[m-rev.] diff: typecheck speedup

[Zoltan Somogyi]

Avoid some redundant work in typechecking. This diff speeds up tools/speedtest
by about 2.5%.

compiler/typeclasses.m:
	Do not perform context reduction for typeclass checking if the current
	context has no typeclass constraints.

	Iterate over lists of constraints using det code, not nondet.
	Move the predicate concerned next to the one place where it is used.
	Give it a more meaningful name.

	Use the new capability in typecheck_info.m.

compiler/typecheck_info.m:
	Provide a predicate for setting several fields of a type_assign at
	once.

Zoltan.

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cvs diff: Diffing compiler
Index: compiler/typecheck_info.m
===================================================================
RCS file: /home/mercury/mercury1/repository/mercury/compiler/typecheck_info.m,v
retrieving revision 1.37
diff -u -b -r1.37 typecheck_info.m
--- compiler/typecheck_info.m	18 Apr 2012 02:25:00 -0000	1.37
+++ compiler/typecheck_info.m	23 Apr 2012 09:51:07 -0000
@@ -280,6 +280,10 @@
 :- pred type_assign_set_constraint_map(constraint_map::in,
     type_assign::in, type_assign::out) is det.
 
+:- pred type_assign_set_reduce_results(tsubst::in, tvarset::in,
+    hlds_constraints::in, constraint_proof_map::in, constraint_map::in,
+    type_assign::in, type_assign::out) is det.
+
 %-----------------------------------------------------------------------------%
 
 :- type args_type_assign_set == list(args_type_assign).
@@ -678,13 +682,31 @@
 type_assign_get_constraint_proofs(TA, TA ^ constraint_proofs).
 type_assign_get_constraint_map(TA, TA ^ constraint_map).
 
-type_assign_set_var_types(X, TA, TA ^ var_types := X).
-type_assign_set_typevarset(X, TA, TA ^ type_varset := X).
-type_assign_set_head_type_params(X, TA, TA ^ head_type_params := X).
-type_assign_set_type_bindings(X, TA, TA ^ type_bindings := X).
-type_assign_set_typeclass_constraints(X, TA, TA ^ class_constraints := X).
-type_assign_set_constraint_proofs(X, TA, TA ^ constraint_proofs := X).
-type_assign_set_constraint_map(X, TA, TA ^ constraint_map := X).
+type_assign_set_var_types(VarTypes, !TA) :-
+    !TA ^ var_types := VarTypes.
+type_assign_set_typevarset(TVarSet, !TA) :-
+    !TA ^ type_varset := TVarSet.
+type_assign_set_head_type_params(HeadTypeParams, !TA) :-
+    !TA ^ head_type_params := HeadTypeParams.
+type_assign_set_type_bindings(TypeBindings, !TA) :-
+    !TA ^ type_bindings := TypeBindings.
+type_assign_set_typeclass_constraints(Constraints, !TA) :-
+    !TA ^ class_constraints := Constraints.
+type_assign_set_constraint_proofs(Proofs, !TA) :-
+    !TA ^ constraint_proofs := Proofs.
+type_assign_set_constraint_map(ConstraintMap, !TA) :-
+    !TA ^ constraint_map := ConstraintMap.
+
+type_assign_set_reduce_results(Bindings, TVarSet, Constraints, Proofs,
+        ConstraintMap, !TA) :-
+    % This should allocate just one new type_assign, whereas separate calls
+    % to the predicates above to set each of these fields would allocate
+    % several.
+    !TA ^ type_bindings := Bindings,
+    !TA ^ type_varset := TVarSet,
+    !TA ^ class_constraints := Constraints,
+    !TA ^ constraint_proofs := Proofs,
+    !TA ^ constraint_map := ConstraintMap.
 
 %-----------------------------------------------------------------------------%
 
Index: compiler/typeclasses.m
===================================================================
RCS file: /home/mercury/mercury1/repository/mercury/compiler/typeclasses.m,v
retrieving revision 1.30
diff -u -b -r1.30 typeclasses.m
--- compiler/typeclasses.m	18 Apr 2012 02:25:00 -0000	1.30
+++ compiler/typeclasses.m	23 Apr 2012 10:07:36 -0000
@@ -140,32 +140,83 @@
 
 reduce_type_assign_context(ClassTable, InstanceTable, !.TypeAssign,
         !TypeAssignSet, !UnsatTypeAssignSet) :-
+    type_assign_get_typeclass_constraints(!.TypeAssign, Constraints0),
+    (
+        % Optimize the common case of no typeclass constraints at all.
+        Constraints0 =
+            hlds_constraints(Unproven0, Assumed0, Redundant0, Ancestors0),
+        Unproven0 = [],
+        Assumed0 = [],
+        map.is_empty(Redundant0),
+        map.is_empty(Ancestors0)
+    ->
+        !:TypeAssignSet = !.TypeAssignSet ++ [!.TypeAssign]
+    ;
     type_assign_get_head_type_params(!.TypeAssign, HeadTypeParams),
     type_assign_get_type_bindings(!.TypeAssign, Bindings0),
-    type_assign_get_typeclass_constraints(!.TypeAssign, Constraints0),
     type_assign_get_typevarset(!.TypeAssign, TVarSet0),
     type_assign_get_constraint_proofs(!.TypeAssign, Proofs0),
     type_assign_get_constraint_map(!.TypeAssign, ConstraintMap0),
 
-    typeclasses.reduce_context_by_rule_application(ClassTable, InstanceTable,
+        reduce_context_by_rule_application(ClassTable, InstanceTable,
         HeadTypeParams, Bindings0, Bindings, TVarSet0, TVarSet,
         Proofs0, Proofs, ConstraintMap0, ConstraintMap,
         Constraints0, Constraints),
 
-    type_assign_set_type_bindings(Bindings, !TypeAssign),
-    type_assign_set_typeclass_constraints(Constraints, !TypeAssign),
-    type_assign_set_typevarset(TVarSet, !TypeAssign),
-    type_assign_set_constraint_proofs(Proofs, !TypeAssign),
-    type_assign_set_constraint_map(ConstraintMap, !TypeAssign),
+        type_assign_set_reduce_results(Bindings, TVarSet, Constraints,
+            Proofs, ConstraintMap, !TypeAssign),
+
 
-    ( check_satisfiability(Constraints ^ hcs_unproven, HeadTypeParams) ->
+        Unproven = Constraints ^ hcs_unproven,
+        ( all_constraints_are_satisfiable(Unproven, HeadTypeParams) ->
         !:TypeAssignSet = !.TypeAssignSet ++ [!.TypeAssign]
     ;
         % Remember the unsatisfiable type_assign_set so we can produce more
         % specific error messages.
-        list.cons(!.TypeAssign, !UnsatTypeAssignSet)
+            !:UnsatTypeAssignSet = [!.TypeAssign | !.UnsatTypeAssignSet]
+        )
     ).
 
+    % all_constraints_are_satisfiable(Constraints, HeadTypeParams):
+    %
+    % Check that all of the constraints are satisfiable. Fail if any are
+    % definitely not satisfiable.
+    %
+    % We disallow ground constraints for which there are no matching instance
+    % rules, even though the module system means that it would make sense
+    % to allow them: even if there is no instance declaration visible
+    % in the current module, there may be one visible in the caller. The reason
+    % we disallow them is that in practice allowing this causes type inference
+    % to let too many errors slip through, with the error diagnosis being
+    % too far removed from the real cause of the error. Note that ground
+    % constraints *are* allowed if you declare them, since we removed declared
+    % constraints before checking satisfiability.
+    %
+    % Similarly, for constraints on head type params (universally quantified
+    % type vars in this pred's type decl, or existentially quantified type vars
+    % in type decls for callees), we know that the head type params can
+    % never get bound. This means that if the constraint wasn't an assumed
+    % constraint and can't be eliminated by instance rule or class rule
+    % application, then we can report an error now, rather than later.
+    % (For non-head-type-param type variables, we need to wait, in case
+    % the type variable gets bound to a type for which there is a valid
+    % instance declaration.)
+    %
+    % So a constraint is considered satisfiable iff it contains at least one
+    % type variable that is not in the head type params.
+    %
+:- pred all_constraints_are_satisfiable(list(hlds_constraint)::in,
+    head_type_params::in) is semidet.
+
+all_constraints_are_satisfiable([], _).
+all_constraints_are_satisfiable([Constraint | Constraints], HeadTypeParams) :-
+    Constraint = hlds_constraint(_Ids, _ClassName, Types),
+    some [TVar] (
+        type_list_contains_var(Types, TVar),
+        not list.member(TVar, HeadTypeParams)
+    ),
+    all_constraints_are_satisfiable(Constraints, HeadTypeParams).
+
 reduce_context_by_rule_application(ClassTable, InstanceTable, HeadTypeParams,
         !Bindings, !TVarSet, !Proofs, !ConstraintMap, !Constraints) :-
     reduce_context_by_rule_application_2(ClassTable, InstanceTable,
@@ -700,48 +751,6 @@
     map.set(Constraint, superclass(Descendant), !Proofs),
     add_superclass_proofs(Descendant, Descendants, !Proofs).
 
-    % check_satisfiability(Constraints, HeadTypeParams):
-    %
-    % Check that all of the constraints are satisfiable. Fail if any are
-    % definitely not satisfiable.
-    %
-    % We disallow ground constraints for which there are no matching instance
-    % rules, even though the module system means that it would make sense
-    % to allow them: even if there is no instance declaration visible
-    % in the current module, there may be one visible in the caller. The reason
-    % we disallow them is that in practice allowing this causes type inference
-    % to let too many errors slip through, with the error diagnosis being
-    % too far removed from the real cause of the error. Note that ground
-    % constraints *are* allowed if you declare them, since we removed declared
-    % constraints before checking satisfiability.
-    %
-    % Similarly, for constraints on head type params (universally quantified
-    % type vars in this pred's type decl, or existentially quantified type vars
-    % in type decls for callees), we know that the head type params can
-    % never get bound. This means that if the constraint wasn't an assumed
-    % constraint and can't be eliminated by instance rule or class rule
-    % application, then we can report an error now, rather than later.
-    % (For non-head-type-param type variables, we need to wait, in case
-    % the type variable gets bound to a type for which there is a valid
-    % instance declaration.)
-    %
-    % So a constraint is considered satisfiable iff it contains at least one
-    % type variable that is not in the head type params.
-    %
-:- pred check_satisfiability(list(hlds_constraint)::in, head_type_params::in)
-    is semidet.
-
-check_satisfiability(Constraints, HeadTypeParams) :-
-    all [Constraint] (
-        list.member(Constraint, Constraints)
-    =>
-        (
-            Constraint = hlds_constraint(_Ids, _ClassName, Types),
-            type_list_contains_var(Types, TVar),
-            not list.member(TVar, HeadTypeParams)
-        )
-    ).
-
 %-----------------------------------------------------------------------------%
 :- end_module check_hlds.typeclasses.
 %-----------------------------------------------------------------------------%
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