Is Perl's unpack() ever faster than substr()?
Several times I\'ve read that unpack() is faster than substr(), especially as the number of substrings increases. However, this benchmark suggests
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As a matter of fact, your benchmark is flawed, in a really, really interesting way, but what it boils down to is that what you are really comparing is the relative efficiency with which unpack vs. map can throw away a list, because Benchmark::cmpthese() is executing the functions in void context.
The reason your substr comes out on top is this line of code in pp_ctl.c pp_mapwhile():
if (items && gimme != G_VOID) {i.e. perl's map magically skips a bunch of work (namely allocating storage for the results of the map) if it knows it is being called in void context!
(My hunch on the windows vs. other seen above is that windows-based perl memory allocation is awful, so skipping the allocation is a bigger savings there -- just a hunch, though, I don't have a windows box to play with. But the actual unpack implementation is straight-up C code, and shouldn't differ substantially from windows to other.)
I have three different solutions for working around this issue and generating a more fair comparison:
- assign the list to an array
- loop over the list inside the function, and return nothing
- return a reference to the list (hiding the void context)
Here's my version of %methods, with all three versions:
my %methods = ( unpack_assign => sub { my @foo = unpack $format_string, $data; return }, unpack_loop => sub { for my $foo (unpack $format_string, $data) { } }, unpack_return_ref => sub { return [ unpack $format_string, $data ] }, unpack_return_array => sub { return unpack $format_string, $data }, substr_assign => sub { my @foo = map {substr $data, $_, 1} 0 .. ($n_substrings - 1) }, substr_loop => sub { for my $foo ( map {substr $data, $_, 1} 0 .. ($n_substrings - 1)) { } }, substr_return_ref => sub { return [ map {substr $data, $_, 1} 0 .. ($n_substrings - 1) ] }, substr_return_array => sub { return map { substr $data, $_, 1} 0 .. ($n_substrings - 1) }, );And my results:
$ perl -v This is perl, v5.10.0 built for x86_64-linux-gnu-thread-multi $ perl foo.pl 10 Rate substr_assign substr_return_ref substr_loop unpack_assign unpack_return_ref unpack_loop unpack_return_array substr_return_array substr_assign 101915/s -- -20% -21% -28% -51% -51% -65% -69% substr_return_ref 127224/s 25% -- -1% -10% -39% -39% -57% -62% substr_loop 128484/s 26% 1% -- -9% -38% -39% -56% -61% unpack_assign 141499/s 39% 11% 10% -- -32% -32% -52% -57% unpack_return_ref 207144/s 103% 63% 61% 46% -- -1% -29% -37% unpack_loop 209520/s 106% 65% 63% 48% 1% -- -28% -37% unpack_return_array 292713/s 187% 130% 128% 107% 41% 40% -- -12% substr_return_array 330827/s 225% 160% 157% 134% 60% 58% 13% -- 100 Rate substr_assign substr_loop substr_return_ref unpack_assign unpack_return_ref unpack_loop unpack_return_array substr_return_array substr_assign 11818/s -- -25% -25% -26% -53% -55% -63% -70% substr_loop 15677/s 33% -- -0% -2% -38% -40% -51% -60% substr_return_ref 15752/s 33% 0% -- -2% -37% -40% -51% -60% unpack_assign 16061/s 36% 2% 2% -- -36% -39% -50% -59% unpack_return_ref 25121/s 113% 60% 59% 56% -- -4% -22% -35% unpack_loop 26188/s 122% 67% 66% 63% 4% -- -19% -33% unpack_return_array 32310/s 173% 106% 105% 101% 29% 23% -- -17% substr_return_array 38910/s 229% 148% 147% 142% 55% 49% 20% -- 1000 Rate substr_assign substr_return_ref substr_loop unpack_assign unpack_return_ref unpack_loop unpack_return_array substr_return_array substr_assign 1309/s -- -23% -25% -28% -52% -54% -62% -67% substr_return_ref 1709/s 31% -- -3% -6% -38% -41% -51% -57% substr_loop 1756/s 34% 3% -- -3% -36% -39% -49% -56% unpack_assign 1815/s 39% 6% 3% -- -34% -37% -48% -55% unpack_return_ref 2738/s 109% 60% 56% 51% -- -5% -21% -32% unpack_loop 2873/s 120% 68% 64% 58% 5% -- -17% -28% unpack_return_array 3470/s 165% 103% 98% 91% 27% 21% -- -14% substr_return_array 4015/s 207% 135% 129% 121% 47% 40% 16% -- 10000 Rate substr_assign substr_return_ref substr_loop unpack_assign unpack_return_ref unpack_loop unpack_return_array substr_return_array substr_assign 131/s -- -23% -27% -28% -52% -55% -63% -67% substr_return_ref 171/s 30% -- -5% -6% -38% -42% -52% -57% substr_loop 179/s 37% 5% -- -1% -35% -39% -50% -55% unpack_assign 181/s 38% 6% 1% -- -34% -38% -49% -55% unpack_return_ref 274/s 109% 60% 53% 51% -- -6% -23% -32% unpack_loop 293/s 123% 71% 63% 62% 7% -- -18% -27% unpack_return_array 356/s 171% 108% 98% 96% 30% 21% -- -11% substr_return_array 400/s 205% 134% 123% 121% 46% 37% 13% -- 100000 Rate substr_assign substr_return_ref substr_loop unpack_assign unpack_return_ref unpack_loop unpack_return_array substr_return_array substr_assign 13.0/s -- -22% -26% -29% -51% -55% -63% -67% substr_return_ref 16.7/s 29% -- -5% -8% -37% -43% -52% -58% substr_loop 17.6/s 36% 5% -- -3% -33% -40% -50% -56% unpack_assign 18.2/s 40% 9% 3% -- -31% -37% -48% -54% unpack_return_ref 26.4/s 103% 58% 50% 45% -- -9% -25% -34% unpack_loop 29.1/s 124% 74% 65% 60% 10% -- -17% -27% unpack_return_array 35.1/s 170% 110% 99% 93% 33% 20% -- -12% substr_return_array 39.7/s 206% 137% 125% 118% 50% 36% 13% --So back to the original question: "is unpack() ever faster than substr()?" Answer: always, for this type of application -- unless you don't care about the return values ;)
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