in reply to $_ vs. argument passing
One of the reasons why the arg version is so much faster is because of a little-known feature of Perl: the arguments to functions are passed by-reference. That means when you use $_[0], you're actually referring to the constant 'foobar' which is embedded in the Perl bytecode.
But if you're worried about performance, keep in mind that your slowest one executed in an average of 5.5 microseconds, which on a short-running script isn't terrible.
On the other hand, your $_ = 'foobar' induces a string copy of 'foobar' every single time that statement is executed. Since calculating the length is easy, the additional overhead of the extra copying *really* stands out.
I added these two subs to the benchmark:
And reran it (dropping the time from 60s each to 10s each, because I'm far too impatient to wait six minutes for the test to rerun):sub xarg { my $tmp = $_[0]; length $tmp } sub xarg_shift { my $tmp = shift; length $tmp }
Note that they're still not on equal footing: the additional symbol table lookup needed to find the location of $_ *really* hurts. This is because the work needed to find $_ in the symbol table is far greater than the work needed to find the length of it (because the length is precomputed).Benchmark: running arg, arg_shift, noarg, noarg2, xarg, xarg_shift for + at least 10 CPU seconds... arg: 10 wallclock secs (10.52 usr + -0.01 sys = 10.51 CPU) @ 48 +9653.09/s (n=5146254) arg_shift: 10 wallclock secs (10.28 usr + 0.00 sys = 10.28 CPU) @ 41 +4663.33/s (n=4262739) noarg: 9 wallclock secs (10.00 usr + 0.00 sys = 10.00 CPU) @ 28 +7455.40/s (n=2874554) noarg2: 11 wallclock secs (10.55 usr + 0.00 sys = 10.55 CPU) @ 28 +2535.73/s (n=2980752) xarg: 10 wallclock secs (10.59 usr + 0.00 sys = 10.59 CPU) @ 33 +7761.28/s (n=3576892) xarg_shift: 10 wallclock secs (10.39 usr + 0.01 sys = 10.40 CPU) @ 29 +7917.40/s (n=3098341) Rate noarg2 noarg xarg_shift xarg arg_shift + arg noarg2 282536/s -- -2% -5% -16% -32% + -42% noarg 287455/s 2% -- -4% -15% -31% + -41% xarg_shift 297917/s 5% 4% -- -12% -28% + -39% xarg 337761/s 20% 18% 13% -- -19% + -31% arg_shift 414663/s 47% 44% 39% 23% -- + -15% arg 489653/s 73% 70% 64% 45% 18% + --
But if you're worried about performance, keep in mind that your slowest one executed in an average of 5.5 microseconds, which on a short-running script isn't terrible.
--Stevie-O
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