Arc Forum | The gist of the papers are mostly this:Use a "Polymorphic Inline Cache" (PIC). B...

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2 points by almkglor 5980 days ago | link | parent

The gist of the papers are mostly this:

Use a "Polymorphic Inline Cache" (PIC). Basically if a piece of code could call several different methods, we figure out which one it is, then we create a copy of the calling function which does the type checking at the top and specialize all method calls to that type:

  (defm method ((t n my-type))
    (my-type::method n))
  (defm method ((t n your-type))
    (your-type::method n))
  (defm other-meth ((t n my-type))
    (my-type::other-meth n))
  (defm other-meth ((t n your-type))
    (your-type::other-meth n))

  (def general-function (n)
    (+ (method n) (other-meth n)))

  (general-function (your-type-creator 42))
  ===>
    (do
      (def general-function (n)
        (if (isa n 'your-type)
          (+ (your-type::method n) (your-type::other-meth n))
          (+ (method n) (other-meth n))))
      (general-function (your-type-creator 42)))

Everything else in the papers that go beyond the PIC is mostly about debugging and making the PIC lazy.

Edit:

Okay, I've been thinking. Basically the call* table is about specializing on 'apply, and we might think of 'defcall as:

  (defcall type (val . params)
    (code))
  ==>
  (defm apply ((t val type) . params)
    (code))

Could we possibly generalize this at the language level and make a PIC, say in arc2c/SNAP?

1 point by stefano 5980 days ago | link

To do the optimization when we see

  (general-function (your-type-creator 42))

we need a type inferencer to discover the return type of your-type-creator. The cache should also be able to change. For example I could write:

(general-function (your-type-creator 42))

(set your-type-creator another-type-creator)

(general-function (your-type-creator 42))

Now the optimization doesn't work if the cache doesn't change. This seems a rare case, though, and it's useless to optimize rare cases.

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1 point by almkglor 5979 days ago | link

Actually we don't: general-function is actually defined this way:

  (with (PIC (table) ;init empty table
         num-calls 0
         orig-fn
         (fn (n)
           (+ (method n) (other-meth n))))
    (def general-function (n)
      ((aif
         ; don't infer type: just look at the runtime type
         (PIC:type n)
            it
         (is optimization-trigger-level** (++ num-calls))
            (do (= num-calls 0)
                (= (PIC:type n)
                   (optimize* orig-fn (type n))))
            orig-fn)
        n)))

Basically s/type inference/just look at it directly/

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