Clojure

locking expressions cause vars to be dereferenced, even if not executed, unless wrapped in let

Details

  • Type: Enhancement Enhancement
  • Status: Open Open
  • Priority: Minor Minor
  • Resolution: Unresolved
  • Affects Version/s: Release 1.5
  • Fix Version/s: None
  • Component/s: None
  • Labels:

Description

Description of one example with poor performance discovered by MichaƂ Marczyk in the discussion thread linked below.

The difference between the compiled versions of:

(defn foo [x]
  (if (> x 0)
    (inc x)
    (locking o
      (dec x))))

and

(defn bar [x]
  (if (> x 0)
    (inc x)
    (let [res (locking o
                (dec x))]
      res)))

is quite significant. foo gets compiled to a single class, with invocations handled by a single invoke method; bar gets compiled to a class for bar + an extra class for an inner function which handles the (locking o (dec x)) part – probably very similar to the output for the version with the hand-coded locking-part (although I haven't really looked at that yet). The inner function is a closure, so calling it involves an allocation of a closure object; its ctor receives the closed-over locals as arguments and stores them in two fields (lockee and x). Then they get loaded from the fields in the body of the closure's invoke method etc.

Note: The summary line may be too narrow a description of the root cause, and simply the first example of a case where this issue was noticed and examined. Please make the summary and this description more accurate if you diagnose this issue.

See discussion thread on Clojure group here: https://groups.google.com/forum/#!topic/clojure/x86VygZYf4Y

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  • Created:
    Updated: