mercury/compiler/delay_construct.m

%-----------------------------------------------------------------------------%
% vim: ft=mercury ts=4 sw=4 et
%-----------------------------------------------------------------------------%
% Copyright (C) 2001-2005 The University of Melbourne.
% This file may only be copied under the terms of the GNU General
% Public License - see the file COPYING in the Mercury distribution.
%-----------------------------------------------------------------------------%
%
% File: delay_construct.m
%
% Author: zs.
%
% This module transforms sequences of goals in procedure bodies.
% It looks for a unification that constructs a ground term followed by
% primitive goals, at least one of which can fail, and none of which take
% the variable representing the cell as their input. Such code sequences
% cause the cell to be constructed even if the following goal would fail,
% which is wasteful. This module therefore reorders the sequence, moving the
% construction unification past all the semidet primitives it can.
%
% The reason we don't move the construction past calls or composite goals
% is that this may require storing the input arguments of the construction on
% the stack, which may cause a slowdown bigger than the speedup available from
% not having to construct the cell on some execution paths.
%
%-----------------------------------------------------------------------------%

:- module transform_hlds__delay_construct.

:- interface.

:- import_module hlds__hlds_module.
:- import_module hlds__hlds_pred.

:- import_module io.

:- pred delay_construct_proc(pred_id::in, proc_id::in, module_info::in,
    proc_info::in, proc_info::out, io::di, io::uo) is det.

%-----------------------------------------------------------------------------%

:- implementation.

:- import_module check_hlds__inst_match.
:- import_module hlds__hlds_data.
:- import_module hlds__hlds_goal.
:- import_module hlds__instmap.
:- import_module hlds__passes_aux.
:- import_module libs__globals.
:- import_module parse_tree__error_util.
:- import_module parse_tree__prog_data.

:- import_module bool.
:- import_module list.
:- import_module require.
:- import_module set.
:- import_module std_util.

%-----------------------------------------------------------------------------%

delay_construct_proc(PredId, ProcId, ModuleInfo, !ProcInfo, !IO) :-
    write_proc_progress_message("% Delaying construction unifications in ",
        PredId, ProcId, ModuleInfo, !IO),
    globals__io_get_globals(Globals, !IO),
    module_info_pred_info(ModuleInfo, PredId, PredInfo),
    delay_construct_proc_no_io(PredInfo, ModuleInfo, Globals, !ProcInfo).

:- pred delay_construct_proc_no_io(pred_info::in, module_info::in, globals::in,
    proc_info::in, proc_info::out) is det.

delay_construct_proc_no_io(PredInfo, ModuleInfo, Globals, !ProcInfo) :-
    body_should_use_typeinfo_liveness(PredInfo, Globals, BodyTypeinfoLiveness),
    proc_info_vartypes(!.ProcInfo, VarTypes),
    proc_info_rtti_varmaps(!.ProcInfo, RttiVarMaps),
    proc_info_get_initial_instmap(!.ProcInfo, ModuleInfo, InstMap0),
    DelayInfo = delay_construct_info(ModuleInfo, BodyTypeinfoLiveness,
        VarTypes, RttiVarMaps),
    proc_info_goal(!.ProcInfo, Goal0),
    delay_construct_in_goal(Goal0, InstMap0, DelayInfo, Goal),
    proc_info_set_goal(Goal, !ProcInfo).

:- type delay_construct_info
    --->    delay_construct_info(
                module_info             :: module_info,
                body_typeinfo_liveness  :: bool,
                vartypes                :: vartypes,
                rtti_varmaps            :: rtti_varmaps
            ).

%-----------------------------------------------------------------------------%

:- pred delay_construct_in_goal(hlds_goal::in, instmap::in,
    delay_construct_info::in, hlds_goal::out) is det.

delay_construct_in_goal(GoalExpr0 - GoalInfo0, InstMap0, DelayInfo, Goal) :-
    (
        GoalExpr0 = conj(Goals0),
        goal_info_get_determinism(GoalInfo0, Detism),
        determinism_components(Detism, CanFail, MaxSoln),
        (
            % If the conjunction cannot fail, then its conjuncts cannot fail
            % either, so we have no hope of pushing a construction past a
            % failing goal.
            %
            % If the conjuntion contains goals that can succeed more than once,
            % which is possible if MaxSoln is at_most_many or at_most_many_cc,
            % then moving a construction to the right may increase the number
            % of times the construction is executed. We are therefore careful
            % to make sure delay_construct_in_conj doesn't move constructions
            % across goals that succeed more than once.
            %
            % If the conjunction cannot succeed, i.e. MaxSoln is at_most_zero,
            % there is no point in trying to speed it up.

            CanFail = can_fail,
            MaxSoln \= at_most_zero
        ->
            delay_construct_in_conj(Goals0, InstMap0, DelayInfo, set__init, [],
                Goals1)
        ;
            Goals1 = Goals0
        ),
        delay_construct_in_goals(Goals1, InstMap0, DelayInfo, Goals),
        Goal = conj(Goals) - GoalInfo0
    ;
        GoalExpr0 = par_conj(Goals0),
        delay_construct_in_goals(Goals0, InstMap0, DelayInfo, Goals),
        Goal = par_conj(Goals) - GoalInfo0
    ;
        GoalExpr0 = disj(Goals0),
        delay_construct_in_goals(Goals0, InstMap0, DelayInfo, Goals),
        Goal = disj(Goals) - GoalInfo0
    ;
        GoalExpr0 = not(NegGoal0),
        delay_construct_in_goal(NegGoal0, InstMap0, DelayInfo, NegGoal),
        Goal = not(NegGoal) - GoalInfo0
    ;
        GoalExpr0 = switch(Var, CanFail, Cases0),
        delay_construct_in_cases(Cases0, InstMap0, DelayInfo, Cases),
        Goal = switch(Var, CanFail, Cases) - GoalInfo0
    ;
        GoalExpr0 = if_then_else(Vars, Cond0, Then0, Else0),
        Cond0 = _ - CondInfo0,
        goal_info_get_instmap_delta(CondInfo0, CondInstMapDelta),
        instmap__apply_instmap_delta(InstMap0, CondInstMapDelta, InstMapThen),
        delay_construct_in_goal(Cond0, InstMap0, DelayInfo, Cond),
        delay_construct_in_goal(Then0, InstMapThen, DelayInfo, Then),
        delay_construct_in_goal(Else0, InstMap0, DelayInfo, Else),
        Goal = if_then_else(Vars, Cond, Then, Else) - GoalInfo0
    ;
        GoalExpr0 = scope(Reason, SubGoal0),
        delay_construct_in_goal(SubGoal0, InstMap0, DelayInfo, SubGoal),
        Goal = scope(Reason, SubGoal) - GoalInfo0
    ;
        GoalExpr0 = generic_call(_, _, _, _),
        Goal = GoalExpr0 - GoalInfo0
    ;
        GoalExpr0 = call(_, _, _, _, _, _),
        Goal = GoalExpr0 - GoalInfo0
    ;
        GoalExpr0 = unify(_, _, _, _, _),
        Goal = GoalExpr0 - GoalInfo0
    ;
        GoalExpr0 = foreign_proc(_, _, _, _, _, _),
        Goal = GoalExpr0 - GoalInfo0
    ;
        GoalExpr0 = shorthand(_),
        % These should have been expanded out by now.
        unexpected(this_file, "delay_construct_in_goal: unexpected shorthand")
    ).

%-----------------------------------------------------------------------------%

% We maintain a list of delayed construction unifications that construct ground
% terms, and the set of variables they define.
%
% When we find other construction unifications, we add them to the list.
% It does not matter if they depend on other delayed construction unifications;
% when we put them back into the conjunction, we do so in the original order.
%
% There are several reasons why we may not be able to delay a construction
% unification past a conjunct. The conjunct may not be a primitive goal,
% or it may be impure; in either case, we must insert all the delayed
% construction unifications before it. The conjunct may also require the value
% of a variable defined by a construction unification. In such cases, we could
% drop before that goal only the construction unifications that define the
% variables needed by the conjunct, either directly or indirectly through
% the values required by some of those construction unifications. However,
% separating out this set of delayed constructions from the others would
% require somewhat complex code, and it is not clear that there would be any
% significant benefit. We therefore insert *all* the delayed constructions
% before a goal if the goal requires *any* of the variables bound by the
% constructions.
%
% The instmap we pass around is the one that we construct from the original
% conjunction order. At each point, it reflects the bindings made by the
% conjuncts so far *plus* the bindings made by the delayed goals.

:- pred delay_construct_in_conj(list(hlds_goal)::in, instmap::in,
    delay_construct_info::in, set(prog_var)::in, list(hlds_goal)::in,
    list(hlds_goal)::out) is det.

delay_construct_in_conj([], _, _, _, RevDelayedGoals, DelayedGoals) :-
    list__reverse(RevDelayedGoals, DelayedGoals).
delay_construct_in_conj([Goal0 | Goals0], InstMap0, DelayInfo,
        ConstructedVars0, RevDelayedGoals0, Goals) :-
    Goal0 = GoalExpr0 - GoalInfo0,
    goal_info_get_instmap_delta(GoalInfo0, InstMapDelta0),
    instmap__apply_instmap_delta(InstMap0, InstMapDelta0, InstMap1),
    (
        GoalExpr0 = unify(_, _, _, Unif, _),
        Unif = construct(Var, _, Args, _, _, _, _),
        Args = [_ | _], % We are constructing a cell, not a constant
        instmap__lookup_var(InstMap0, Var, Inst0),
        inst_is_free(DelayInfo ^ module_info, Inst0),
        instmap__lookup_var(InstMap1, Var, Inst1),
        inst_is_ground(DelayInfo ^ module_info, Inst1)
    ->
        set__insert(ConstructedVars0, Var, ConstructedVars1),
        RevDelayedGoals1 = [Goal0 | RevDelayedGoals0],
        delay_construct_in_conj(Goals0, InstMap1, DelayInfo,
            ConstructedVars1, RevDelayedGoals1, Goals)
    ;
        Goal0 = GoalExpr0 - GoalInfo0,
        delay_construct_skippable(GoalExpr0, GoalInfo0),
        goal_info_get_nonlocals(GoalInfo0, NonLocals),
        proc_info_maybe_complete_with_typeinfo_vars(NonLocals,
            DelayInfo ^ body_typeinfo_liveness,
            DelayInfo ^ vartypes,
            DelayInfo ^ rtti_varmaps, CompletedNonLocals),
        set__intersect(CompletedNonLocals, ConstructedVars0,
            Intersection),
        set__empty(Intersection),
        \+ goal_info_has_feature(GoalInfo0, impure_goal),
        \+ goal_info_has_feature(GoalInfo0, semipure_goal)
    ->
        delay_construct_in_conj(Goals0, InstMap1, DelayInfo,
            ConstructedVars0, RevDelayedGoals0, Goals1),
        Goals = [Goal0 | Goals1]
    ;
        list__reverse(RevDelayedGoals0, DelayedGoals),
        delay_construct_in_conj(Goals0, InstMap1, DelayInfo,
            set__init, [], Goals1),
        list__append(DelayedGoals, [Goal0 | Goals1], Goals)
    ).

:- pred delay_construct_skippable(hlds_goal_expr::in, hlds_goal_info::in)
    is semidet.

delay_construct_skippable(GoalExpr, GoalInfo) :-
    (
        GoalExpr = unify(_, _, _, _, _)
    ;
        GoalExpr = call(_, _, _, inline_builtin, _, _)
    ),
    goal_info_get_determinism(GoalInfo, Detism),
    determinism_components(Detism, _CanFail, MaxSoln),
    MaxSoln \= at_most_many.

%-----------------------------------------------------------------------------%

:- pred delay_construct_in_goals(list(hlds_goal)::in, instmap::in,
    delay_construct_info::in, list(hlds_goal)::out) is det.

delay_construct_in_goals([], _, _, []).
delay_construct_in_goals([Goal0 | Goals0], InstMap0, DelayInfo,
        [Goal | Goals]) :-
    delay_construct_in_goal(Goal0, InstMap0, DelayInfo, Goal),
    delay_construct_in_goals(Goals0, InstMap0, DelayInfo, Goals).

:- pred delay_construct_in_cases(list(case)::in, instmap::in,
    delay_construct_info::in, list(case)::out) is det.

delay_construct_in_cases([], _, _, []).
delay_construct_in_cases([case(Cons, Goal0) | Cases0], InstMap0, DelayInfo,
        [case(Cons, Goal) | Cases]) :-
    delay_construct_in_goal(Goal0, InstMap0, DelayInfo, Goal),
    delay_construct_in_cases(Cases0, InstMap0, DelayInfo, Cases).

%-----------------------------------------------------------------------------%

:- func this_file = string.

this_file = "delay_construct.m".

%-----------------------------------------------------------------------------%