Should multiplication by integer be avoided in favour of division for performance reasons?

vr has asked for the wisdom of the Perl Monks concerning the following question:

use strict;
use warnings;
use Benchmark 'cmpthese';

my @a = map log, 1..1e6;

cmpthese( -2, {
    1 => sub {[ map $_ * 4, @a ]},
    2 => sub {[ map $_ / (1/4), @a ]},
});

__END__

    Rate    1    2
1 7.70/s   -- -27%
2 10.6/s  38%   --
[download]

Fooling around with integers and floats (inspired by this question: Clean log_10 ?), I found (again!) unknown to me Perl behaviour, which can actually lead to performance issues. Maybe it's open secret and was described 20 years ago. Couldn't come up with good Google query, found nothing, hence this question/observation.

It looks like if scalar contains NV, and though it never has been IV, and another operand is IV, and operator is e.g. multiplication (and some other operators, but not, important, division), then Perl checks if this scalar can be promoted (or is it "demoted"?) to IV, and if so, then result will be IV.

>perl -MDevel::Peek -E "$x=2/1; Dump$x; $y=$x*2; Dump$x; Dump$y"
SV = NV(0xd75628) at 0xd75640
  REFCNT = 1
  FLAGS = (NOK,pNOK)
  NV = 2
SV = PVNV(0xe26078) at 0xd75640
  REFCNT = 1
  FLAGS = (IOK,NOK,pIOK,pNOK)
  IV = 2
  NV = 2
  PV = 0
SV = IV(0xd75a80) at 0xd75a90
  REFCNT = 1
  FLAGS = (IOK,pIOK)
  IV = 4
[download]

Though it means some amount of extra work, it could be argued that Perl tries to do "a right thing". However, with arrays, this helpful (?) optimization becomes an anti-DWIM, I think. If array contains just a single element which "looks like integer" (log 1, above), then "optimization" is attempted for all elements. Try range 2..1e6, then there's no penalty. I understand, that (a) as it's said somewhere in FAQ, Perl is not well suited for number crunching, and (b), if properly profiled, difference between multiplication and division by reciprocal somewhere in real program will probably be tiny. Fun fact, nevertheless.

Comment on Should multiplication by integer be avoided in favour of division for performance reasons? Select or Download Code

Replies are listed 'Best First'.
Re: Should multiplication by integer be avoided in favour of division for performance reasons? by Anonymous Monk on Nov 29, 2019 at 01:44 UTC
Use strings instead of subs and the benchmark numbers will be closer to reality	[reply]
Re^2: Should multiplication by integer be avoided in favour of division for performance reasons? (benchmark pitfalls) by Anonymous Monk on Nov 29, 2019 at 03:37 UTC
Its your basic benchmark pitfall, the overhead overshadows the thing you're timing #!/usr/bin/perl -- use strict; use warnings; use Benchmark 'cmpthese'; my @a = map log, 1..1e6; cmpthese( -2, { 1 => sub {[ map $_ * 4, @a ]}, 2 => sub {[ map $_ / (1/4), @a ]}, }); cmpthese( -2, { 1 => sub {[ map $_ * 4, @a ]}, 2 => sub {[ map $_ / (1/4), @a ]}, }); cmpthese( -2, { 1 => sub {[ map $_ * 4, @a ]}, 2 => sub {[ map $_ / (1/4), @a ]}, }); cmpthese( -2, { 1 => '[ map $_ * 4, @a ]', 2 => '[ map $_ / (1/4), @a ]', }); cmpthese( -2, { 1 => '[ map $_ * 4, @a ]', 2 => '[ map $_ / (1/4), @a ]', }); cmpthese( -2, { 1 => '[ map $_ * 4, @a ]', 2 => '[ map $_ / (1/4), @a ]', }); __END__ [download] But as you can see, there is no difference between the two Rate 1 2 1 1.73/s -- -18% 2 2.11/s 22% -- Rate 1 2 1 1.70/s -- -19% 2 2.11/s 24% -- Rate 1 2 1 1.75/s -- -17% 2 2.10/s 20% -- Rate 1 2 1 2371151/s -- -3% 2 2438111/s 3% -- Rate 2 1 2 2272556/s -- -8% 1 2473993/s 9% -- Rate 2 1 2 2140327/s -- -8% 1 2313868/s 8% --	[reply] [d/l]
Re^3: Should multiplication by integer be avoided in favour of division for performance reasons? (benchmark pitfalls) by swl (Parson) on Nov 29, 2019 at 07:30 UTC
I know there are overheads to calling subs, but did not think they were sufficiently large that inline code runs millions of times per second while code refs run at fewer than ten times per second. Purely out of curiosity, are the process and overheads documented anywhere? I can think of things like setting up @_ and other bookkeeping processes, but perhaps there are other steps.	[reply]
Re^4: Should multiplication by integer be avoided in favour of division for performance reasons? (benchmark pitfalls) by tobyink (Canon) on Nov 29, 2019 at 08:19 UTC
Re^5: Should multiplication by integer be avoided in favour of division for performance reasons? (benchmark pitfalls) by swl (Parson) on Nov 29, 2019 at 22:53 UTC
Re^3: Should multiplication by integer be avoided in favour of division for performance reasons? (benchmark pitfalls) by vr (Curate) on Nov 29, 2019 at 08:57 UTC
There is no `@main::a`. You are benchmarking couple of no-ops. `use strict; use warnings; use Benchmark qw/ cmpthese timeit /; our @b = my @a = map log, 1..1e6; timeit( 1, 'print "\$#a = $#a\n";' ); timeit( 1, 'print "\$#b = $#b\n";' ); cmpthese( -2, { a => '[ map $_ * 4, @a ]', b => '[ map $_ * 4, @b ]', }); cmpthese( -2, { 1 => '[ map $_ * 4, @b ]', 2 => '[ map $_ / (1/4), @b ]', }); __END__ $#a = -1 $#b = 999999 Rate b a b 7.70/s -- -100% a 10680591/s 138780824% -- Rate 1 2 1 7.76/s -- -28% 2 10.7/s 39% --` [download]	[reply] [d/l] [select]
Re^4: Should multiplication by integer be avoided in favour of division for performance reasons? (benchmark pitfalls) by dave_the_m (Monsignor) on Nov 29, 2019 at 14:35 UTC
Re^5: Should multiplication by integer be avoided in favour of division for performance reasons? (benchmark pitfalls) by vr (Curate) on Nov 29, 2019 at 16:24 UTC
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