[racket-dev] Inline caching (was Re: my '312' this semester, how we compare to others)

On 2011-05-04 12:04 PM, Matthias Felleisen wrote:
> I still believe that the Java implementation (just under 1s without
> their 'Google' contract) benefits from typed dispatches.

Maybe it does, but it's almost certain that it is benefiting from inline 
caching at send sites (i.e. dynamic type information) much more than it 
will be benefiting from static type information.

A quick-and-dirty comparison of raw send performance on my Mac:

   Language     Virtual machine          Nanoseconds per send
  ------------------------------------------------------------
   Java         Hotspot 64-bit 1.6.0_24      1.4
   Smalltalk    Cog r2382                   21
   Smalltalk    SqueakVM 4.2.4beta1U       122
   Racket       Racket v5.1               ~350

Note that Cog is a JITting VM and SqueakVM is a plain (but very well 
optimised) interpreter. Both Cog and SqueakVM use a couple of levels of 
method lookup cache.

A simple experiment I just performed suggests that a monomorphic inline 
cache hit can reduce the time needed for a send in Racket from 350ns to 
around 60ns, which is a massive win. I've attached the program I used to 
measure this, FWIW. (Run it using command-line Racket, not DrRacket: I 
got some *very* weird timing artifacts out of DrRacket during this 
experiment!)

The question, then, is: how do we implement MICs or PICs using Racket's 
macro system? Each send site needs to expand into

  - a piece of global state
  - a test involving that global state
  - a possible update to that global state

Hypothesising some kind of (let-static) form that introduces a 
lexically-scoped piece of global state, this kind of thing might Just 
Work to provide a speedup of almost six-fold on almost-monomorphic 
send-heavy code:

(define-syntax cached-send
   (syntax-rules ()
     ((_ obj msg arg ...)
      (let-static ((bc (box #f))
	          (bm (box #f)))
        (let* ((tmp obj)
	      (cls (object-ref tmp)))
	 (if (eq? (unbox bc) cls)
	     ((unbox bm) tmp arg ...)
	     (let-values (((method _)
                            (find-method/who 'send tmp 'msg)))
	       (set-box! bc cls)
	       (set-box! bm method)
	       (method tmp arg ...))))))))

Regards,
   Tony
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