%%
%% %CopyrightBegin%
%%
%% Copyright Ericsson AB 2002-2016. All Rights Reserved.
%%
%% Licensed under the Apache License, Version 2.0 (the "License");
%% you may not use this file except in compliance with the License.
%% You may obtain a copy of the License at
%%
%% http://www.apache.org/licenses/LICENSE-2.0
%%
%% Unless required by applicable law or agreed to in writing, software
%% distributed under the License is distributed on an "AS IS" BASIS,
%% WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
%% See the License for the specific language governing permissions and
%% limitations under the License.
%%
%% %CopyrightEnd%
%%
-module(beam_dead).
-export([module/2]).
%%% Dead code is code that is executed but has no effect. This
%%% optimization pass either removes dead code or jumps around it,
%%% potentially making it unreachable and a target for the
%%% the beam_jump pass.
-import(lists, [mapfoldl/3,reverse/1]).
module({Mod,Exp,Attr,Fs0,_}, _Opts) ->
{Fs1,Lc1} = beam_clean:clean_labels(Fs0),
{Fs,Lc} = mapfoldl(fun function/2, Lc1, Fs1),
%%{Fs,Lc} = {Fs1,Lc1},
{ok,{Mod,Exp,Attr,Fs,Lc}}.
function({function,Name,Arity,CLabel,Is0}, Lc0) ->
try
Is1 = beam_jump:remove_unused_labels(Is0),
%% Initialize label information with the code
%% for the func_info label. Without it, a register
%% may seem to be live when it is not.
[{label,L}|FiIs] = Is1,
D0 = beam_utils:empty_label_index(),
D = beam_utils:index_label(L, FiIs, D0),
%% Optimize away dead code.
{Is2,Lc} = forward(Is1, Lc0),
Is3 = backward(Is2, D),
Is = move_move_into_block(Is3, []),
{{function,Name,Arity,CLabel,Is},Lc}
catch
Class:Error ->
Stack = erlang:get_stacktrace(),
io:fwrite("Function: ~w/~w\n", [Name,Arity]),
erlang:raise(Class, Error, Stack)
end.
%% 'move' instructions outside of blocks may thwart the jump optimizer.
%% Move them back into the block.
move_move_into_block([{block,Bl0},{move,S,D}|Is], Acc) ->
Bl = Bl0 ++ [{set,[D],[S],move}],
move_move_into_block([{block,Bl}|Is], Acc);
move_move_into_block([{move,S,D}|Is], Acc) ->
Bl = [{set,[D],[S],move}],
move_move_into_block([{block,Bl}|Is], Acc);
move_move_into_block([I|Is], Acc) ->
move_move_into_block(Is, [I|Acc]);
move_move_into_block([], Acc) -> reverse(Acc).
%%%
%%% Scan instructions in execution order and remove redundant 'move'
%%% instructions. 'move' instructions are redundant if we know that
%%% the register already contains the value being assigned, as in the
%%% following code:
%%%
%%% test is_eq_exact SomeLabel Src Dst
%%% move Src Dst
%%%
%%% or in:
%%%
%%% test is_nil SomeLabel Dst
%%% move nil Dst
%%%
%%% or in:
%%%
%%% select_val Register FailLabel [... Literal => L1...]
%%% .
%%% .
%%% .
%%% L1: move Literal Register
%%%
%%% Also add extra labels to help the second backward pass.
%%%
forward(Is, Lc) ->
forward(Is, #{}, Lc, []).
forward([{move,_,_}=Move|[{label,L}|_]=Is], D, Lc, Acc) ->
%% move/2 followed by jump/1 is optimized by backward/3.
forward([Move,{jump,{f,L}}|Is], D, Lc, Acc);
forward([{bif,_,_,_,_}=Bif|[{label,L}|_]=Is], D, Lc, Acc) ->
%% bif/4 followed by jump/1 is optimized by backward/3.
forward([Bif,{jump,{f,L}}|Is], D, Lc, Acc);
forward([{block,[]}|Is], D, Lc, Acc) ->
%% Empty blocks can prevent optimizations.
forward(Is, D, Lc, Acc);
forward([{select,select_val,Reg,_,List}=I|Is], D0, Lc, Acc) ->
D = update_value_dict(List, Reg, D0),
forward(Is, D, Lc, [I|Acc]);
forward([{label,Lbl}=LblI,{block,[{set,[Dst],[Lit],move}|BlkIs]}=Blk|Is], D, Lc, Acc) ->
%% Assumption: The target labels in a select_val/3 instruction
%% cannot be reached in any other way than through the select_val/3
%% instruction (i.e. there can be no fallthrough to such label and
%% it cannot be referenced by, for example, a jump/1 instruction).
Key = {Lbl,Dst},
Block = case D of
#{Key := Lit} -> {block,BlkIs}; %Safe to remove move instruction.
_ -> Blk %Must keep move instruction.
end,
forward([Block|Is], D, Lc, [LblI|Acc]);
forward([{label,Lbl}=LblI|[{move,Lit,Dst}|Is1]=Is0], D, Lc, Acc) ->
%% Assumption: The target labels in a select_val/3 instruction
%% cannot be reached in any other way than through the select_val/3
%% instruction (i.e. there can be no fallthrough to such label and
%% it cannot be referenced by, for example, a jump/1 instruction).
Is = case maps:find({Lbl,Dst}, D) of
{ok,Lit} -> Is1; %Safe to remove move instruction.
_ -> Is0 %Keep move instruction.
end,
forward(Is, D, Lc, [LblI|Acc]);
forward([{test,is_eq_exact,_,[Same,Same]}|Is], D, Lc, Acc) ->
forward(Is, D, Lc, Acc);
forward([{test,is_eq_exact,_,[Dst,Src]}=I,
{block,[{set,[Dst],[Src],move}|Bl]}|Is], D, Lc, Acc) ->
forward([I,{block,Bl}|Is], D, Lc, Acc);
forward([{test,is_nil,_,[Dst]}=I,
{block,[{set,[Dst],[nil],move}|Bl]}|Is], D, Lc, Acc) ->
forward([I,{block,Bl}|Is], D, Lc, Acc);
forward([{test,is_eq_exact,_,[Dst,Src]}=I,{move,Src,Dst}|Is], D, Lc, Acc) ->
forward([I|Is], D, Lc, Acc);
forward([{test,is_nil,_,[Dst]}=I,{move,nil,Dst}|Is], D, Lc, Acc) ->
forward([I|Is], D, Lc, Acc);
forward([{test,_,_,_}=I|Is]=Is0, D, Lc, Acc) ->
%% Help the second, backward pass to by inserting labels after
%% relational operators so that they can be skipped if they are
%% known to be true.
case useful_to_insert_label(Is0) of
false -> forward(Is, D, Lc, [I|Acc]);
true -> forward(Is, D, Lc+1, [{label,Lc},I|Acc])
end;
forward([I|Is], D, Lc, Acc) ->
forward(Is, D, Lc, [I|Acc]);
forward([], _, Lc, Acc) -> {Acc,Lc}.
update_value_dict([Lit,{f,Lbl}|T], Reg, D0) ->
Key = {Lbl,Reg},
D = case D0 of
#{Key := inconsistent} -> D0;
#{Key := _} -> D0#{Key := inconsistent};
_ -> D0#{Key => Lit}
end,
update_value_dict(T, Reg, D);
update_value_dict([], _, D) -> D.
useful_to_insert_label([_,{label,_}|_]) ->
false;
useful_to_insert_label([{test,Op,_,_}|_]) ->
case Op of
is_lt -> true;
is_ge -> true;
is_eq_exact -> true;
is_ne_exact -> true;
_ -> false
end.
%%%
%%% Scan instructions in reverse execution order and try to
%%% shortcut branch instructions.
%%%
%%% For example, in this code:
%%%
%%% move Literal Register
%%% jump L1
%%% .
%%% .
%%% .
%%% L1: test is_{integer,atom} FailLabel Register
%%% select_val {x,0} FailLabel [... Literal => L2...]
%%% .
%%% .
%%% .
%%% L2: ...
%%%
%%% the 'selectval' instruction will always transfer control to L2,
%%% so we can just as well jump to L2 directly by rewriting the
%%% first part of the sequence like this:
%%%
%%% move Literal Register
%%% jump L2
%%%
%%% If register Register is killed at label L2, we can remove the
%%% 'move' instruction, leaving just the 'jump' instruction:
%%%
%%% jump L2
%%%
%%% These transformations may leave parts of the code unreachable.
%%% The beam_jump pass will remove the unreachable code.
backward(Is, D) ->
backward(Is, D, []).
backward([{test,is_eq_exact,Fail,[Dst,{integer,Arity}]}=I|
[{bif,tuple_size,Fail,[Reg],Dst}|Is]=Is0], D, Acc) ->
%% Provided that Dst is killed following this sequence,
%% we can rewrite the instructions like this:
%%
%% bif tuple_size Fail Reg Dst ==> is_tuple Fail Reg
%% is_eq_exact Fail Dst Integer test_arity Fail Reg Integer
%%
%% (still two instructions, but they they will be combined to
%% one by the loader).
case beam_utils:is_killed(Dst, Acc, D) andalso (Arity bsr 32) =:= 0 of
false ->
%% Not safe because the register Dst is not killed
%% (probably cannot not happen in practice) or the arity
%% does not fit in 32 bits (the loader will fail to load
%% the module). We must move the first instruction to the
%% accumulator to avoid an infinite loop.
backward(Is0, D, [I|Acc]);
true ->
%% Safe.
backward([{test,test_arity,Fail,[Reg,Arity]},
{test,is_tuple,Fail,[Reg]}|Is], D, Acc)
end;
backward([{label,Lbl}=L|Is], D, Acc) ->
backward(Is, beam_utils:index_label(Lbl, Acc, D), [L|Acc]);
backward([{select,select_val,Reg,{f,Fail0},List0}|Is], D, Acc) ->
List1 = shortcut_select_list(List0, Reg, D, []),
Fail1 = shortcut_label(Fail0, D),
Fail = shortcut_bs_test(Fail1, Is, D),
List = prune_redundant(List1, Fail),
case List of
[] ->
Jump = {jump,{f,Fail}},
backward([Jump|Is], D, Acc);
[V,F] ->
Test = {test,is_eq_exact,{f,Fail},[Reg,V]},
Jump = {jump,F},
backward([Jump,Test|Is], D, Acc);
[{atom,B1},F,{atom,B2},F] when B1 =:= not B2 ->
Test = {test,is_boolean,{f,Fail},[Reg]},
Jump = {jump,F},
backward([Jump,Test|Is], D, Acc);
[_|_] ->
Sel = {select,select_val,Reg,{f,Fail},List},
backward(Is, D, [Sel|Acc])
end;
backward([{jump,{f,To0}},{move,Src,Reg}=Move|Is], D, Acc) ->
To = shortcut_select_label(To0, Reg, Src, D),
Jump = {jump,{f,To}},
case is_killed_at(Reg, To, D) of
false -> backward([Move|Is], D, [Jump|Acc]);
true -> backward([Jump|Is], D, Acc)
end;
backward([{jump,{f,To}}=J|[{bif,Op,_,Ops,Reg}|Is]=Is0], D, Acc) ->
try replace_comp_op(To, Reg, Op, Ops, D) of
I -> backward(Is, D, I++Acc)
catch
throw:not_possible -> backward(Is0, D, [J|Acc])
end;
backward([{test,bs_start_match2,F,Live,[R,_]=Args,Ctxt}|Is], D,
[{test,bs_match_string,F,[Ctxt,Bs]},
{test,bs_test_tail2,F,[Ctxt,0]}|Acc0]=Acc) ->
{f,To0} = F,
case beam_utils:is_killed(Ctxt, Acc0, D) of
true ->
To = shortcut_bs_context_to_binary(To0, R, D),
Eq = {test,is_eq_exact,{f,To},[R,{literal,Bs}]},
backward(Is, D, [Eq|Acc0]);
false ->
To = shortcut_bs_start_match(To0, R, D),
I = {test,bs_start_match2,{f,To},Live,Args,Ctxt},
backward(Is, D, [I|Acc])
end;
backward([{test,bs_start_match2,{f,To0},Live,[Src|_]=Info,Dst}|Is], D, Acc) ->
To = shortcut_bs_start_match(To0, Src, D),
I = {test,bs_start_match2,{f,To},Live,Info,Dst},
backward(Is, D, [I|Acc]);
backward([{test,Op,{f,To0},Ops0}|Is], D, Acc) ->
To1 = shortcut_bs_test(To0, Is, D),
To2 = shortcut_label(To1, D),
To3 = shortcut_rel_op(To2, Op, Ops0, D),
%% Try to shortcut a repeated test:
%%
%% test Op {f,Fail1} Operands test Op {f,Fail2} Operands
%% . . . ==> ...
%% Fail1: test Op {f,Fail2} Operands Fail1: test Op {f,Fail2} Operands
%%
To = case beam_utils:code_at(To3, D) of
[{test,Op,{f,To4},Ops}|_] ->
case equal_ops(Ops0, Ops) of
true -> To4;
false -> To3
end;
_Code ->
To3
end,
I = case Op of
is_eq_exact -> combine_eqs(To, Ops0, D, Acc);
_ -> {test,Op,{f,To},Ops0}
end,
case {I,Acc} of
{{test,is_atom,Fail,Ops0},[{test,is_boolean,Fail,Ops0}|_]} ->
%% An is_atom test before an is_boolean test (with the
%% same failure label) is redundant.
backward(Is, D, Acc);
{{test,is_atom,Fail,[R]},
[{test,is_eq_exact,Fail,[R,{atom,_}]}|_]} ->
%% An is_atom test before a comparison with an atom (with
%% the same failure label) is redundant.
backward(Is, D, Acc);
{{test,is_integer,Fail,[R]},
[{test,is_eq_exact,Fail,[R,{integer,_}]}|_]} ->
%% An is_integer test before a comparison with an integer
%% (with the same failure label) is redundant.
backward(Is, D, Acc);
{{test,_,_,_},_} ->
%% Still a test instruction. Done.
backward(Is, D, [I|Acc]);
{_,_} ->
%% Rewritten to a select_val. Rescan.
backward([I|Is], D, Acc)
end;
backward([{test,Op,{f,To0},Live,Ops0,Dst}|Is], D, Acc) ->
To1 = shortcut_bs_test(To0, Is, D),
To2 = shortcut_label(To1, D),
%% Try to shortcut a repeated test:
%%
%% test Op {f,Fail1} _ Ops _ test Op {f,Fail2} _ Ops _
%% . . . ==> ...
%% Fail1: test Op {f,Fail2} _ Ops _ Fail1: test Op {f,Fail2} _ Ops _
%%
To = case beam_utils:code_at(To2, D) of
[{test,Op,{f,To3},_,Ops,_}|_] ->
case equal_ops(Ops0, Ops) of
true -> To3;
false -> To2
end;
_Code ->
To2
end,
I = {test,Op,{f,To},Live,Ops0,Dst},
backward(Is, D, [I|Acc]);
backward([{kill,_}=I|Is], D, [{line,_},Exit|_]=Acc) ->
case beam_jump:is_exit_instruction(Exit) of
false -> backward(Is, D, [I|Acc]);
true -> backward(Is, D, Acc)
end;
backward([I|Is], D, Acc) ->
backward(Is, D, [I|Acc]);
backward([], _D, Acc) -> Acc.
equal_ops([{field_flags,FlA0}|T0], [{field_flags,FlB0}|T1]) ->
FlA = lists:keydelete(anno, 1, FlA0),
FlB = lists:keydelete(anno, 1, FlB0),
FlA =:= FlB andalso equal_ops(T0, T1);
equal_ops([Op|T0], [Op|T1]) ->
equal_ops(T0, T1);
equal_ops([], []) -> true;
equal_ops(_, _) -> false.
shortcut_select_list([Lit,{f,To0}|T], Reg, D, Acc) ->
To = shortcut_select_label(To0, Reg, Lit, D),
shortcut_select_list(T, Reg, D, [{f,To},Lit|Acc]);
shortcut_select_list([], _, _, Acc) -> reverse(Acc).
shortcut_label(To0, D) ->
case beam_utils:code_at(To0, D) of
[{jump,{f,To}}|_] -> shortcut_label(To, D);
_ -> To0
end.
shortcut_select_label(To, Reg, Lit, D) ->
shortcut_rel_op(To, is_ne_exact, [Reg,Lit], D).
prune_redundant([_,{f,Fail}|T], Fail) ->
prune_redundant(T, Fail);
prune_redundant([V,F|T], Fail) ->
[V,F|prune_redundant(T, Fail)];
prune_redundant([], _) -> [].
%% Replace a comparison operator with a test instruction and a jump.
%% For example, if we have this code:
%%
%% bif '=:=' Fail Src1 Src2 {x,0}
%% jump L1
%% .
%% .
%% .
%% L1: select_val {x,0} FailLabel [... true => L2..., ...false => L3...]
%%
%% the first two instructions can be replaced with
%%
%% test is_eq_exact L3 Src1 Src2
%% jump L2
%%
%% provided that {x,0} is killed at both L2 and L3.
replace_comp_op(To, Reg, Op, Ops, D) ->
False = comp_op_find_shortcut(To, Reg, {atom,false}, D),
True = comp_op_find_shortcut(To, Reg, {atom,true}, D),
[bif_to_test(Op, Ops, False),{jump,{f,True}}].
comp_op_find_shortcut(To0, Reg, Val, D) ->
case shortcut_select_label(To0, Reg, Val, D) of
To0 ->
not_possible();
To ->
case is_killed_at(Reg, To, D) of
false -> not_possible();
true -> To
end
end.
bif_to_test(Name, Args, Fail) ->
try
beam_utils:bif_to_test(Name, Args, {f,Fail})
catch
error:_ -> not_possible()
end.
not_possible() -> throw(not_possible).
%% combine_eqs(To, Operands, Acc) -> Instruction.
%% Combine two is_eq_exact instructions or (an is_eq_exact
%% instruction and a select_val instruction) to a select_val
%% instruction if possible.
%%
%% Example:
%%
%% is_eq_exact F1 Reg Lit1 select_val Reg F2 [ Lit1 L1
%% L1: . Lit2 L2 ]
%% .
%% . ==>
%% .
%% F1: is_eq_exact F2 Reg Lit2 F1: is_eq_exact F2 Reg Lit2
%% L2: .... L2:
%%
combine_eqs(To, [Reg,{Type,_}=Lit1]=Ops, D, [{label,L1}|_])
when Type =:= atom; Type =:= integer ->
case beam_utils:code_at(To, D) of
[{test,is_eq_exact,{f,F2},[Reg,{Type,_}=Lit2]},
{label,L2}|_] when Lit1 =/= Lit2 ->
{select,select_val,Reg,{f,F2},[Lit1,{f,L1},Lit2,{f,L2}]};
[{select,select_val,Reg,{f,F2},[{Type,_}|_]=List0}|_] ->
List = remove_from_list(Lit1, List0),
{select,select_val,Reg,{f,F2},[Lit1,{f,L1}|List]};
_Is ->
{test,is_eq_exact,{f,To},Ops}
end;
combine_eqs(To, Ops, _D, _Acc) ->
{test,is_eq_exact,{f,To},Ops}.
remove_from_list(Lit, [Lit,{f,_}|T]) ->
T;
remove_from_list(Lit, [Val,{f,_}=Fail|T]) ->
[Val,Fail|remove_from_list(Lit, T)];
remove_from_list(_, []) -> [].
%% shortcut_bs_test(TargetLabel, ReversedInstructions, D) -> TargetLabel'
%% Try to shortcut the failure label for bit syntax matching.
shortcut_bs_test(To, Is, D) ->
shortcut_bs_test_1(beam_utils:code_at(To, D), Is, To, D).
shortcut_bs_test_1([{bs_restore2,Reg,SavePoint},
{label,_},
{test,bs_test_tail2,{f,To},[_,TailBits]}|_],
PrevIs, To0, D) ->
case count_bits_matched(PrevIs, {Reg,SavePoint}, 0) of
Bits when Bits > TailBits ->
%% This instruction will fail. We know because a restore has been
%% done from the previous point SavePoint in the binary, and we
%% also know that the binary contains at least Bits bits from
%% SavePoint.
%%
%% Since we will skip a bs_restore2 if we shortcut to label To,
%% we must now make sure that code at To does not depend on
%% the position in the context in any way.
case shortcut_bs_pos_used(To, Reg, D) of
false -> To;
true -> To0
end;
_Bits ->
To0
end;
shortcut_bs_test_1([_|_], _, To, _) -> To.
%% counts_bits_matched(ReversedInstructions, SavePoint, Bits) -> Bits'
%% Given a reversed instruction stream, determine the minimum number
%% of bits that will be matched by bit syntax instructions up to the
%% given save point.
count_bits_matched([{test,bs_get_utf8,{f,_},_,_,_}|Is], SavePoint, Bits) ->
count_bits_matched(Is, SavePoint, Bits+8);
count_bits_matched([{test,bs_get_utf16,{f,_},_,_,_}|Is], SavePoint, Bits) ->
count_bits_matched(Is, SavePoint, Bits+16);
count_bits_matched([{test,bs_get_utf32,{f,_},_,_,_}|Is], SavePoint, Bits) ->
count_bits_matched(Is, SavePoint, Bits+32);
count_bits_matched([{test,_,_,_,[_,Sz,U,{field_flags,_}],_}|Is], SavePoint, Bits) ->
case Sz of
{integer,N} -> count_bits_matched(Is, SavePoint, Bits+N*U);
_ -> count_bits_matched(Is, SavePoint, Bits)
end;
count_bits_matched([{test,bs_match_string,_,[_,Bs]}|Is], SavePoint, Bits) ->
count_bits_matched(Is, SavePoint, Bits+bit_size(Bs));
count_bits_matched([{test,_,_,_}|Is], SavePoint, Bits) ->
count_bits_matched(Is, SavePoint, Bits);
count_bits_matched([{bs_save2,Reg,SavePoint}|_], {Reg,SavePoint}, Bits) ->
%% The save point we are looking for - we are done.
Bits;
count_bits_matched([_|_], _, Bits) -> Bits.
shortcut_bs_pos_used(To, Reg, D) ->
shortcut_bs_pos_used_1(beam_utils:code_at(To, D), Reg, D).
shortcut_bs_pos_used_1([{bs_context_to_binary,Reg}|_], Reg, _) ->
false;
shortcut_bs_pos_used_1(Is, Reg, D) ->
not beam_utils:is_killed(Reg, Is, D).
%% shortcut_bs_start_match(TargetLabel, Reg) -> TargetLabel
%% A failing bs_start_match2 instruction means that the source (Reg)
%% cannot be a binary. That means that it is safe to skip
%% bs_context_to_binary instructions operating on Reg, and
%% bs_start_match2 instructions operating on Reg.
shortcut_bs_start_match(To, Reg, D) ->
shortcut_bs_start_match_1(beam_utils:code_at(To, D), Reg, To, D).
shortcut_bs_start_match_1([{bs_context_to_binary,Reg}|Is], Reg, To, D) ->
shortcut_bs_start_match_1(Is, Reg, To, D);
shortcut_bs_start_match_1([{jump,{f,To}}|_], Reg, _, D) ->
Code = beam_utils:code_at(To, D),
shortcut_bs_start_match_1(Code, Reg, To, D);
shortcut_bs_start_match_1([{test,bs_start_match2,{f,To},_,[Reg|_],_}|_],
Reg, _, D) ->
Code = beam_utils:code_at(To, D),
shortcut_bs_start_match_1(Code, Reg, To, D);
shortcut_bs_start_match_1(_, _, To, _) ->
To.
%% shortcut_bs_context_to_binary(TargetLabel, Reg) -> TargetLabel
%% If a bs_start_match2 instruction has been eliminated, the
%% bs_context_to_binary instruction can be eliminated too.
shortcut_bs_context_to_binary(To, Reg, D) ->
shortcut_bs_ctb_1(beam_utils:code_at(To, D), Reg, To, D).
shortcut_bs_ctb_1([{bs_context_to_binary,Reg}|Is], Reg, To, D) ->
shortcut_bs_ctb_1(Is, Reg, To, D);
shortcut_bs_ctb_1([{jump,{f,To}}|_], Reg, _, D) ->
Code = beam_utils:code_at(To, D),
shortcut_bs_ctb_1(Code, Reg, To, D);
shortcut_bs_ctb_1(_, _, To, _) ->
To.
%% shortcut_rel_op(FailLabel, Operator, [Operand], D) -> FailLabel'
%% Try to shortcut the given test instruction. Example:
%%
%% is_ge L1 {x,0} 48
%% .
%% .
%% .
%% L1: is_ge L2 {x,0} 65
%%
%% The first test instruction can be rewritten to "is_ge L2 {x,0} 48"
%% since the instruction at L1 will also fail.
%%
%% If there are instructions between L1 and the other test instruction
%% it may still be possible to do the shortcut. For example:
%%
%% L1: is_eq_exact L3 {x,0} 92
%% is_ge L2 {x,0} 65
%%
%% Since the first test instruction failed, we know that {x,0} must
%% be less than 48; therefore, we know that {x,0} cannot be equal to
%% 92 and the jump to L3 cannot happen.
shortcut_rel_op(To, Op, Ops, D) ->
case normalize_op({test,Op,{f,To},Ops}) of
{{NormOp,A,B},_} ->
Normalized = {negate_op(NormOp),A,B},
shortcut_rel_op_fp(To, Normalized, D);
{_,_} ->
To;
error ->
To
end.
shortcut_rel_op_fp(To0, Normalized, D) ->
Code = beam_utils:code_at(To0, D),
case shortcut_any_label(Code, Normalized) of
error ->
To0;
To ->
shortcut_rel_op_fp(To, Normalized, D)
end.
%% shortcut_any_label([Instruction], PrevCondition) -> FailLabel | error
%% Using PrevCondition (a previous condition known to be true),
%% try to shortcut to another failure label.
shortcut_any_label([{jump,{f,Lbl}}|_], _Prev) ->
Lbl;
shortcut_any_label([{label,Lbl}|_], _Prev) ->
Lbl;
shortcut_any_label([{select,select_val,R,{f,Fail},L}|_], Prev) ->
shortcut_selectval(L, R, Fail, Prev);
shortcut_any_label([I|Is], Prev) ->
case normalize_op(I) of
error ->
error;
{Normalized,Fail} ->
%% We have a relational operator.
case will_succeed(Prev, Normalized) of
no ->
%% This test instruction will always branch
%% to Fail.
Fail;
yes ->
%% This test instruction will never branch,
%% so we will look at the next instruction.
shortcut_any_label(Is, Prev);
maybe ->
%% May or may not branch. From now on, we can only
%% shortcut to the this specific failure label
%% Fail.
shortcut_specific_label(Is, Fail, Prev)
end
end.
%% shortcut_specific_label([Instruction], FailLabel, PrevCondition) ->
%% FailLabel | error
%% We have previously encountered a test instruction that may or
%% may not branch to FailLabel. Therefore we are only allowed
%% to do the shortcut to the same fail label (FailLabel).
shortcut_specific_label([{label,_}|Is], Fail, Prev) ->
shortcut_specific_label(Is, Fail, Prev);
shortcut_specific_label([{select,select_val,R,{f,F},L}|_], Fail, Prev) ->
case shortcut_selectval(L, R, F, Prev) of
Fail -> Fail;
_ -> error
end;
shortcut_specific_label([I|Is], Fail, Prev) ->
case normalize_op(I) of
error ->
error;
{Normalized,Fail} ->
case will_succeed(Prev, Normalized) of
no ->
%% Will branch to FailLabel.
Fail;
yes ->
%% Will definitely never branch.
shortcut_specific_label(Is, Fail, Prev);
maybe ->
%% May branch, but still OK since it will branch
%% to FailLabel.
shortcut_specific_label(Is, Fail, Prev)
end;
{Normalized,_} ->
%% This test instruction will branch to a different
%% fail label, if it branches at all.
case will_succeed(Prev, Normalized) of
yes ->
%% Still OK, since the branch will never be
%% taken.
shortcut_specific_label(Is, Fail, Prev);
no ->
%% Give up. The branch will definitely be taken
%% to a different fail label.
error;
maybe ->
%% Give up. If the branch is taken, it will be
%% to a different fail label.
error
end
end.
%% shortcut_selectval(List, Reg, Fail, PrevCond) -> FailLabel | error
%% Try to shortcut a selectval instruction. A selectval instruction
%% is equivalent to the following instruction sequence:
%%
%% is_ne_exact L1 Reg Value1
%% .
%% .
%% .
%% is_ne_exact LN Reg ValueN
%% jump DefaultFailLabel
%%
shortcut_selectval([Val,{f,Lbl}|T], R, Fail, Prev) ->
case will_succeed(Prev, {'=/=',R,get_literal(Val)}) of
yes -> shortcut_selectval(T, R, Fail, Prev);
no -> Lbl;
maybe -> error
end;
shortcut_selectval([], _, Fail, _) -> Fail.
%% will_succeed(PrevCondition, Condition) -> yes | no | maybe
%% PrevCondition is a condition known to be true. This function
%% will tell whether Condition will succeed.
will_succeed({Op1,Reg,A}, {Op2,Reg,B}) ->
will_succeed_1(Op1, A, Op2, B);
will_succeed({'=:=',Reg,{literal,A}}, {TypeTest,Reg}) ->
case erlang:TypeTest(A) of
false -> no;
true -> yes
end;
will_succeed({_,_,_}, maybe) ->
maybe;
will_succeed({_,_,_}, Test) when is_tuple(Test) ->
maybe.
will_succeed_1('=:=', A, '<', B) ->
if
B =< A -> no;
true -> yes
end;
will_succeed_1('=:=', A, '=<', B) ->
if
B < A -> no;
true -> yes
end;
will_succeed_1('=:=', A, '=:=', B) ->
if
A =:= B -> yes;
true -> no
end;
will_succeed_1('=:=', A, '=/=', B) ->
if
A =:= B -> no;
true -> yes
end;
will_succeed_1('=:=', A, '>=', B) ->
if
B > A -> no;
true -> yes
end;
will_succeed_1('=:=', A, '>', B) ->
if
B >= A -> no;
true -> yes
end;
will_succeed_1('=/=', A, '=/=', B) when A =:= B -> yes;
will_succeed_1('=/=', A, '=:=', B) when A =:= B -> no;
will_succeed_1('<', A, '=:=', B) when B >= A -> no;
will_succeed_1('<', A, '=/=', B) when B >= A -> yes;
will_succeed_1('<', A, '<', B) when B >= A -> yes;
will_succeed_1('<', A, '=<', B) when B > A -> yes;
will_succeed_1('<', A, '>=', B) when B > A -> no;
will_succeed_1('<', A, '>', B) when B >= A -> no;
will_succeed_1('=<', A, '=:=', B) when B > A -> no;
will_succeed_1('=<', A, '=/=', B) when B > A -> yes;
will_succeed_1('=<', A, '<', B) when B > A -> yes;
will_succeed_1('=<', A, '=<', B) when B >= A -> yes;
will_succeed_1('=<', A, '>=', B) when B > A -> no;
will_succeed_1('=<', A, '>', B) when B >= A -> no;
will_succeed_1('>=', A, '=:=', B) when B < A -> no;
will_succeed_1('>=', A, '=/=', B) when B < A -> yes;
will_succeed_1('>=', A, '<', B) when B =< A -> no;
will_succeed_1('>=', A, '=<', B) when B < A -> no;
will_succeed_1('>=', A, '>=', B) when B =< A -> yes;
will_succeed_1('>=', A, '>', B) when B < A -> yes;
will_succeed_1('>', A, '=:=', B) when B =< A -> no;
will_succeed_1('>', A, '=/=', B) when B =< A -> yes;
will_succeed_1('>', A, '<', B) when B =< A -> no;
will_succeed_1('>', A, '=<', B) when B < A -> no;
will_succeed_1('>', A, '>=', B) when B =< A -> yes;
will_succeed_1('>', A, '>', B) when B < A -> yes;
will_succeed_1(_, _, _, _) -> maybe.
%% normalize_op(Instruction) -> {Normalized,FailLabel} | error
%% Normalized = {Operator,Register,Literal} |
%% {TypeTest,Register} |
%% maybe
%% Operation = '<' | '=<' | '=:=' | '=/=' | '>=' | '>'
%% TypeTest = is_atom | is_integer ...
%% Literal = {literal,Term}
%%
%% Normalize a relational operator to facilitate further
%% comparisons between operators. Always make the register
%% operand the first operand. Thus the following instruction:
%%
%% {test,is_ge,{f,99},{integer,13},{x,0}}
%%
%% will be normalized to:
%%
%% {'=<',{x,0},{literal,13}}
%%
%% NOTE: Bit syntax test instructions are scary. They may change the
%% state of match contexts and update registers, so we don't dare
%% mess with them.
normalize_op({test,is_ge,{f,Fail},Ops}) ->
normalize_op_1('>=', Ops, Fail);
normalize_op({test,is_lt,{f,Fail},Ops}) ->
normalize_op_1('<', Ops, Fail);
normalize_op({test,is_eq_exact,{f,Fail},Ops}) ->
normalize_op_1('=:=', Ops, Fail);
normalize_op({test,is_ne_exact,{f,Fail},Ops}) ->
normalize_op_1('=/=', Ops, Fail);
normalize_op({test,is_nil,{f,Fail},[R]}) ->
normalize_op_1('=:=', [R,nil], Fail);
normalize_op({test,Op,{f,Fail},[R]}) ->
case erl_internal:new_type_test(Op, 1) of
true -> {{Op,R},Fail};
false -> {maybe,Fail}
end;
normalize_op({test,_,{f,Fail},_}=I) ->
case beam_utils:is_pure_test(I) of
true -> {maybe,Fail};
false -> error
end;
normalize_op(_) ->
error.
normalize_op_1(Op, [Op1,Op2], Fail) ->
case {get_literal(Op1),get_literal(Op2)} of
{error,error} ->
%% Both operands are registers.
{maybe,Fail};
{error,Lit} ->
{{Op,Op1,Lit},Fail};
{Lit,error} ->
{{turn_op(Op),Op2,Lit},Fail};
{_,_} ->
%% Both operands are literals. Can probably only
%% happen if the Core Erlang optimizations passes were
%% turned off, so don't bother trying to do something
%% smart here.
{maybe,Fail}
end.
turn_op('<') -> '>';
turn_op('>=') -> '=<';
turn_op('=:='=Op) -> Op;
turn_op('=/='=Op) -> Op.
negate_op('>=') -> '<';
negate_op('<') -> '>=';
negate_op('=<') -> '>';
negate_op('>') -> '=<';
negate_op('=:=') -> '=/=';
negate_op('=/=') -> '=:='.
get_literal({atom,Val}) ->
{literal,Val};
get_literal({integer,Val}) ->
{literal,Val};
get_literal({float,Val}) ->
{literal,Val};
get_literal(nil) ->
{literal,[]};
get_literal({literal,_}=Lit) ->
Lit;
get_literal({_,_}) -> error.
%%%
%%% Removing stores to Y registers is not always safe
%%% if there is an instruction that causes an exception
%%% within a catch. In practice, there are few or no
%%% opportunities for removing stores to Y registers anyway
%%% if sys_core_fold has been run.
%%%
is_killed_at({x,_}=Reg, Lbl, D) ->
beam_utils:is_killed_at(Reg, Lbl, D);
is_killed_at({y,_}, _, _) ->
false.
