diff: code generation bug fix

[Fergus Henderson]

Tom and Zoltan, can you both please review this change?

Fix a code generation bug that broke Tom's mediancut.m program.

compiler/code_exprn.m:
	Change code_exprn__lval_in_use/3 to also check the registers marked
	in use, not just the variables marked in use. 
	
	This avoids problems in the following case:
		- a register R was marked in use, since it was the target
		  register we decided to use to compute variable X;
		- before variable X was given status `evaled(R)' we had to
		  first produce some other variable Y that X depended on;
		- Y was placed in register R (since that was not considered
		  in use);
		- we then assigned X to R, because that was
		  the location we had decided to place X in,
		  thus clobbering Y;
		- subsequently we used R, expecting it to hold Y.

	This change increases the code size of the compiler on DEC Alpha
	by 100k (2.7%).  :-(

Index: code_exprn.m
===================================================================
RCS file: /home/mercury1/repository/mercury/compiler/code_exprn.m,v
retrieving revision 1.51
diff -u -u -r1.51 code_exprn.m
--- code_exprn.m	1997/07/27 14:59:55	1.51
+++ code_exprn.m	1997/11/21 15:32:25
@@ -494,33 +494,21 @@
 %------------------------------------------------------------------------------%
 
 code_exprn__lval_in_use(Lval) -->
-	code_exprn__lval_in_use_except(Lval, no).
-
-:- pred code_exprn__lval_in_use_except(lval, maybe(var),
-	exprn_info, exprn_info).
-:- mode code_exprn__lval_in_use_except(in, in, in, out) is semidet.
-
-code_exprn__lval_in_use_except(Lval, MaybeVar) -->
 	code_exprn__get_vars(Vars),
-	{ map__to_assoc_list(Vars, VarStatList) },
-	{ code_exprn__lval_in_use_2(Lval, MaybeVar, VarStatList) }.
-
-	% The auxiliary predicate is needed to allow Mercury to recognize
-	% that the subcomputation has no outputs without using existential
-	% quantification, whose syntax SICStus doesn't like.
+	code_exprn__get_regs(Regs),
+	(
+		{ bag__contains(Regs, Lval) }
+	;
+		{ map__to_assoc_list(Vars, VarStatList) },
+		{ code_exprn__lval_in_use_by_vars(Lval, VarStatList) }
+	).
 
-:- pred code_exprn__lval_in_use_2(lval, maybe(var), assoc_list(var, var_stat)).
-:- mode code_exprn__lval_in_use_2(in, in, in) is semidet.
+:- pred code_exprn__lval_in_use_by_vars(lval, assoc_list(var, var_stat)).
+:- mode code_exprn__lval_in_use_by_vars(in, in) is semidet.
 
-code_exprn__lval_in_use_2(Lval, MaybeVar, VarStatList) :-
+code_exprn__lval_in_use_by_vars(Lval, VarStatList) :-
 	list__member(VarStat, VarStatList),
-	VarStat = Var - Stat,
-	(
-		MaybeVar = no
-	;
-		MaybeVar = yes(ExceptionVar),
-		\+ Var = ExceptionVar
-	),
+	VarStat = _Var - Stat,
 	(
 		Stat = cached(Rval),
 		exprn_aux__rval_contains_lval(Rval, Lval)
@@ -1163,7 +1151,7 @@
 	code_exprn__add_lval_reg_dependencies(Lval),
 	code_exprn__maybe_get_var_name(MaybeVar, VarName),
 	( { IsConst = yes } ->
-		{ string__append("Assigning from ", VarName, Comment) },
+		{ string__append("Assigning from const ", VarName, Comment) },
 		{ ExprnCode = node([assign(Lval, Rval2) - Comment]) }
 	;
 		( { StandAlone = yes } ->

-- 
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.