slicing does not seem to include the preallocation optimization

I don't know how you can say given that the test shows no benefit from pre-allocating[1].

But the reason the test shows no benefit from pre-allocating because the test is still flawed.

Lexical variables aren't freed when they go out of scope; they are kept around for use the next time the scope is entered. That means the hash is effectively pre-allocated for all tests![2]. 

$ perl -MDevel::Peek -e'
   sub x {
      my %h;
      Dump(%h, 0);
      keys(%h) = 100;
      Dump(%h, 0);
   }

   x() for 1..3;
' 2>&1 | grep MAX
  MAX = 7
  MAX = 127
  MAX = 127  <-- Preallocated even before C<< keys(%h) = 100; >>!
  MAX = 127
  MAX = 127  <-- Preallocated even before C<< keys(%h) = 100; >>!
  MAX = 127

[download]

Adding undef %h; should provide better results. 

$ perl -MDevel::Peek -e'
   sub x {
      my %h;
      Dump(%h, 0);
      keys(%h) = 100;
      Dump(%h, 0);
      undef %h;
   }

   x() for 1..3;
' 2>&1 | grep MAX
  MAX = 7
  MAX = 127
  MAX = 7
  MAX = 127
  MAX = 7
  MAX = 127

[download]
  1. The number are far too small to be meaningful, and one would expect the difference to grow as the hash size increases. (1 vs 2: 3%, 4%, 4%, 6%; 3 vs 4: -3%, 5%, 3%, 4%)
  2. Well, except on the first pass of a given size of a given test.

In reply to Re^3: Does "preallocating hash improve performance"? Or "using a hash slice"? by ikegami
in thread Does "preallocating hash improve performance"? Or "using a hash slice"? by vr

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.