[m-rev.] For review: Fix dummy type handling in the IL backend

[Jonathan Morgan]

With the below patch mercury-0.13 handles the IL backend correctly (as
well as it ever did).  I cannot test the patch on main, as
foreign_export does not yet work properly with the IL backend, but it
should work without change.

Jon
================================================================
Branches: release, main

Previously in the IL backend dummy variables were assigned to
private_builtin.dummy_var, which did not work when values were
assigned to it using the high-level data representation, as their type
was not the low-level object[].  This change changes the
representation of dummy variables from object[] (the low-level
representation) to object (suitable for high-level data), and causes
the IL backend to detect assignments to dummy_var and cast them to
type object before assigning to them.

compiler/mlds_to_il.m:
    Detect when an object is constructed and assigned to
private_builtin.dummy_var, and cast to il_generic_type (object) before
assigning the constructed object.

library/private_builtin.m:
    Change dummy_var's type from object[] to object.

Index: compiler/mlds_to_il.m
===================================================================
RCS file: /home/mercury1/repository/mercury/compiler/mlds_to_il.m,v
retrieving revision 1.161
diff -u -u -r1.161 mlds_to_il.m
--- compiler/mlds_to_il.m	29 Mar 2006 08:07:05 -0000	1.161
+++ compiler/mlds_to_il.m	27 Sep 2006 11:06:28 -0000
@@ -2056,7 +2056,23 @@
         ILArgTypes = list.map(mlds_type_to_ilds_type(DataRep), ArgTypes),
         list.map_foldl(load, Args, ArgsLoadInstrsTrees, !Info),
         ArgsLoadInstrs = tree_list(ArgsLoadInstrsTrees),
-        get_load_store_lval_instrs(Target, LoadMemRefInstrs,
+        % check if it's assigning to private_builtin.dummy_var
+        % If so, then it needs to be cast to il_generic_type.
+        (
+            Target = var(qual(MLDS_Module, QualKind, VarName), _),
+            VarName = mlds_var_name("dummy_var", _),
+            mercury_private_builtin_module(PrivateBuiltin),
+            MLDS_PrivateBuiltin = mercury_module_name_to_mlds(PrivateBuiltin),
+            mlds_append_wrapper_class(MLDS_PrivateBuiltin) = MLDS_Module
+        ->
+            MaybeCastInstrs = node([castclass(il_generic_type)]),
+            Target1 = var(qual(MLDS_Module, QualKind, VarName),
+                mlds_generic_type)
+        ;
+            MaybeCastInstrs = empty,
+            Target1 = Target
+        ),
+        get_load_store_lval_instrs(Target1, LoadMemRefInstrs,
             StoreLvalInstrs, !Info),
         CallCtor = newobj_constructor(ClassName, ILArgTypes),
         Instrs = tree_list([
@@ -2064,6 +2080,7 @@
             comment_node("new object (call constructor)"),
             ArgsLoadInstrs,
             instr_node(CallCtor),
+            MaybeCastInstrs,
             StoreLvalInstrs
         ])
     ;
@@ -3112,7 +3129,7 @@
     il_object_array_type.
 mlds_mercury_type_to_ilds_type(_, _, type_cat_tuple) = il_object_array_type.
 mlds_mercury_type_to_ilds_type(_, _, type_cat_enum) =  il_object_array_type.
-mlds_mercury_type_to_ilds_type(_, _, type_cat_dummy) =  il_object_array_type.
+mlds_mercury_type_to_ilds_type(_, _, type_cat_dummy) =  il_generic_type.
 mlds_mercury_type_to_ilds_type(_, _, type_cat_variable) = il_generic_type.
 mlds_mercury_type_to_ilds_type(DataRep, MercuryType, type_cat_type_info) =
     mlds_mercury_type_to_ilds_type(DataRep, MercuryType, type_cat_user_ctor).
Index: library/private_builtin.m
===================================================================
RCS file: /home/mercury1/repository/mercury/library/private_builtin.m,v
retrieving revision 1.154.2.3
diff -u -u -r1.154.2.3 private_builtin.m
--- library/private_builtin.m	21 Aug 2006 05:37:25 -0000	1.154.2.3
+++ library/private_builtin.m	27 Sep 2006 11:07:54 -0000
@@ -124,7 +124,7 @@
 // will be used by the code generator as an lval, so we use
 // private_builtin:dummy_var as that lval.

-public static object[] dummy_var;
+public static object dummy_var;

 ").
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