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Date: -- (:)
From: Jacques Garrigue <garrigue@k...>
Subject: Re: [Caml-list] Stupid question
From: skaller <skaller@tpg.com.au>
> On Thu, 2004-01-15 at 11:03, Brian Hurt wrote:
> > I was poking around in the produced assembly code of some ocaml code of 
> > mine, and I noticed something.  The construct:
> > 	if (x < m) <> (y < m) then
> > 		...
> > 
> > when the compiler knows that x, y, and m are all ints, it calls an 
> > external C routine to compare the two booleans.  But the construct:
> > 
> > 	if (x < m) != (y < m) then
> > 		...
> > 
> > does not.  Now, this isn't a big deal- I just replaced <> with != and no 
> > problem.  But I was kind of interested in why the compiler didn't catch 
> > and optimize this.  Is there some sort of subtle semantic difference I'm 
> > too dense to see?
> 
> Well, <> is a polymorphic value comparison.
> Whereas != is a physical inequality comparison.

Yes indeed.
But the compiler is clever enough to use specific comparison functions
when the type is known to be int or float.
Since bool looks like a subset of int, one might expect it to be
optimised also. However, bool is just a normal sum type, and since
some sum types have non-constant constructors, in general sum types
require a recursive comparison operation.
For types where constructors are constant, this might be optimised...

Jacques Garrigue

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