%%
%% %CopyrightBegin%
%%
%% Copyright Ericsson AB 1999-2018. 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%
%%
%%% Purpose : Optimise jumps and remove unreachable code.
-module(beam_jump).
-export([module/2,
is_unreachable_after/1,is_exit_instruction/1,
remove_unused_labels/1]).
%%% The following optimisations are done:
%%%
%%% (1) This code with two identical instruction sequences
%%%
%%% L1: <Instruction sequence>
%%% L2:
%%% . . .
%%% L3: <Instruction sequence>
%%% L4:
%%%
%%% can be replaced with
%%%
%%% L1: jump L3
%%% L2:
%%% . . .
%%% L3: <Instruction sequence>
%%% L4
%%%
%%% Note: The instruction sequence must end with an instruction
%%% such as a jump that never transfers control to the instruction
%%% following it.
%%%
%%% (2) Short sequences starting with a label and ending in case_end, if_end,
%%% and badmatch, and function calls that cause an exit (such as calls
%%% to exit/1) are moved to the end of the function, but only if the
%%% the block is not entered via a fallthrough. The purpose of this move
%%% is to allow further optimizations at the place from which the
%%% code was moved (a jump around the block could be replaced with a
%%% fallthrough).
%%%
%%% (3) Any unreachable code is removed. Unreachable code is code
%%% after jump, call_last and other instructions which never
%%% transfer control to the following instruction. Code is
%%% unreachable up to the next *referenced* label. Note that the
%%% optimisations below might generate more possibilities for
%%% removing unreachable code.
%%%
%%% (4) This code:
%%% L1: jump L2
%%% . . .
%%% L2: ...
%%%
%%% will be changed to
%%%
%%% jump L2
%%% . . .
%%% L2: ...
%%%
%%% and all preceding uses of L1 renamed to L2.
%%% If the jump is unreachable, it will be removed according to (1).
%%%
%%% (5) In
%%%
%%% jump L1
%%% L1:
%%%
%%% the jump (but not the label) will be removed.
%%%
%%% (6) If test instructions are used to skip a single jump instruction,
%%% the test is inverted and the jump is eliminated (provided that
%%% the test can be inverted). Example:
%%%
%%% is_eq L1 {x,1} {x,2}
%%% jump L2
%%% L1:
%%%
%%% will be changed to
%%%
%%% is_ne L2 {x,1} {x,2}
%%% L1:
%%%
%%% Because there may be backward references to the label L1
%%% (for instance from the wait_timeout/1 instruction), we will
%%% always keep the label. (beam_clean will remove any unused
%%% labels.)
%%%
%%% Note: This modules depends on (almost) all branches and jumps only
%%% going forward, so that we can remove instructions (including definition
%%% of labels) after any label that has not been referenced by the code
%%% preceeding the labels. Regarding the few instructions that have backward
%%% references to labels, we assume that they only transfer control back
%%% to an instruction that has already been executed. That is, code such as
%%%
%%% jump L_entry
%%%
%%% L_again:
%%% .
%%% .
%%% .
%%% L_entry:
%%% .
%%% .
%%% .
%%% jump L_again;
%%%
%%% is NOT allowed (and such code is never generated by the code generator).
%%%
%%% Terminology note: The optimisation done here is called unreachable-code
%%% removal, NOT dead-code elimination. Dead code elimination means the
%%% removal of instructions that are executed, but have no visible effect
%%% on the program state.
%%%
-import(lists, [reverse/1,reverse/2,foldl/3]).
-type instruction() :: beam_utils:instruction().
-spec module(beam_utils:module_code(), [compile:option()]) ->
{'ok',beam_utils:module_code()}.
module({Mod,Exp,Attr,Fs0,Lc}, _Opt) ->
Fs = [function(F) || F <- Fs0],
{ok,{Mod,Exp,Attr,Fs,Lc}}.
%% function(Function) -> Function'
%% Optimize jumps and branches.
%%
%% NOTE: This function assumes that there are no labels inside blocks.
function({function,Name,Arity,CLabel,Asm0}) ->
Asm1 = share(Asm0),
Asm2 = move(Asm1),
Asm3 = opt(Asm2, CLabel),
Asm = remove_unused_labels(Asm3),
{function,Name,Arity,CLabel,Asm}.
%%%
%%% (1) We try to share the code for identical code segments by replacing all
%%% occurrences except the last with jumps to the last occurrence.
%%%
share(Is0) ->
%% We will get more sharing if we never fall through to a label.
Is = eliminate_fallthroughs(Is0, []),
share_1(Is, #{}, [], []).
share_1([{label,L}=Lbl|Is], Dict0, [_|_]=Seq, Acc) ->
case maps:find(Seq, Dict0) of
error ->
Dict = maps:put(Seq, L, Dict0),
share_1(Is, Dict, [], [Lbl|Seq ++ Acc]);
{ok,Label} ->
share_1(Is, Dict0, [], [Lbl,{jump,{f,Label}}|Acc])
end;
share_1([{func_info,_,_,_}=I|Is], _, [], Acc) ->
reverse(Is, [I|Acc]);
share_1([{'catch',_,_}=I|Is], Dict0, Seq, Acc) ->
Dict = clean_non_sharable(Dict0),
share_1(Is, Dict, [I|Seq], Acc);
share_1([{'try',_,_}=I|Is], Dict0, Seq, Acc) ->
Dict = clean_non_sharable(Dict0),
share_1(Is, Dict, [I|Seq], Acc);
share_1([{try_case,_}=I|Is], Dict0, Seq, Acc) ->
Dict = clean_non_sharable(Dict0),
share_1(Is, Dict, [I|Seq], Acc);
share_1([{catch_end,_}=I|Is], Dict0, Seq, Acc) ->
Dict = clean_non_sharable(Dict0),
share_1(Is, Dict, [I|Seq], Acc);
share_1([I|Is], Dict, Seq, Acc) ->
case is_unreachable_after(I) of
false ->
share_1(Is, Dict, [I|Seq], Acc);
true ->
share_1(Is, Dict, [I], Acc)
end.
clean_non_sharable(Dict) ->
%% We are passing in or out of a 'catch' or 'try' block. Remove
%% sequences that should not be shared over the boundaries of the
%% block. Since the end of the sequence must match, the only
%% possible match between a sequence outside and a sequence inside
%% the 'catch'/'try' block is a sequence that ends with an
%% instruction that causes an exception. Any sequence that causes
%% an exception must contain a line/1 instruction.
maps:filter(fun(K, _V) -> sharable_with_try(K) end, Dict).
sharable_with_try([{line,_}|_]) ->
%% This sequence may cause an exception and may potentially
%% match a sequence on the other side of the 'catch'/'try' block
%% boundary.
false;
sharable_with_try([_|Is]) ->
sharable_with_try(Is);
sharable_with_try([]) -> true.
%% Eliminate all fallthroughs. Return the result reversed.
eliminate_fallthroughs([{label,L}=Lbl|Is], [I|_]=Acc) ->
case is_unreachable_after(I) of
false ->
%% Eliminate fallthrough.
eliminate_fallthroughs(Is, [Lbl,{jump,{f,L}}|Acc]);
true ->
eliminate_fallthroughs(Is, [Lbl|Acc])
end;
eliminate_fallthroughs([I|Is], Acc) ->
eliminate_fallthroughs(Is, [I|Acc]);
eliminate_fallthroughs([], Acc) -> Acc.
%%%
%%% (2) Move short code sequences ending in an instruction that causes an exit
%%% to the end of the function.
%%%
%%% Implementation note: Since share/1 eliminated fallthroughs to labels,
%%% we don't have to test whether instructions before labels may fail through.
%%%
move(Is) ->
move_1(Is, [], []).
move_1([I|Is], Ends, Acc0) ->
case is_exit_instruction(I) of
false ->
move_1(Is, Ends, [I|Acc0]);
true ->
case extract_seq(Acc0, [I]) of
no ->
move_1(Is, Ends, [I|Acc0]);
{yes,End,Acc} ->
move_1(Is, [End|Ends], Acc)
end
end;
move_1([], Ends, Acc) -> reverse(Acc, lists:append(reverse(Ends))).
extract_seq([{line,_}=Line|Is], Acc) ->
extract_seq(Is, [Line|Acc]);
extract_seq([{block,_}=Bl|Is], Acc) ->
extract_seq_1(Is, [Bl|Acc]);
extract_seq([{bs_context_to_binary,_}=I|Is], Acc) ->
extract_seq_1(Is, [I|Acc]);
extract_seq([{label,_}|_]=Is, Acc) ->
extract_seq_1(Is, Acc);
extract_seq(_, _) -> no.
extract_seq_1([{line,_}=Line|Is], Acc) ->
extract_seq_1(Is, [Line|Acc]);
extract_seq_1([{label,_},{func_info,_,_,_}|_], _) ->
no;
extract_seq_1([{label,Lbl},{jump,{f,Lbl}}|_], _) ->
%% Don't move a sequence which have a fallthrough entering it.
no;
extract_seq_1([{label,_}=Lbl|Is], Acc) ->
{yes,[Lbl|Acc],Is};
extract_seq_1(_, _) -> no.
%%%
%%% (3) (4) (5) (6) Jump and unreachable code optimizations.
%%%
-record(st,
{
entry :: beam_asm:label(), %Entry label (must not be moved).
replace :: #{beam_asm:label() := beam_asm:label()}, %Labels to replace.
labels :: cerl_sets:set(), %Set of referenced labels.
index :: beam_utils:code_index() | {lazy,[beam_utils:instruction()]} %Index built lazily only if needed
}).
opt(Is0, CLabel) ->
find_fixpoint(fun(Is) ->
Lbls = initial_labels(Is),
St = #st{entry=CLabel,replace=#{},labels=Lbls,index={lazy,Is}},
opt(Is, [], St)
end, Is0).
find_fixpoint(OptFun, Is0) ->
case OptFun(Is0) of
Is0 -> Is0;
Is -> find_fixpoint(OptFun, Is)
end.
opt([{test,_,{f,L}=Lbl,_}=I|[{jump,{f,L}}|_]=Is], Acc0, St0) ->
%% We have
%% Test Label Ops
%% jump Label
%% The test instruction is not needed if the test is pure
%% (it modifies neither registers nor bit syntax state).
case beam_utils:is_pure_test(I) of
false ->
%% Test is not pure; we must keep it.
opt(Is, [I|Acc0], label_used(Lbl, St0));
true ->
%% The test is pure and its failure label is the same
%% as in the jump that follows -- thus it is not needed.
%% Check if any of the previous instructions could also be eliminated.
{Acc,St} = opt_useless_loads(Acc0, L, St0),
opt(Is, Acc, St)
end;
opt([{test,_,{f,L}=Lbl,_}=I|[{label,L}|_]=Is], Acc0, St0) ->
%% Similar to the above, except we have a fall-through rather than jump
%% Test Label Ops
%% label Label
case beam_utils:is_pure_test(I) of
false ->
opt(Is, [I|Acc0], label_used(Lbl, St0));
true ->
{Acc,St} = opt_useless_loads(Acc0, L, St0),
opt(Is, Acc, St)
end;
opt([{test,Test0,{f,L}=Lbl,Ops}=I|[{jump,To}|Is]=Is0], Acc, St) ->
case is_label_defined(Is, L) of
false ->
opt(Is0, [I|Acc], label_used(Lbl, St));
true ->
case invert_test(Test0) of
not_possible ->
opt(Is0, [I|Acc], label_used(Lbl, St));
Test ->
%% Invert the test and remove the jump.
opt([{test,Test,To,Ops}|Is], Acc, St)
end
end;
opt([{test,_,{f,_}=Lbl,_}=I|Is], Acc, St) ->
opt(Is, [I|Acc], label_used(Lbl, St));
opt([{test,_,{f,_}=Lbl,_,_,_}=I|Is], Acc, St) ->
opt(Is, [I|Acc], label_used(Lbl, St));
opt([{select,_,_R,Fail,Vls}=I|Is], Acc, St) ->
skip_unreachable(Is, [I|Acc], label_used([Fail|Vls], St));
opt([{label,From}=I,{label,To}|Is], Acc, #st{replace=Replace}=St) ->
opt([I|Is], Acc, St#st{replace=Replace#{To => From}});
opt([{jump,{f,_}=X}|[{label,_},{jump,X}|_]=Is], Acc, St) ->
opt(Is, Acc, St);
opt([{jump,{f,Lbl}}|[{label,Lbl}|_]=Is], Acc, St) ->
opt(Is, Acc, St);
opt([{jump,{f,L}=Lbl}=I|Is], Acc0, St0) ->
%% Replace all labels before this jump instruction into the
%% location of the jump's target.
{Acc,St} = collect_labels(Acc0, L, St0),
skip_unreachable(Is, [I|Acc], label_used(Lbl, St));
%% Optimization: quickly handle some common instructions that don't
%% have any failure labels and where is_unreachable_after(I) =:= false.
opt([{block,_}=I|Is], Acc, St) ->
opt(Is, [I|Acc], St);
opt([{kill,_}=I|Is], Acc, St) ->
opt(Is, [I|Acc], St);
opt([{call,_,_}=I|Is], Acc, St) ->
opt(Is, [I|Acc], St);
opt([{deallocate,_}=I|Is], Acc, St) ->
opt(Is, [I|Acc], St);
%% All other instructions.
opt([I|Is], Acc, #st{labels=Used0}=St0) ->
Used = ulbl(I, Used0),
St = St0#st{labels=Used},
case is_unreachable_after(I) of
true -> skip_unreachable(Is, [I|Acc], St);
false -> opt(Is, [I|Acc], St)
end;
opt([], Acc, #st{replace=Replace0}) when Replace0 =/= #{} ->
Replace = normalize_replace(maps:to_list(Replace0), Replace0, []),
beam_utils:replace_labels(Acc, [], Replace, fun(Old) -> Old end);
opt([], Acc, #st{replace=Replace}) when Replace =:= #{} ->
reverse(Acc).
normalize_replace([{From,To0}|Rest], Replace, Acc) ->
case Replace of
#{To0 := To} ->
normalize_replace([{From,To}|Rest], Replace, Acc);
_ ->
normalize_replace(Rest, Replace, [{From,To0}|Acc])
end;
normalize_replace([], _Replace, Acc) ->
maps:from_list(Acc).
%% After eliminating a test, it might happen, that a register was only used
%% in this test. Let's check if that was the case and if it was so, we can
%% eliminate the load into the register completely.
opt_useless_loads([{block,_}|_]=Is, L, #st{index={lazy,FIs}}=St) ->
opt_useless_loads(Is, L, St#st{index=beam_utils:index_labels(FIs)});
opt_useless_loads([{block,Block0}|Is], L, #st{index=Index}=St) ->
case opt_useless_block_loads(Block0, L, Index) of
[] ->
opt_useless_loads(Is, L, St);
[_|_]=Block ->
{[{block,Block}|Is],St}
end;
%% After eliminating the test and useless blocks, it might happen,
%% that the previous test could also be eliminated.
%% It might be that the label was already marked as used, even if ultimately,
%% it never will be - we can't do much about it at that point, though
opt_useless_loads([{test,_,{f,L},_}=I|Is], L, St) ->
case beam_utils:is_pure_test(I) of
false ->
{[I|Is],St};
true ->
opt_useless_loads(Is, L, St)
end;
opt_useless_loads(Is, _L, St) ->
{Is,St}.
opt_useless_block_loads([{set,[Dst],_,_}=I|Is], L, Index) ->
BlockJump = [{block,Is},{jump,{f,L}}],
case beam_utils:is_killed(Dst, BlockJump, Index) of
true ->
%% The register is killed and not used, we can remove the load
opt_useless_block_loads(Is, L, Index);
false ->
[I|opt_useless_block_loads(Is, L, Index)]
end;
opt_useless_block_loads([I|Is], L, Index) ->
[I|opt_useless_block_loads(Is, L, Index)];
opt_useless_block_loads([], _L, _Index) ->
[].
collect_labels(Is, Label, #st{entry=Entry,replace=Replace} = St) ->
collect_labels_1(Is, Label, Entry, Replace, St).
collect_labels_1([{label,Entry}|_]=Is, _Label, Entry, Acc, St) ->
%% Never move the entry label.
{Is,St#st{replace=Acc}};
collect_labels_1([{label,L}|Is], Label, Entry, Acc, St) ->
collect_labels_1(Is, Label, Entry, Acc#{L => Label}, St);
collect_labels_1(Is, _Label, _Entry, Acc, St) ->
{Is,St#st{replace=Acc}}.
%% label_defined(Is, Label) -> true | false.
%% Test whether the label Label is defined at the start of the instruction
%% sequence, possibly preceeded by other label definitions.
%%
is_label_defined([{label,L}|_], L) -> true;
is_label_defined([{label,_}|Is], L) -> is_label_defined(Is, L);
is_label_defined(_, _) -> false.
%% invert_test(Test0) -> not_possible | Test
invert_test(is_ge) -> is_lt;
invert_test(is_lt) -> is_ge;
invert_test(is_eq) -> is_ne;
invert_test(is_ne) -> is_eq;
invert_test(is_eq_exact) -> is_ne_exact;
invert_test(is_ne_exact) -> is_eq_exact;
invert_test(_) -> not_possible.
%% skip_unreachable([Instruction], St).
%% Remove all instructions (including definitions of labels
%% that have not been referenced yet) up to the next
%% referenced label, then call opt/3 to optimize the rest
%% of the instruction sequence.
%%
skip_unreachable([{label,L}|_Is]=Is0, [{jump,{f,L}}|Acc], St) ->
opt(Is0, Acc, St);
skip_unreachable([{label,L}|Is]=Is0, Acc, St) ->
case is_label_used(L, St) of
true -> opt(Is0, Acc, St);
false -> skip_unreachable(Is, Acc, St)
end;
skip_unreachable([_|Is], Acc, St) ->
skip_unreachable(Is, Acc, St);
skip_unreachable([], Acc, St) ->
opt([], Acc, St).
%% Add one or more label to the set of used labels.
label_used({f,L}, St) -> St#st{labels=cerl_sets:add_element(L,St#st.labels)};
label_used([H|T], St0) -> label_used(T, label_used(H, St0));
label_used([], St) -> St;
label_used(_Other, St) -> St.
%% Test if label is used.
is_label_used(L, St) ->
cerl_sets:is_element(L, St#st.labels).
%% is_unreachable_after(Instruction) -> boolean()
%% Test whether the code after Instruction is unreachable.
-spec is_unreachable_after(instruction()) -> boolean().
is_unreachable_after({func_info,_M,_F,_A}) -> true;
is_unreachable_after(return) -> true;
is_unreachable_after({jump,_Lbl}) -> true;
is_unreachable_after({select,_What,_R,_Lbl,_Cases}) -> true;
is_unreachable_after({loop_rec_end,_}) -> true;
is_unreachable_after({wait,_}) -> true;
is_unreachable_after(I) -> is_exit_instruction(I).
%% is_exit_instruction(Instruction) -> boolean()
%% Test whether the instruction Instruction always
%% causes an exit/failure.
-spec is_exit_instruction(instruction()) -> boolean().
is_exit_instruction({call_ext,_,{extfunc,M,F,A}}) ->
erl_bifs:is_exit_bif(M, F, A);
is_exit_instruction(if_end) -> true;
is_exit_instruction({case_end,_}) -> true;
is_exit_instruction({try_case_end,_}) -> true;
is_exit_instruction({badmatch,_}) -> true;
is_exit_instruction(_) -> false.
%% remove_unused_labels(Instructions0) -> Instructions
%% Remove all unused labels. Also remove unreachable
%% instructions following labels that are removed.
-spec remove_unused_labels([instruction()]) -> [instruction()].
remove_unused_labels(Is) ->
Used0 = initial_labels(Is),
Used = foldl(fun ulbl/2, Used0, Is),
rem_unused(Is, Used, []).
rem_unused([{label,Lbl}=I|Is0], Used, [Prev|_]=Acc) ->
case cerl_sets:is_element(Lbl, Used) of
false ->
Is = case is_unreachable_after(Prev) of
true -> drop_upto_label(Is0);
false -> Is0
end,
rem_unused(Is, Used, Acc);
true ->
rem_unused(Is0, Used, [I|Acc])
end;
rem_unused([I|Is], Used, Acc) ->
rem_unused(Is, Used, [I|Acc]);
rem_unused([], _, Acc) -> reverse(Acc).
initial_labels(Is) ->
initial_labels(Is, []).
initial_labels([{line,_}|Is], Acc) ->
initial_labels(Is, Acc);
initial_labels([{label,Lbl}|Is], Acc) ->
initial_labels(Is, [Lbl|Acc]);
initial_labels([{func_info,_,_,_},{label,Lbl}|_], Acc) ->
cerl_sets:from_list([Lbl|Acc]).
drop_upto_label([{label,_}|_]=Is) -> Is;
drop_upto_label([_|Is]) -> drop_upto_label(Is);
drop_upto_label([]) -> [].
%% ulbl(Instruction, UsedGbSet) -> UsedGbSet'
%% Update the gb_set UsedGbSet with any function-local labels
%% (i.e. not with labels in call instructions) referenced by
%% the instruction Instruction.
%%
%% NOTE: This function does NOT look for labels inside blocks.
ulbl({test,_,Fail,_}, Used) ->
mark_used(Fail, Used);
ulbl({test,_,Fail,_,_,_}, Used) ->
mark_used(Fail, Used);
ulbl({select,_,_,Fail,Vls}, Used) ->
mark_used_list(Vls, mark_used(Fail, Used));
ulbl({'try',_,Lbl}, Used) ->
mark_used(Lbl, Used);
ulbl({'catch',_,Lbl}, Used) ->
mark_used(Lbl, Used);
ulbl({jump,Lbl}, Used) ->
mark_used(Lbl, Used);
ulbl({loop_rec,Lbl,_}, Used) ->
mark_used(Lbl, Used);
ulbl({loop_rec_end,Lbl}, Used) ->
mark_used(Lbl, Used);
ulbl({wait,Lbl}, Used) ->
mark_used(Lbl, Used);
ulbl({wait_timeout,Lbl,_To}, Used) ->
mark_used(Lbl, Used);
ulbl({bif,_Name,Lbl,_As,_R}, Used) ->
mark_used(Lbl, Used);
ulbl({gc_bif,_Name,Lbl,_Live,_As,_R}, Used) ->
mark_used(Lbl, Used);
ulbl({bs_init,Lbl,_,_,_,_}, Used) ->
mark_used(Lbl, Used);
ulbl({bs_put,Lbl,_,_}, Used) ->
mark_used(Lbl, Used);
ulbl({put_map,Lbl,_Op,_Src,_Dst,_Live,_List}, Used) ->
mark_used(Lbl, Used);
ulbl({get_map_elements,Lbl,_Src,_List}, Used) ->
mark_used(Lbl, Used);
ulbl(_, Used) -> Used.
mark_used({f,0}, Used) -> Used;
mark_used({f,L}, Used) -> cerl_sets:add_element(L, Used).
mark_used_list([{f,L}|T], Used) ->
mark_used_list(T, cerl_sets:add_element(L, Used));
mark_used_list([_|T], Used) ->
mark_used_list(T, Used);
mark_used_list([], Used) -> Used.
