diff: unify_gen.m bug fix

[Fergus Henderson]

Hi Zoltan,

It would be good if you could have a look at this one and
verify that the code does what it is supposed to do.

compiler/unify_gen.m:
	Fix another bug in the optimization of partial application
	of higher-order calls: the test for when to apply the optimization
	was wrong.  Also, generate better code for the case when you add
	zero arguments.

Index: unify_gen.m
===================================================================
RCS file: /home/staff/zs/imp/mercury/compiler/unify_gen.m,v
retrieving revision 1.78
diff -u -r1.78 unify_gen.m
--- unify_gen.m	1997/02/23 01:13:14	1.78
+++ unify_gen.m	1997/05/26 20:14:05
@@ -317,14 +317,14 @@
 % We handle currying of a higher-order pred variable as a special case.
 % We recognize
 %
-%	P = l(P0, A, B, C)
+%	P = l(P0, X, Y, Z)
 %
 %  where
 %
-%	l(P0, A, B, C, ...) :- call(P0, A, B, C, ...). % higher-order call
+%	l(P0, A, B, C, ...) :- P0(A, B, C, ...).  % higher-order call
 %
 % as a special case, and generate special code to construct the
-% new closure P from the old closure P0 by appending the args A, B, C.
+% new closure P from the old closure P0 by appending the args X, Y, Z.
 % The advantage of this optimization is that when P is called, we
 % will only need to do one indirect call rather than two.
 % (Hmm... is this optimization really worth it?  It probably
@@ -332,10 +332,12 @@
 %
 	{ proc_info_goal(ProcInfo, ProcInfoGoal) },
 	{ proc_info_interface_code_model(ProcInfo, CodeModel) },
+	{ proc_info_headvars(ProcInfo, ProcHeadVars) },
 	(
-		{ ProcInfoGoal = higher_order_call(_, _, _, _,
-					CallDeterminism) - _GoalInfo },
 		{ Args = [CallPred | CallArgs] },
+		{ ProcHeadVars = [ProcPred | ProcArgs] },
+		{ ProcInfoGoal = higher_order_call(ProcPred, ProcArgs, _, _,
+					CallDeterminism) - _GoalInfo },
 		{ determinism_to_code_model(CallDeterminism, CallCodeModel) },
 			% Check that the code models are compatible.
 			% Note that det is not compatible with semidet,
@@ -346,6 +348,11 @@
 		; CodeModel = model_non, CallCodeModel = model_det
 		}
 	->
+	    ( { CallArgs = [] } ->
+		% if there are no new arguments, we can just use the old
+		% closure
+		code_info__produce_variable(CallPred, Code, Value)
+	    ;
 		code_info__get_next_label(LoopEnd),
 		code_info__get_next_label(LoopStart),
 		code_info__acquire_reg(r, LoopCounter),
@@ -394,6 +401,7 @@
 		code_info__release_reg(NewClosure),
 		{ Code = tree(Code1, tree(Code2, Code3)) },
 		{ Value = lval(NewClosure) }
+	    )
 	;
 		{ Code = empty },
 		{ proc_info_arg_info(ProcInfo, ArgInfo) },

-- 
Fergus Henderson <fjh at cs.mu.oz.au>   |  "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh>   |  of excellence is a lethal habit"
PGP: finger fjh at 128.250.37.3         |     -- the last words of T. S. Garp.