keys builds the whole list prior to usage

I thought I remember reading somewhere that keys has an optimization in this case, and the whole list is not actually built up front. I couldn't find any documentation, so I decided to do a few tests. Perhaps my test code is flawed in some way I don't see, but the results seem to agree with my premise.

Here are my snippets:

Snippet a

perl -le '%hash = (1 .. COUNT); print ">"; <STDIN>'

Snippet b

perl -le '%hash = (1 .. COUNT); @keys = keys %hash; print ">"; <STDIN>'

Snippet c

perl -le '%hash = (1 .. COUNT); for (keys %hash) { print ">"; <STDIN> }'

Here's a table with the results:

COUNTa memb memc mema-b changea-c change
100,00011,612 13,008 12,212 1,396 (12.0%) 600 (5.17%)
1,000,00097,320111,288103,32413,968 (14.4%)6,004 (6.17%)

Update: I should have mentioned earlier, these are from perl 5.005_03 on FreeBSD 4.9-STABLE. I got the value from the VSZ column in ps aux output.

The change from snippet a to b is much larger than the change from a to c. This seems consistent with the optimization I thought should occur. There may be overhead in the array @keys, but I wouldn't think it would be so much more than building an equivalent list. Maybe someone can explain this another way?

In reply to Re^3: Optimization, side-effects and wrong code by revdiablo
in thread Optimization, side-effects and wrong code by cbraga

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