aboutsummaryrefslogtreecommitdiffstats
path: root/lib/compiler/src/beam_dead.erl
blob: 0afb027d3a4eca78e6514ec12d2aace8ab3e617a (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
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
%%
%% %CopyrightBegin%
%%
%% Copyright Ericsson AB 2002-2010. 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%
%%

-module(beam_dead).

-export([module/2]).

%%% The following optimisations are done:
%%%
%%% (1) In this code
%%%
%%%     	move DeadValue {x,0}
%%%     	jump L2
%%%        .
%%%        .
%%%        .
%%%     L2:	move Anything {x,0}
%%%        .
%%%        .
%%%        .
%%%
%%%     the first assignment to {x,0} has no effect (is dead),
%%%     so it can be removed. Besides removing a move instruction,
%%%     if the move was preceeded by a label, the resulting code
%%%	will look this
%%%
%%%     L1:	jump L2
%%%        .
%%%        .
%%%        .
%%%     L2:	move Anything {x,0}
%%%        .
%%%        .
%%%        .
%%%
%%%	which can be further optimized by the jump optimizer (beam_jump).
%%%
%%% (2) In this code
%%%
%%%     L1:	move AtomLiteral {x,0}
%%%     	jump L2
%%%        .
%%%        .
%%%        .
%%%     L2:	test is_atom FailLabel {x,0}
%%%    		select_val {x,0}, FailLabel [... AtomLiteral => L3...]
%%%        .
%%%        .
%%%        .
%%%	L3:	...
%%%
%%%     FailLabel: ...
%%%
%%%	the first code fragment can be changed to
%%%
%%%     L1:	move AtomLiteral {x,0}
%%%     	jump L3
%%%
%%%     If the literal is not included in the table of literals in the
%%%     select_val instruction, the first code fragment will instead be
%%%     rewritten as:
%%%
%%%     L1:	move AtomLiteral {x,0}
%%%     	jump FailLabel
%%%
%%%	The move instruction will be removed by optimization (1) above,
%%%	if the code following the L3 label overwrites {x,0}.
%%%
%%% 	The code following the L2 label will be kept, but it will be removed later
%%%	by the jump optimizer.
%%%
%%% (3) In this code
%%%
%%%     	test is_eq_exact ALabel Src Dst
%%%     	move Src Dst
%%%
%%%	the move instruction can be removed.
%%%     Same thing for
%%%
%%%     	test is_nil ALabel Dst
%%%     	move [] Dst
%%%
%%%
%%% (4) In this code
%%%
%%%    		select_val {x,Reg}, ALabel [... Literal => L1...]
%%%        .
%%%        .
%%%        .
%%%	L1:	move Literal {x,Reg}
%%%
%%%     we can remove the move instruction.
%%%
%%% (5) In the following code
%%%
%%%     	bif '=:=' Fail Src1 Src2 {x,0}
%%%    		jump L1
%%%	   .
%%%	   .
%%%	   .
%%%        L1:	select_val {x,0}, ALabel [... true => L2..., ...false => L3...]
%%%	   .
%%%	   .
%%%	   .
%%%        L2: ....      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.
%%%

-import(lists, [mapfoldl/3,reverse/1]).

module({Mod,Exp,Attr,Fs0,_}, _Opts) ->
    Fs1 = [split_blocks(F) || F <- Fs0],
    {Fs2,Lc1} = beam_clean:clean_labels(Fs1),
    {Fs,Lc} = mapfoldl(fun function/2, Lc1, Fs2),
    %%{Fs,Lc} = {Fs2,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},{func_info,_,_,_}=FI|_] = Is1,
	D0 = beam_utils:empty_label_index(),
	D = beam_utils:index_label(L, [FI], 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.

%% We must split the basic block when we encounter instructions with labels,
%% such as catches and BIFs. All labels must be visible outside the blocks.
%% Also remove empty blocks.

split_blocks({function,Name,Arity,CLabel,Is0}) ->
    Is = split_blocks(Is0, []),
    {function,Name,Arity,CLabel,Is}.

split_blocks([{block,[]}|Is], Acc) ->
    split_blocks(Is, Acc);
split_blocks([{block,Bl}|Is], Acc0) ->
    Acc = split_block(Bl, [], Acc0),
    split_blocks(Is, Acc);
split_blocks([I|Is], Acc) ->
    split_blocks(Is, [I|Acc]);
split_blocks([], Acc) -> reverse(Acc).

split_block([{set,[R],[_,_,_]=As,{bif,is_record,{f,Lbl}}}|Is], Bl, Acc) ->
    %% is_record/3 must be translated by beam_clean; therefore,
    %% it must be outside of any block.
    split_block(Is, [], [{bif,is_record,{f,Lbl},As,R}|make_block(Bl, Acc)]);
split_block([{set,[R],As,{bif,N,{f,Lbl}=Fail}}|Is], Bl, Acc) when Lbl =/= 0 ->
    split_block(Is, [], [{bif,N,Fail,As,R}|make_block(Bl, Acc)]);
split_block([{set,[R],As,{alloc,Live,{gc_bif,N,{f,Lbl}=Fail}}}|Is], Bl, Acc)
  when Lbl =/= 0 ->
    split_block(Is, [], [{gc_bif,N,Fail,Live,As,R}|make_block(Bl, Acc)]);
split_block([{set,[R],[],{'catch',L}}|Is], Bl, Acc) ->
    split_block(Is, [], [{'catch',R,L}|make_block(Bl, Acc)]);
split_block([I|Is], Bl, Acc) ->
    split_block(Is, [I|Bl], Acc);
split_block([], Bl, Acc) -> make_block(Bl, Acc).

make_block([], Acc) -> Acc;
make_block([{set,[D],Ss,{bif,Op,Fail}}|Bl]=Bl0, Acc) ->
    %% If the last instruction in the block is a comparison or boolean operator
    %% (such as '=:='), move it out of the block to facilitate further
    %% optimizations.
    Arity = length(Ss),
    case erl_internal:comp_op(Op, Arity) orelse
	erl_internal:new_type_test(Op, Arity) orelse
	erl_internal:bool_op(Op, Arity) of
	false ->
	    [{block,reverse(Bl0)}|Acc];
	true ->
	    I = {bif,Op,Fail,Ss,D},
	    case Bl =:= [] of
		true -> [I|Acc];
		false -> [I,{block,reverse(Bl)}|Acc]
	    end
    end;
make_block([{set,[Dst],[Src],move}|Bl], Acc) ->
    %% Make optimization of {move,Src,Dst}, {jump,...} possible.
    I = {move,Src,Dst},
    case Bl =:= [] of
	true -> [I|Acc];
	false -> [I,{block,reverse(Bl)}|Acc]
    end;
make_block(Bl, Acc) -> [{block,reverse(Bl)}|Acc].

%% '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 dead code.
%%%

forward(Is, Lc) ->
    forward(Is, gb_trees:empty(), Lc, []).

forward([{block,[]}|Is], D, Lc, Acc) ->
    %% Empty blocks can prevent optimizations.
    forward(Is, D, Lc, Acc);
forward([{select_val,Reg,_,{list,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).
    Block = case gb_trees:lookup({Lbl,Dst}, D) of
		{value,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 gb_trees:lookup({Lbl,Dst}, D) of
	     {value,Lit} -> Is1;     %Safe to remove move instruction.
	     _ -> Is0		     %Keep move instruction.
	 end,
    forward(Is, D, Lc, [LblI|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,is_eq_exact,_,_}=I|Is], D, Lc, Acc) ->
    case Is of
	[{label,_}|_] -> forward(Is, D, Lc, [I|Acc]);
	_ -> forward(Is, D, Lc+1, [{label,Lc},I|Acc])
    end;
forward([{test,is_ne_exact,_,_}=I|Is], D, Lc, Acc) ->
    case Is of
	[{label,_}|_] -> forward(Is, D, Lc, [I|Acc]);
	_ -> 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 gb_trees:lookup(Key, D0) of
	    none -> gb_trees:insert(Key, Lit, D0); %New.
	    {value,inconsistent} -> D0;		%Inconsistent.
	    {value,_} -> gb_trees:update(Key, inconsistent, D0)
	end,
    update_value_dict(T, Reg, D);
update_value_dict([], _, D) -> D.

%%%
%%% Scan instructions in reverse execution order and remove dead 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_val,Reg,{f,Fail0},{list,List0}}|Is], D, Acc) ->
    List = shortcut_select_list(List0, Reg, D, []),
    Fail1 = shortcut_label(Fail0, D),
    Fail = shortcut_bs_test(Fail1, Is, D),
    Sel = {select_val,Reg,{f,Fail},{list,List}},
    backward(Is, D, [Sel|Acc]);
backward([{jump,{f,To0}},{move,Src,Reg}=Move0|Is], D, Acc) ->
    {To,Move} = case Src of
		    {atom,Val0} ->
			To1 = shortcut_select_label(To0, Reg, Val0, D),
			{To2,Val} = shortcut_boolean_label(To1, Reg, Val0, D),
			{To2,{move,{atom,Val},Reg}};
		    _ ->
			{shortcut_label(To0, D),Move0}
		end,
    Jump = {jump,{f,To}},
    case beam_utils: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,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,is_eq_exact,{f,To0},[Reg,{atom,Val}]=Ops}|Is], D, Acc) ->
    To1 = shortcut_bs_test(To0, Is, D),
    To = shortcut_fail_label(To1, Reg, Val, D),
    I = combine_eqs(To, Ops, D, Acc),
    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),
    %% 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(To2, D) of
	     [{test,Op,{f,To3},Ops}|_] ->
		 case equal_ops(Ops0, Ops) of
		     true -> To3;
		     false -> To2
		 end;
	     _Code ->
		 To2
	 end,
    I = case Op of
	    is_eq_exact -> combine_eqs(To, Ops0, D, Acc);
	    _ -> {test,Op,{f,To},Ops0}
	end,
    backward(Is, D, [I|Acc]);
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, [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([{_,Val}=Lit,{f,To0}|T], Reg, D, Acc) ->
    To = shortcut_select_label(To0, Reg, Val, 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(To0, Reg, Val, D) ->
    case beam_utils:code_at(To0, D) of
 	[{jump,{f,To}}|_] ->
 	    shortcut_select_label(To, Reg, Val, D);
	[{test,is_atom,_,[Reg]},{select_val,Reg,{f,Fail},{list,Map}}|_] ->
	    To = find_select_val(Map, Val, Fail),
	    shortcut_select_label(To, Reg, Val, D);
  	[{test,is_eq_exact,{f,_},[Reg,{atom,Val}]},{label,To}|_] when is_atom(Val) ->
	    shortcut_select_label(To, Reg, Val, D);
  	[{test,is_eq_exact,{f,_},[Reg,{atom,Val}]},{jump,{f,To}}|_] when is_atom(Val) ->
	    shortcut_select_label(To, Reg, Val, D);
  	[{test,is_eq_exact,{f,To},[Reg,{atom,AnotherVal}]}|_]
	when is_atom(Val), Val =/= AnotherVal ->
	    shortcut_select_label(To, Reg, Val, D);
  	[{test,is_ne_exact,{f,To},[Reg,{atom,Val}]}|_] when is_atom(Val) ->
	    shortcut_select_label(To, Reg, Val, D);
  	[{test,is_ne_exact,{f,_},[Reg,{atom,_}]},{label,To}|_] when is_atom(Val) ->
	    shortcut_select_label(To, Reg, Val, D);
	[{test,is_tuple,{f,To},[Reg]}|_] when is_atom(Val) ->
	    shortcut_select_label(To, Reg, Val, D);
	_ ->
	    To0
    end.

shortcut_fail_label(To0, Reg, Val, D) ->
    case beam_utils:code_at(To0, D) of
 	[{jump,{f,To}}|_] ->
	    shortcut_fail_label(To, Reg, Val, D);
  	[{test,is_eq_exact,{f,To},[Reg,{atom,Val}]}|_] when is_atom(Val) ->
	    shortcut_fail_label(To, Reg, Val, D);
	_ ->
	    To0
    end.

shortcut_boolean_label(To0, Reg, Bool0, D) when is_boolean(Bool0) ->
    case beam_utils:code_at(To0, D) of
	[{bif,'not',_,[Reg],Reg},{jump,{f,To}}|_] ->
	    Bool = not Bool0,
	    {shortcut_select_label(To, Reg, Bool, D),Bool};
	_ ->
	    {To0,Bool0}
    end;
shortcut_boolean_label(To, _, Bool, _) -> {To,Bool}.

find_select_val([{_,Val},{f,To}|_], Val, _) -> To;
find_select_val([{_,_}, {f,_}|T], Val, Fail) ->
    find_select_val(T, Val, Fail);
find_select_val([], _, Fail) -> Fail.

replace_comp_op(To, Reg, Op, Ops, D) ->
    False = comp_op_find_shortcut(To, Reg, false, D),
    True = comp_op_find_shortcut(To, Reg, 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 beam_utils: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_val,Reg,{f,F2},{list,[Lit1,{f,L1},Lit2,{f,L2}]}};
	[{select_val,Reg,{f,F2},{list,[{Type,_}|_]=List0}}|_] ->
	    List = remove_from_list(Lit1, List0),
	    {select_val,Reg,{f,F2},{list,[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, [Instruction], D) -> TargetLabel'
%%  Try to shortcut the failure label for a bit syntax matching.
%%  We know that the binary contains at least Bits bits after
%%  the latest save point.

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}|Is], PrevIs, To, D) ->
    shortcut_bs_test_2(Is, {Reg,SavePoint}, PrevIs, To, D);
shortcut_bs_test_1([_|_], _, To, _) -> To.

shortcut_bs_test_2([{label,_}|Is], Save, PrevIs, To, D) ->
    shortcut_bs_test_2(Is, Save, PrevIs, To, D);
shortcut_bs_test_2([{test,bs_test_tail2,{f,To},[_,TailBits]}|_],
		   {Reg,_Point} = RP, PrevIs, To0, D) ->
    case count_bits_matched(PrevIs, RP, 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_2([_|_], _, _, To, _) -> To.

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,_,_,_}|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([{bs_save2,_,_}|Is], SavePoint, Bits) ->
    %% Another save point - keep counting.
    count_bits_matched(Is, SavePoint, 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_restore2,Reg,_}|_], Reg, _) ->
    false;
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
%%  cannot be a binary, so there is no need to jump bs_context_to_binary/1
%%  or another bs_start_match2 instruction.

shortcut_bs_start_match(To, Reg, D) ->
    shortcut_bs_start_match_1(beam_utils:code_at(To, D), Reg, To).

shortcut_bs_start_match_1([{bs_context_to_binary,Reg}|Is], Reg, To) ->
    shortcut_bs_start_match_2(Is, Reg, To);
shortcut_bs_start_match_1(_, _, To) -> To.

shortcut_bs_start_match_2([{jump,{f,To}}|_], _, _) ->
    To;
shortcut_bs_start_match_2([{test,bs_start_match2,{f,To},_,[Reg|_],_}|_], Reg, _) ->
    To;
shortcut_bs_start_match_2(_Is, _Reg, To) ->
    To.