispatch{ default } } } Rate ifelse dispatch ifelse 8092/s -- -6% dispatch 8566/s 6% --

I thought I read somewhere that subroutine call performance had been improved in bleedperl, but I cannot find the reference?


Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal?
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In reply to Re: When should I use a dispatch table? by BrowserUk
in thread When should I use a dispatch table? by Limbic~Region

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