I've been working on an infrastructure module that needs to be as fast as possible. As I don't have the time to trudge the XS learning curve, I've been working on speeding the Perl up.

One of the things I found when examining usage is that there's an awful lot of the following: 

sub foo
{
    .....

    return @foo;
}

my $num_foo = foo( @args );

[download]

So, I was thinking - would using wantarray be faster than not? Or, alternately, does Perl implicitly use wantarray for you? Out comes Benchmark!

Using 5.8.0 on Solaris9 ... 

my @x = 1 .. 1_000_000;
sub want { return wantarray ? @x : $#x + 1; }
sub nowant { return @x; }

cmpthese( -10, {
    '1nowant' => sub { my $x = nowant(); },
    '2want'   => sub { my $x = want(); },
    '3nowant_list' => sub { my $x = () = nowant(); },
    '4want_list'   => sub { my $x = () = want(); },
});
--------
Benchmark: running 1nowant, 2want, 3nowant_list, 4want_list for at lea
+st 10 CPU seconds...
   1nowant: 10 wallclock secs (10.25 usr +  0.00 sys = 10.25 CPU) @ 58
+2570.73/s (n=5971350)
     2want: 10 wallclock secs (10.05 usr +  0.00 sys = 10.05 CPU) @ 57
+8237.81/s (n=5811290)
3nowant_list: 11 wallclock secs (10.84 usr +  0.00 sys = 10.84 CPU) @ 
+ 1.11/s (n=12)
4want_list: 11 wallclock secs (10.66 usr +  0.00 sys = 10.66 CPU) @  1
+.13/s (n=12)
                 Rate 3nowant_list   4want_list        2want      1now
+ant
3nowant_list   1.11/s           --          -2%        -100%        -1
+00%
4want_list     1.13/s           2%           --        -100%        -1
+00%
2want        578238/s    52234049%    51366692%           --          
+-1%
1nowant      582571/s    52625456%    51751600%           1%          
+ --

[download]

The results seem to say that Perl puts a wantarray in for you, if it seems appropriate. What looks to be proof to me is that if you force list context, but still assign to a scalar, you lose a ton of speed. In fact, wantarray seems to be slightly slower. (It actually isn't, if you reverse the order of running. Plus, my personal rule of thumb is that anything which doesn't provide 5% or more of difference is a wash.)

So, it seems that this micro-optimization actually doesn't help.

Update: Fixed the code and re-ran it. The values changed a bit. 

Benchmark: running 1nowant, 2want, 3nowant_list, 4want_list for at lea
+st 10 CPU seconds...
   1nowant: 13 wallclock secs (10.67 usr +  0.00 sys = 10.67 CPU) @ 55
+8715.46/s (n=5961494)
     2want: 12 wallclock secs (10.66 usr +  0.00 sys = 10.66 CPU) @ 39
+0887.71/s (n=4166863)
3nowant_list: 11 wallclock secs (10.75 usr +  0.00 sys = 10.75 CPU) @ 
+ 1.12/s (n=12)
4want_list: 11 wallclock secs (10.72 usr +  0.00 sys = 10.72 CPU) @  1
+.12/s (n=12)
                 Rate 3nowant_list   4want_list        2want      1now
+ant
3nowant_list   1.12/s           --          -0%        -100%        -1
+00%
4want_list     1.12/s           0%           --        -100%        -1
+00%
2want        390888/s    35016924%    34919202%           --         -
+30%
1nowant      558715/s    50051494%    49911815%          43%          
+ --

[download]

Using wantarray actually seems to be a penalty, not an improvement.

------
We are the carpenters and bricklayers of the Information Age.

Then there are Damian modules.... *sigh* ... that's not about being less-lazy -- that's about being on some really good drugs -- you know, there is no spoon. - flyingmoose

I shouldn't have to say this, but any code, unless otherwise stated, is untested

In reply to Optimizing with wantarray by dragonchild

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