On Sun Jun 16 16:51:41 2013, Colin Fleming wrote:
Hi all,
This is slightly tangential to the current discussion on unnecessary
type checks - does anyone have any good links to information about the
JIT optimisations performed by Hotspot? One question I've been
interested in recently is how well it can optimise Clojure function
calls. The compiled classes of Clojure fns cache references to the
vars of functions that they call with the bytecode equivalent of:
private static Var const__0 = RT.var("namespace", "var")
and the calls are like:
((IFn) const__0.getRawRoot()).invoke(args)
for non-dynamic bindings (the default from 1.3+ IIRC), and
((IFn) const__0.get()).invoke(args)
Does anyone know if Hotspot is capable of optimising these calls?
Another question would be why in the non-dynamic case the IFn
contained in the var is not cached since the root binding should never
be modified. Is this just to support alter-var-root, or is it to
support declare'd vars which may not contain an IFn at compilation
time? I would imagine that Hotspot would optimise this case much
better since this mimics what objects in Java tend to do.
Cheers,
Colin
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Fogus touched on this issue in this post about invoke dynamic:
http://blog.fogus.me/2011/10/14/why-clojure-doesnt-need-invokedynamic-but-it-might-be-nice/
From the post:
"To provide this level of flexibility Clojure establishes a level of
indirection.3 Specifically, all function lookups through a Var occur,
at the lowest level, through an atomic volatile.4 This happens every
time that a function bound using the def/defn special forms is called.
This indirection is not amenable to HotSpot optimizations.
This is a great place for invokedynamic, especially during production
scenarios where the root value of Vars remain stable. That is,
invokedynamic would eliminate the volatile lookup on every call and
likely lead to HotSpot inlining."
-Ben
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