%%
%% %CopyrightBegin%
%%
%% Copyright Ericsson AB 1999-2013. 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_reorder).
-export([module/2]).
-import(lists, [member/2,reverse/1]).
module({Mod,Exp,Attr,Fs0,Lc}, _Opt) ->
Fs = [function(F) || F <- Fs0],
{ok,{Mod,Exp,Attr,Fs,Lc}}.
function({function,Name,Arity,CLabel,Is0}) ->
try
Is = reorder(Is0),
{function,Name,Arity,CLabel,Is}
catch
Class:Error ->
Stack = erlang:get_stacktrace(),
io:fwrite("Function: ~w/~w\n", [Name,Arity]),
erlang:raise(Class, Error, Stack)
end.
%% reorder(Instructions0) -> Instructions
%% Reorder instructions before the beam_block pass, because reordering
%% will be more cumbersome when the blocks are in place.
%%
%% Execution of get_tuple_element instructions can be delayed until
%% they are actually needed. Consider the sequence:
%%
%% get_tuple_element Tuple Pos Dst
%% test Test Fail Operands
%%
%% If Dst is killed at label Fail (and not referenced in Operands),
%% we can can swap the instructions:
%%
%% test Test Fail Operands
%% get_tuple_element Tuple Pos Dst
%%
%% That can be beneficial in two ways: Firstly, if the branch is taken
%% we have avoided execution of the get_tuple_element instruction.
%% Secondly, even if the branch is not taken, subsequent optimization
%% (opt_blocks/1) may be able to change Dst to the final destination
%% register and eliminate a 'move' instruction.
reorder(Is) ->
D = beam_utils:index_labels(Is),
reorder_1(Is, D, []).
reorder_1([{Op,_,_}=TryCatch|[I|Is]=Is0], D, Acc)
when Op =:= 'catch'; Op =:= 'try' ->
%% Don't allow 'try' or 'catch' instructions to split blocks if
%% it can be avoided.
case is_safe(I) of
false ->
reorder_1(Is0, D, [TryCatch|Acc]);
true ->
reorder_1([TryCatch|Is], D, [I|Acc])
end;
reorder_1([{label,L}=I|_], D, Acc) ->
Is = beam_utils:code_at(L, D),
reorder_1(Is, D, [I|Acc]);
reorder_1([{test,is_nonempty_list,_,_}=I|Is], D, Acc) ->
%% The run-time system may combine the is_nonempty_list test with
%% the following get_list instruction.
reorder_1(Is, D, [I|Acc]);
reorder_1([{test,_,_,_}=I,
{select,_,_,_,_}=S|Is], D, Acc) ->
%% There is nothing to gain by inserting a get_tuple_element
%% instruction between the test instruction and the select
%% instruction.
reorder_1(Is, D, [S,I|Acc]);
reorder_1([{test,_,{f,L},Ss}=I|Is0], D0,
[{get_tuple_element,_,_,El}=G|Acc0]=Acc) ->
case member(El, Ss) of
true ->
reorder_1(Is0, D0, [I|Acc]);
false ->
case beam_utils:is_killed_at(El, L, D0) of
true ->
Is = [I,G|Is0],
reorder_1(Is, D0, Acc0);
false ->
case beam_utils:is_killed(El, Is0, D0) of
true ->
Code0 = beam_utils:code_at(L, D0),
Code = [G|Code0],
D = beam_utils:index_label(L, Code, D0),
Is = [I|Is0],
reorder_1(Is, D, Acc0);
false ->
reorder_1(Is0, D0, [I|Acc])
end
end
end;
reorder_1([{allocate_zero,N,Live}=I0|Is], D,
[{get_tuple_element,{x,Tup},_,{x,Dst}}=G|Acc]=Acc0) ->
case Tup < Dst andalso Dst+1 =:= Live of
true ->
%% Move allocation instruction upwards past
%% get_tuple_element instructions to create more
%% opportunities for moving get_tuple_element
%% instructions.
I = {allocate_zero,N,Dst},
reorder_1([I,G|Is], D, Acc);
false ->
reorder_1(Is, D, [I0|Acc0])
end;
reorder_1([I|Is], D, Acc) ->
reorder_1(Is, D, [I|Acc]);
reorder_1([], _, Acc) -> reverse(Acc).
%% is_safe(Instruction) -> true|false
%% Test whether an instruction is safe (cannot cause an exception).
is_safe({kill,_}) -> true;
is_safe({move,_,_}) -> true;
is_safe({put,_}) -> true;
is_safe({put_list,_,_,_}) -> true;
is_safe({put_tuple,_,_}) -> true;
is_safe({test_heap,_,_}) -> true;
is_safe(_) -> false.
