aboutsummaryrefslogtreecommitdiffstats
path: root/lib/compiler/src/beam_jump.erl
blob: 100acaede3e3e3947e52687ebfc85ba064895292 (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
%%
%% %CopyrightBegin%
%%
%% Copyright Ericsson AB 1999-2013. All Rights Reserved.
%%
%% The contents of this file are subject to the Erlang Public License,
%% Version 1.1, (the "License"); you may not use this file except in
%% compliance with the License. You should have received a copy of the
%% Erlang Public License along with this software. If not, it can be
%% retrieved online at http://www.erlang.org/.
%%
%% Software distributed under the License is distributed on an "AS IS"
%% basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
%% the License for the specific language governing rights 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,is_label_used_in/2]).

%%% 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
%%%          . . .
%%%    L1:
%%%    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,dropwhile/2]).

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, dict:new(), [], []).

share_1([{label,_}=Lbl|Is], Dict, [], Acc) ->
    share_1(Is, Dict, [], [Lbl|Acc]);
share_1([{label,L}=Lbl|Is], Dict0, Seq, Acc) ->
    case dict:find(Seq, Dict0) of
	error ->
	    Dict = dict:store(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([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.


%% Eliminate all fallthroughs. Return the result reversed.

eliminate_fallthroughs([I,{label,L}=Lbl|Is], Acc) ->
    case is_unreachable_after(I) orelse is_label(I) of
	false ->
	    %% Eliminate fallthrough.
	    eliminate_fallthroughs(Is, [Lbl,{jump,{f,L}},I|Acc]);
	true ->
	    eliminate_fallthroughs(Is, [Lbl,I|Acc])
    end;
eliminate_fallthroughs([I|Is], Acc) ->
    eliminate_fallthroughs(Is, [I|Acc]);
eliminate_fallthroughs([], Acc) -> Acc.

is_label({label,_}) -> true;
is_label(_) -> false.
    
%%%
%%% (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, {fc,				%Label for function class errors.
	     entry,				%Entry label (must not be moved).
	     mlbl,				%Moved labels.
	     labels				%Set of referenced labels.
	    }).

opt([{label,Fc}|_]=Is0, CLabel) ->
    Lbls = initial_labels(Is0),
    find_fixpoint(fun(Is) ->
			  St = #st{fc=Fc,entry=CLabel,mlbl=dict:new(),
				   labels=Lbls},
			  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], Acc, St) ->
    %% 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|Acc], label_used(Lbl, St));
	true ->
	    %% The test is pure and its failure label is the same
	    %% as in the jump that follows -- thus it is not needed.
	    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,L}=I|Is], Acc, #st{entry=L}=St) ->
    %% NEVER move the entry label.
    opt(Is, [I|Acc], St);
opt([{label,L1},{jump,{f,L2}}=I|Is], [Prev|Acc], St0) ->
    St = St0#st{mlbl=dict:append(L2, L1, St0#st.mlbl)},
    opt([Prev,I|Is], Acc, label_used({f,L2}, St));
opt([{label,Lbl}=I|Is], Acc, #st{mlbl=Mlbl}=St0) ->
    case dict:find(Lbl, Mlbl) of
	{ok,Lbls} ->
	    %% Essential to remove the list of labels from the dictionary,
	    %% since we will rescan the inserted labels.  We MUST rescan.
	    St = St0#st{mlbl=dict:erase(Lbl, Mlbl)},
	    insert_labels([Lbl|Lbls], Is, Acc, St);
	error -> opt(Is, [I|Acc], St0)
    end;
opt([{jump,{f,Lbl}},{label,Lbl}=I|Is], Acc, St) ->
    opt([I|Is], Acc, St);
opt([{jump,Lbl}=I|Is], Acc, St) ->
    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{fc=Fc,mlbl=Mlbl}) ->
    Code = reverse(Acc),
    case dict:find(Fc, Mlbl) of
 	{ok,Lbls} -> insert_fc_labels(Lbls, Mlbl, Code);
 	error -> Code
    end.

insert_fc_labels([L|Ls], Mlbl, Acc0) ->
    Acc = [{label,L}|Acc0],
    case dict:find(L, Mlbl) of
	error ->
	    insert_fc_labels(Ls, Mlbl, Acc);
	{ok,Lbls} ->
	    insert_fc_labels(Lbls++Ls, Mlbl, Acc)
    end;
insert_fc_labels([], _, Acc) -> 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.

insert_labels([L|Ls], Is, [{jump,{f,L}}|Acc], St) ->
    insert_labels(Ls, [{label,L}|Is], Acc, St);
insert_labels([L|Ls], Is, Acc, St) ->
    insert_labels(Ls, [{label,L}|Is], Acc, St);
insert_labels([], Is, Acc, St) ->
    opt(Is, Acc, St).

%% 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=gb_sets:add(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) ->
    gb_sets:is_member(L, St#st.labels).

%% is_unreachable_after(Instruction) -> boolean()
%%  Test whether the code after Instruction is unreachable.

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.

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.

%% is_label_used_in(LabelNumber, [Instruction]) -> boolean()
%%  Check whether the label is used in the instruction sequence
%%  (including inside blocks).

is_label_used_in(Lbl, Is) ->
    is_label_used_in_1(Is, Lbl, gb_sets:empty()).

is_label_used_in_1([{block,Block}|Is], Lbl, Empty) ->
    lists:any(fun(I) -> is_label_used_in_2(I, Lbl) end, Block)
	orelse is_label_used_in_1(Is, Lbl, Empty);
is_label_used_in_1([I|Is], Lbl, Empty) ->
    Used = ulbl(I, Empty),
    gb_sets:is_member(Lbl, Used) orelse is_label_used_in_1(Is, Lbl, Empty);
is_label_used_in_1([], _, _) -> false.

is_label_used_in_2({set,_,_,Info}, Lbl) ->
    case Info of
	{bif,_,{f,F}} -> F =:= Lbl;
	{alloc,_,{gc_bif,_,{f,F}}} -> F =:= Lbl;
	{'catch',{f,F}} -> F =:= Lbl;
	{alloc,_,_} -> false;
	{put_tuple,_} -> false;
	{get_tuple_element,_} -> false;
	{set_tuple_element,_} -> false;
        {get_map_elements,{f,F}} -> F =:= Lbl;
	{line,_} -> false;
	_ when is_atom(Info) -> false
    end.

%% remove_unused_labels(Instructions0) -> Instructions
%%  Remove all unused labels. Also remove unreachable
%%  instructions following labels that are removed.

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 gb_sets:is_member(Lbl, Used) of
	false ->
	    Is = case is_unreachable_after(Prev) of
		     true ->
			 dropwhile(fun({label,_}) -> false;
				      (_) -> true
				   end, 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) ->
    gb_sets:from_list([Lbl|Acc]).

%% 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) -> gb_sets:add(L, Used).

mark_used_list([{f,L}|T], Used) ->
    mark_used_list(T, gb_sets:add(L, Used));
mark_used_list([_|T], Used) ->
    mark_used_list(T, Used);
mark_used_list([], Used) -> Used.