On Fri, Apr 17, 2009 at 12:16, Jim Gibson <jimsgib...@gmail.com> wrote:
> On 4/16/09 Thu Apr 16, 2009 1:39 PM, "edw...@yandex.ru" <edw...@yandex.ru>
> scribbled:
>
>> Hello,
>>
>> I would like to know, how to force perl unfold "foreach" expression during
>> the
>> compilation, i.e. I want next code:
snip
> Can you explain why you want to do this?
snip
It is called loop unrolling and is a performance hack that many
compilers perform when passed a switch to optimize the code it
generates.
Perl does do some optimization (such as constant folding), but I do
not believe you have any control over it and I am nearly 100% certain
it does not unroll loops for you. If you have need of that sort of
micro-optimization you probably shouldn't be using Perl, or you should
be looking into using XS or Inline::C for the high cost loops.
snip
> I suggest you write a program that measures the execution times for the
> iterative-loop case and the unfolded case and compare them to see if you
> will gain enough execution time (if that is indeed what you are seeking) to
> justify the additional memory needed to contain the unwrapped block
> statements. (Were I to do this, I would write a Perl program to generate the
> Perl program!) See the Benchmark module.
snip
Manually unrolling the loop gives you a modest increase in speed for
1,000 items:
forloop: 500500
unrolled: 500500
Rate forloop unrolled
forloop 4034/s -- -31%
unrolled 5852/s 45% --
But almost no benefit for 100,000 items:
forloop: 5000050000
unrolled: 5000050000
Rate forloop unrolled
forloop 38.6/s -- -1%
unrolled 38.9/s 1% --
#!/usr/bin/perl
use strict;
use warnings;
use List::Util qw/sum/;
use Benchmark;
my $unrolled = 'sub unrolled { my @a; ';
for my $i (0 .. 1_000) {
$unrolled .= "\$a[$i] = $i;";
}
$unrolled .= 'return \...@a; }; 1';
eval $unrolled or die $@;
my %subs = (
forloop => sub {
my @a;
for my $i (0 .. 1_000) {
$a[$i] = $i;
}
return \...@a;
},
unrolled => \&unrolled
);
for my $sub (sort keys %subs) {
print "$sub: ", sum(@{$subs{$sub}->()}), "\n";
}
Benchmark::cmpthese(-2, \%subs);
--
Chas. Owens
wonkden.net
The most important skill a programmer can have is the ability to read.
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