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

Description

Description of one example with poor performance discovered by Michał 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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Kevin Downey April 18, 2014 at 8:17 AM

maybe it is already clear to others, but this was not immediately clear to me:

the reason

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

generates a second class is locking is a macro that contains a try/finally form in it's expansion.

binding the result of a try/finally form to a result (as in the let) would require some real tricky code gen without adding the extra function, so of course the clojure compile adds the extra function.

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Created November 17, 2013 at 10:07 PM
Updated April 18, 2014 at 8:17 AM

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