Re^4: Efficient bit-twiddling in Perl.

in reply to Re^3: Efficient bit-twiddling in Perl.
in thread Efficient bit-twiddling in Perl.

Your benchmark is not very realistic. As $n doesn't vary, you are completely ignoring the effect of the CPU cache misses that the lookup table may introduce.

The version of my benchmark I posted was the one I use -- in conjunction with -N=1 and uncommenting the print "@_"; -- to check that the output from all the methods is correct.

For timing runs, I operate on the loop counter $_, which when -N=10e6, is enough to exercise the full range of the table 38 times.

And I still get 20%+ improvement from the AoAs lookup:

Results

C:\test>1021064
Shift&and took: 0.000000606833 seconds
  Lookup  took: 0.000000485629 seconds
  Lookup2 took: 0.000000669586 seconds
(un)pack  took: 0.000007564075 seconds
[download]

In general, I've found it very difficult to detect, much less measure any discernible effect from cpu caching in perl code on my machine. It shows up readily in tight loops in C code, but Perl's memory allocation is so cache unfriendly that it rarely seems to come into play.

I would be interested to see the output from the above on your machine if you have a coupe of minutes.

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