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Inside the mind of the inliner
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Date: -- (:)
From: Yaron Minsky <yminsky@g...>
Subject: Inside the mind of the inliner
I've been doing some experiments with the OCaml inliner, and have
walked away from the process very confused.  It seems like inlining
can be prevented by very simple changes to the code of a function.
The main surprise for me is that adding a quite trivial allocation of
a list or a string literal defeats the inliner.

Does anyone have a better understanding of what's going on here?  I
feel like my intuition for this stuff is terrible.

I checked inlining using the following command line:

  ocamlopt -S -inline 10000 z.ml ; egrep 'call.*camlZ__f' z.s

And here are the different variants of z.ml I tried.

(* Simple arithmetic.  f is inlined *)
let f x = x + x
let g x = f x + f x

(* Add in allocation of a list, not inlined *)
let f x = ignore [1]; x + x
let g x = f x + f x

(* allocate a string, not inlined *)
let f x = ignore "foo"; x + x
let g x = f x + f x

(* reference to the empty list, inlined *)
let f x = ignore []; x + x
let g x = f x + f x

(* call a function that iterates over a list, inlined *)
let list = [1;2;3]
let plus x y = x + y
let f x = x * List.fold_left plus 0 list
let g x = f x + f x

(* Call a function that includes an infix operator in prefix form,
   not inlined. *)
let list = [1;2;3]
let f x = x * List.fold_left (+) 0 list
let g x = f x + f x

(* Allocate the list in the function, not inlined *)
let plus x y = x + y
let f x = x * List.fold_left plus 0 [1;2;3]
let g x = f x + f x

(* call a function to allocate your list, inlined *)
let plus x y = x + y
let create_list x = x :: x + 1 :: x + 2 :: []
let f x = x * List.fold_left plus 0 (create_list 1)
let g x = f x + f x

I've tried these experiments with ocaml 3.10.1 and 3.11.1, with similar
results.

y