Most of the above thread is, err, basically wrong! The two Concise outputs show that the only two ops executed in each case are the GV (to get the glob containing the global hash variable) followed by a MULTIDEREF op. The ex-helem and ex-exists are ops that were optimised away and replaced with the MULTIDEREF op (on perl 5.22.0 and later, anyway) - they are no longer on the execution path.

The difference in performance between exists and a plain hash lookup is negligible, although exists should be slightly faster. The slowdown being seen is due to assigning a boolean return value to a scalar. Perl's boolean values (as returned by exists etc) are multi-valued scalars containing an int value of 0/1, a floating value of 0.0/1.0 and a string value of ""/"1". If this value is assigned to a scalar, all three parts need to be copied, including the string. Boolean values are optimised for use in conditionals. If you replace the assignment with something like $xx_global = exists $hash{$key2} ? 1 : 2 you'll see exists becomes faster than a hash lookup.

Dave.

In reply to Re^7: "exists $hash{key}" is slower than "$hash{key}" by dave_the_m
in thread "exists $hash{key}" is slower than "$hash{key}" by swl

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.