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%-----------------------------------------------------------------------------%
% Copyright (C) 1994-1999 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.
%-----------------------------------------------------------------------------%
% labelopt.m - module to eliminate useless labels and dead code.
% Author: zs.
%-----------------------------------------------------------------------------%
:- module labelopt.
:- interface.
:- import_module bool, list, set.
:- import_module llds.
% Build up a set showing which labels are branched to,
% then traverse the instruction list removing unnecessary labels.
% If the instruction before the label branches away, we also
% remove the instruction block following the label.
:- pred labelopt_main(list(instruction)::in, bool::in, set(label)::in,
list(instruction)::out, bool::out) is det.
% Build up a set showing which labels are referred to.
% The input set is the list of labels referred to from outside
% the given list of instructions.
:- pred labelopt__build_useset(list(instruction)::in, set(label)::in,
set(label)::out) is det.
%-----------------------------------------------------------------------------%
:- implementation.
:- import_module opt_util.
:- import_module std_util.
labelopt_main(Instrs0, Final, LayoutLabelSet, Instrs, Mod) :-
labelopt__build_useset(Instrs0, LayoutLabelSet, Useset),
labelopt__instr_list(Instrs0, yes, Useset, Instrs1, Mod),
( Final = yes, Mod = yes ->
labelopt_main(Instrs1, Final, LayoutLabelSet, Instrs, _)
;
Instrs = Instrs1
).
%-----------------------------------------------------------------------------%
labelopt__build_useset([], Useset, Useset).
labelopt__build_useset([Instr | Instructions], Useset0, Useset) :-
Instr = Uinstr - _Comment,
opt_util__instr_labels(Uinstr, Labels, _CodeAddresses),
set__insert_list(Useset0, Labels, Useset1),
labelopt__build_useset(Instructions, Useset1, Useset).
%-----------------------------------------------------------------------------%
% Go through the given instruction sequence. When we find a label,
% we check whether the label can be branched to either from within
% the procedure or from the outside. If yes, we leave it alone.
% If not, we delete it. We delete the following code as well if
% the label was preceded by code that cannot fall through.
:- pred labelopt__instr_list(list(instruction)::in, bool::in, set(label)::in,
list(instruction)::out, bool::out) is det.
labelopt__instr_list([], _Fallthrough, _Useset, [], no).
labelopt__instr_list([Instr0 | MoreInstrs0],
Fallthrough, Useset, MoreInstrs, Mod) :-
Instr0 = Uinstr0 - _Comment,
( Uinstr0 = label(Label) ->
(
( Label = exported(_)
; Label = local(_)
; set__member(Label, Useset)
)
->
ReplInstrs = [Instr0],
Fallthrough1 = yes,
Mod0 = no
;
labelopt__eliminate(Instr0, yes(Fallthrough),
ReplInstrs, Mod0),
Fallthrough1 = Fallthrough
)
;
( Fallthrough = yes ->
ReplInstrs = [Instr0],
Mod0 = no
;
labelopt__eliminate(Instr0, no, ReplInstrs, Mod0)
),
opt_util__can_instr_fall_through(Uinstr0, Canfallthrough),
( Canfallthrough = yes ->
Fallthrough1 = Fallthrough
;
Fallthrough1 = no
)
),
labelopt__instr_list(MoreInstrs0, Fallthrough1, Useset,
MoreInstrs1, Mod1),
list__append(ReplInstrs, MoreInstrs1, MoreInstrs),
( Mod0 = no, Mod1 = no ->
Mod = no
;
Mod = yes
).
% Instead of removing eliminated instructions from the instruction list,
% we can replace them by placeholder comments. The original comment
% field on the instruction is often enough to deduce what the
% eliminated instruction was.
:- pred labelopt__eliminate(instruction::in, maybe(bool)::in,
list(instruction)::out, bool::out) is det.
labelopt__eliminate(Uinstr0 - Comment0, Label, Instr, Mod) :-
labelopt_eliminate_total(Total),
(
Total = yes,
Instr = [],
Mod = yes
;
Total = no,
( Uinstr0 = comment(_) ->
Comment = Comment0,
Uinstr = Uinstr0,
Mod = no
;
( Label = yes(Follow) ->
( Follow = yes ->
Uinstr = comment("eliminated label only")
;
% Follow = no,
Uinstr = comment("eliminated label and block")
)
;
% Label = no,
Uinstr = comment("eliminated instruction")
),
Comment = Comment0,
Mod = yes
),
Instr = [Uinstr - Comment]
).
:- pred labelopt_eliminate_total(bool::out) is det.
labelopt_eliminate_total(yes).
%-----------------------------------------------------------------------------%
|
