common-subexpression elimination (cse)

Stephen Weeks MLton@sourcelight.com
Tue, 24 Jul 2001 16:05:55 -0700


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> But  that  (just replacing latter occurances with the name bound to the first
> result) isn't always correct.

Let me be more precise.  The later occurrences are replaced if the primitive is
functional (or may overflow) and the first occurrence dominates them in the
intraprocedural control-flow graph.  I believe this is safe.

> There is, of course, side effect things  which
> have  to  be duplicated, but the more serious example I was talking about was
> the delay due to lambda.
> 
> Consider something like
> 
>     let val magic: (unit -> int) option ref = ref NONE
>         fun f x =
>                (magic := SOME (fn () => 1 div x);
>                 1 div x)
>     in f 0
>           handle _ => 0;
>        valOf (! magic) ()
>     end
> 
> You have to raise the exception twice.

Agreed.  MLton simplifies this quite nicely.  Just to show off my fine new
control-flow graph feature, here is the control-flow graph that MLton produces
(not suitable for printing, just for viewing).  The important stuff is near the
top, starting at L_38.



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