[m-rev.] Handle polymorphic inequality goals

[Simon Taylor]

On 24-Oct-2002, Fergus Henderson <fjh at cs.mu.OZ.AU> wrote:
> Hmm.  Is the compiler really still capable of generating efficient code
> for `X < 42'?
> 
> It would need to do a quite complicated set of transformations:
> 
> 	X =< Y
> 
> ==> (via your change to simplification)
> 
>  	some [R] (compare(R, X, Y), R \= (>))
> 
> ==> (via specialization)
> 
>  	some [R] (
> 		builtin_compare_int(R, X, Y),
> 		R \= (>))
> 	)
> 
> ==> (via inlining)
> 
>  	some [R] (
> 		( X < Y ->
> 			R = (<)
> 		; X = Y ->
> 			R = (=)
> 		;
> 			R = (>)
> 		),
> 		\+ (R = (>))
> 	)
>
> ==> (how???)
> 
> 	\+ builtin_int_gt(X, Y)

==> (via local constraint propagation)

	some [R] (
 		( X < Y ->
 			R = (<),
 			\+ (R = (>))
 		; X = Y ->
 			R = (=),
 			\+ (R = (>))
 		;
 			R = (>),
 			\+ (R = (>))
 		)
 	)

==> (via simplification, this currently doesn't work because
mode_util__recompute_instmap_delta_unify doesn't do a very good job,
but it should be simple to fix it to handle this case)

	some [R] (
 		( X < Y ->
 			true
 		; X = Y ->
			true
 		;
 			fail		
 		)
 	)

I think this is as far as the Mercury compiler will get.
Unfortunately, gcc doesn't seem to be able to optimize this further.

Simon.
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