Re^5: Should multiplication by integer be avoided in favour of division for performance reasons? (benchmark pitfalls)

Thank you for looking into this, dave_the_m. But there's still performance issue if integer, or float that appears to be integer, is not the first, but last element of array. Then there's just one expensive-to-free element in derived array?

use strict;
use warnings;
use Devel::Peek;
use Benchmark qw/ cmpthese timeit /;

our @a = map log, 2..1e6; 
push @a, 0/1;
print "************   a   ************\n";
Dump $a[-2];
Dump $a[-1];
my @b = map $_ * 4, @a;
print "************   b   ************\n";
Dump $b[-2];
Dump $b[-1];
 
cmpthese( -2, {
    1 => 'my @c = map $_ * 4, @a',
    2 => 'my @c = map $_ / (1/4), @a',
});

__END__

************   a   ************
SV = NV(0x55f0580) at 0x55f0598
  REFCNT = 1
  FLAGS = (NOK,pNOK)
  NV = 13.8155105579643
SV = NV(0x102a388) at 0x102a3a0
  REFCNT = 1
  FLAGS = (NOK,pNOK)
  NV = 0
************   b   ************
SV = NV(0xae879c8) at 0xae879e0
  REFCNT = 1
  FLAGS = (NOK,pNOK)
  NV = 55.2620422318571
SV = PVNV(0x714ec8) at 0xae879f8
  REFCNT = 1
  FLAGS = (IOK,pIOK)
  IV = 0
  NV = 0
  PV = 0
    Rate    1    2
1 10.3/s   -- -20%
2 12.8/s  25%   --
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Comment on Re^5: Should multiplication by integer be avoided in favour of division for performance reasons? (benchmark pitfalls) Download Code

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Re^6: 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 19:32 UTC

eval q[
    for my $i (1..$n) {
        my @c = map $_ * 4, @a;
    }
]
[download]

It's also the case that multiply is fractionally slower at returning a PVNV value rather than an NV value.

Dave.

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