summaryrefslogtreecommitdiff
path: root/dx/src/com/android/dx/ssa/SCCP.java
blob: 1c869e160160fca930d040f88a094f88d31bcb9e (plain)
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
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
/*
 * Copyright (C) 2007 The Android Open Source Project
 *
 * 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.
 */

package com.android.dx.ssa;

import com.android.dx.rop.code.CstInsn;
import com.android.dx.rop.code.Insn;
import com.android.dx.rop.code.PlainInsn;
import com.android.dx.rop.code.RegOps;
import com.android.dx.rop.code.RegisterSpecList;
import com.android.dx.rop.code.Rop;
import com.android.dx.rop.code.RegisterSpec;
import com.android.dx.rop.code.Rops;
import com.android.dx.rop.cst.Constant;
import com.android.dx.rop.cst.CstInteger;
import com.android.dx.rop.cst.TypedConstant;
import com.android.dx.rop.type.TypeBearer;
import com.android.dx.rop.type.Type;

import java.util.ArrayList;
import java.util.BitSet;

/**
 * A small variant of Wegman and Zadeck's Sparse Conditional Constant
 * Propagation algorithm.
 */
public class SCCP {
    /** Lattice values  */
    private static final int TOP = 0;
    private static final int CONSTANT = 1;
    private static final int VARYING = 2;
    /** method we're processing */
    private SsaMethod ssaMeth;
    /** ssaMeth.getRegCount() */
    private int regCount;
    /** Lattice values for each SSA register */
    private int[] latticeValues;
    /** For those registers that are constant, this is the constant value */
    private Constant[] latticeConstants;
    /** Worklist of basic blocks to be processed */
    private ArrayList<SsaBasicBlock> cfgWorklist;
    /** Worklist of executed basic blocks with phis to be processed */
    private ArrayList<SsaBasicBlock> cfgPhiWorklist;
    /** Bitset containing bits for each block that has been found executable */
    private BitSet executableBlocks;
    /** Worklist for SSA edges.  This is a list of registers to process */
    private ArrayList<SsaInsn> ssaWorklist;
    /**
     * Worklist for SSA edges that represent varying values.  It makes the
     * algorithm much faster if you move all values to VARYING as fast as
     * possible.
     */
    private ArrayList<SsaInsn> varyingWorklist;
    /** Worklist of potential branches to convert to gotos */
    private ArrayList<SsaInsn> branchWorklist;

    private SCCP(SsaMethod ssaMeth) {
        this.ssaMeth = ssaMeth;
        this.regCount = ssaMeth.getRegCount();
        this.latticeValues = new int[this.regCount];
        this.latticeConstants = new Constant[this.regCount];
        this.cfgWorklist = new ArrayList<SsaBasicBlock>();
        this.cfgPhiWorklist = new ArrayList<SsaBasicBlock>();
        this.executableBlocks = new BitSet(ssaMeth.getBlocks().size());
        this.ssaWorklist = new ArrayList<SsaInsn>();
        this.varyingWorklist = new ArrayList<SsaInsn>();
        this.branchWorklist = new ArrayList<SsaInsn>();
        for (int i = 0; i < this.regCount; i++) {
            latticeValues[i] = TOP;
            latticeConstants[i] = null;
        }
    }

    /**
     * Performs sparse conditional constant propagation on a method.
     * @param ssaMethod Method to process
     */
    public static void process (SsaMethod ssaMethod) {
        new SCCP(ssaMethod).run();
    }

    /**
     * Adds a SSA basic block to the CFG worklist if it's unexecuted, or
     * to the CFG phi worklist if it's already executed.
     * @param ssaBlock Block to add
     */
    private void addBlockToWorklist(SsaBasicBlock ssaBlock) {
        if (!executableBlocks.get(ssaBlock.getIndex())) {
            cfgWorklist.add(ssaBlock);
            executableBlocks.set(ssaBlock.getIndex());
        } else {
            cfgPhiWorklist.add(ssaBlock);
        }
    }

    /**
     * Adds an SSA register's uses to the SSA worklist.
     * @param reg SSA register
     * @param latticeValue new lattice value for @param reg.
     */
    private void addUsersToWorklist(int reg, int latticeValue) {
        if (latticeValue == VARYING) {
            for (SsaInsn insn : ssaMeth.getUseListForRegister(reg)) {
                varyingWorklist.add(insn);
            }
        } else {
            for (SsaInsn insn : ssaMeth.getUseListForRegister(reg)) {
                ssaWorklist.add(insn);
            }
        }
    }

    /**
     * Sets a lattice value for a register to value.
     * @param reg SSA register
     * @param value Lattice value
     * @param cst Constant value (may be null)
     * @return true if the lattice value changed.
     */
    private boolean setLatticeValueTo(int reg, int value, Constant cst) {
        if (value != CONSTANT) {
            if (latticeValues[reg] != value) {
                latticeValues[reg] = value;
                return true;
            }
            return false;
        } else {
            if (latticeValues[reg] != value
                    || !latticeConstants[reg].equals(cst)) {
                latticeValues[reg] = value;
                latticeConstants[reg] = cst;
                return true;
            }
            return false;
        }
    }

    /**
     * Simulates a PHI node and set the lattice for the result
     * to the appropriate value.
     * Meet values:
     * TOP x anything = TOP
     * VARYING x anything = VARYING
     * CONSTANT x CONSTANT = CONSTANT if equal constants, VARYING otherwise
     * @param insn PHI to simulate.
     */
    private void simulatePhi(PhiInsn insn) {
        int phiResultReg = insn.getResult().getReg();

        if (latticeValues[phiResultReg] == VARYING) {
            return;
        }

        RegisterSpecList sources = insn.getSources();
        int phiResultValue = TOP;
        Constant phiConstant = null;
        int sourceSize = sources.size();

        for (int i = 0; i < sourceSize; i++) {
            int predBlockIndex = insn.predBlockIndexForSourcesIndex(i);
            int sourceReg = sources.get(i).getReg();
            int sourceRegValue = latticeValues[sourceReg];

            if (!executableBlocks.get(predBlockIndex)) {
                continue;
            }

            if (sourceRegValue == CONSTANT) {
                if (phiConstant == null) {
                    phiConstant = latticeConstants[sourceReg];
                    phiResultValue = CONSTANT;
                 } else if (!latticeConstants[sourceReg].equals(phiConstant)){
                    phiResultValue = VARYING;
                    break;
                }
            } else {
                phiResultValue = sourceRegValue;
                break;
            }
        }
        if (setLatticeValueTo(phiResultReg, phiResultValue, phiConstant)) {
            addUsersToWorklist(phiResultReg, phiResultValue);
        }
    }

    /**
     * Simulate a block and note the results in the lattice.
     * @param block Block to visit
     */
    private void simulateBlock(SsaBasicBlock block) {
        for (SsaInsn insn : block.getInsns()) {
            if (insn instanceof PhiInsn) {
                simulatePhi((PhiInsn) insn);
            } else {
                simulateStmt(insn);
            }
        }
    }

    /**
     * Simulate the phis in a block and note the results in the lattice.
     * @param block Block to visit
     */
    private void simulatePhiBlock(SsaBasicBlock block) {
        for (SsaInsn insn : block.getInsns()) {
            if (insn instanceof PhiInsn) {
                simulatePhi((PhiInsn) insn);
            } else {
                return;
            }
        }
    }

    private static String latticeValName(int latticeVal) {
        switch (latticeVal) {
            case TOP: return "TOP";
            case CONSTANT: return "CONSTANT";
            case VARYING: return "VARYING";
            default: return "UNKNOWN";
        }
    }

    /**
     * Simulates branch insns, if possible. Adds reachable successor blocks
     * to the CFG worklists.
     * @param insn branch to simulate
     */
    private void simulateBranch(SsaInsn insn) {
        Rop opcode = insn.getOpcode();
        RegisterSpecList sources = insn.getSources();

        boolean constantBranch = false;
        boolean constantSuccessor = false;

        // Check if the insn is a branch with a constant condition
        if (opcode.getBranchingness() == Rop.BRANCH_IF) {
            Constant cA = null;
            Constant cB = null;

            RegisterSpec specA = sources.get(0);
            int regA = specA.getReg();
            if (!ssaMeth.isRegALocal(specA) &&
                    latticeValues[regA] == CONSTANT) {
                cA = latticeConstants[regA];
            }

            if (sources.size() == 2) {
                RegisterSpec specB = sources.get(1);
                int regB = specB.getReg();
                if (!ssaMeth.isRegALocal(specB) &&
                        latticeValues[regB] == CONSTANT) {
                    cB = latticeConstants[regB];
                }
            }

            // Calculate the result of the condition
            if (cA != null && sources.size() == 1) {
                switch (((TypedConstant) cA).getBasicType()) {
                    case Type.BT_INT:
                        constantBranch = true;
                        int vA = ((CstInteger) cA).getValue();
                        switch (opcode.getOpcode()) {
                            case RegOps.IF_EQ:
                                constantSuccessor = (vA == 0);
                                break;
                            case RegOps.IF_NE:
                                constantSuccessor = (vA != 0);
                                break;
                            case RegOps.IF_LT:
                                constantSuccessor = (vA < 0);
                                break;
                            case RegOps.IF_GE:
                                constantSuccessor = (vA >= 0);
                                break;
                            case RegOps.IF_LE:
                                constantSuccessor = (vA <= 0);
                                break;
                            case RegOps.IF_GT:
                                constantSuccessor = (vA > 0);
                                break;
                            default:
                                throw new RuntimeException("Unexpected op");
                        }
                        break;
                    default:
                        // not yet supported
                }
            } else if (cA != null && cB != null) {
                switch (((TypedConstant) cA).getBasicType()) {
                    case Type.BT_INT:
                        constantBranch = true;
                        int vA = ((CstInteger) cA).getValue();
                        int vB = ((CstInteger) cB).getValue();
                        switch (opcode.getOpcode()) {
                            case RegOps.IF_EQ:
                                constantSuccessor = (vA == vB);
                                break;
                            case RegOps.IF_NE:
                                constantSuccessor = (vA != vB);
                                break;
                            case RegOps.IF_LT:
                                constantSuccessor = (vA < vB);
                                break;
                            case RegOps.IF_GE:
                                constantSuccessor = (vA >= vB);
                                break;
                            case RegOps.IF_LE:
                                constantSuccessor = (vA <= vB);
                                break;
                            case RegOps.IF_GT:
                                constantSuccessor = (vA > vB);
                                break;
                            default:
                                throw new RuntimeException("Unexpected op");
                        }
                        break;
                    default:
                        // not yet supported
                }
            }
        }

        /*
         * If condition is constant, add only the target block to the
         * worklist. Otherwise, add all successors to the worklist.
         */
        SsaBasicBlock block = insn.getBlock();

        if (constantBranch) {
            int successorBlock;
            if (constantSuccessor) {
                successorBlock = block.getSuccessorList().get(1);
            } else {
                successorBlock = block.getSuccessorList().get(0);
            }
            addBlockToWorklist(ssaMeth.getBlocks().get(successorBlock));
            branchWorklist.add(insn);
        } else {
            for (int i = 0; i < block.getSuccessorList().size(); i++) {
                int successorBlock = block.getSuccessorList().get(i);
                addBlockToWorklist(ssaMeth.getBlocks().get(successorBlock));
            }
        }
    }

    /**
     * Simulates math insns, if possible.
     *
     * @param insn non-null insn to simulate
     * @param resultType basic type of the result
     * @return constant result or null if not simulatable.
     */
    private Constant simulateMath(SsaInsn insn, int resultType) {
        Insn ropInsn = insn.getOriginalRopInsn();
        int opcode = insn.getOpcode().getOpcode();
        RegisterSpecList sources = insn.getSources();
        int regA = sources.get(0).getReg();
        Constant cA;
        Constant cB;

        if (latticeValues[regA] != CONSTANT) {
            cA = null;
        } else {
            cA = latticeConstants[regA];
        }

        if (sources.size() == 1) {
            CstInsn cstInsn = (CstInsn) ropInsn;
            cB = cstInsn.getConstant();
        } else { /* sources.size() == 2 */
            int regB = sources.get(1).getReg();
            if (latticeValues[regB] != CONSTANT) {
                cB = null;
            } else {
                cB = latticeConstants[regB];
            }
        }

        if (cA == null || cB == null) {
            //TODO handle a constant of 0 with MUL or AND
            return null;
        }

        switch (resultType) {
            case Type.BT_INT:
                int vR;
                boolean skip=false;

                int vA = ((CstInteger) cA).getValue();
                int vB = ((CstInteger) cB).getValue();

                switch (opcode) {
                    case RegOps.ADD:
                        vR = vA + vB;
                        break;
                    case RegOps.SUB:
                        // 1 source for reverse sub, 2 sources for regular sub
                        if (sources.size() == 1) {
                            vR = vB - vA;
                        } else {
                            vR = vA - vB;
                        }
                        break;
                    case RegOps.MUL:
                        vR = vA * vB;
                        break;
                    case RegOps.DIV:
                        if (vB == 0) {
                            skip = true;
                            vR = 0; // just to hide a warning
                        } else {
                            vR = vA / vB;
                        }
                        break;
                    case RegOps.AND:
                        vR = vA & vB;
                        break;
                    case RegOps.OR:
                        vR = vA | vB;
                        break;
                    case RegOps.XOR:
                        vR = vA ^ vB;
                        break;
                    case RegOps.SHL:
                        vR = vA << vB;
                        break;
                    case RegOps.SHR:
                        vR = vA >> vB;
                        break;
                    case RegOps.USHR:
                        vR = vA >>> vB;
                        break;
                    case RegOps.REM:
                        if (vB == 0) {
                            skip = true;
                            vR = 0; // just to hide a warning
                        } else {
                            vR = vA % vB;
                        }
                        break;
                    default:
                        throw new RuntimeException("Unexpected op");
                }

                return skip ? null : CstInteger.make(vR);

            default:
                // not yet supported
                return null;
        }
    }

    /**
     * Simulates a statement and set the result lattice value.
     * @param insn instruction to simulate
     */
    private void simulateStmt(SsaInsn insn) {
        Insn ropInsn = insn.getOriginalRopInsn();
        if (ropInsn.getOpcode().getBranchingness() != Rop.BRANCH_NONE
                || ropInsn.getOpcode().isCallLike()) {
            simulateBranch(insn);
        }

        int opcode = insn.getOpcode().getOpcode();
        RegisterSpec result = insn.getResult();

        if (result == null) {
            // Find move-result-pseudo result for int div and int rem
            if (opcode == RegOps.DIV || opcode == RegOps.REM) {
                SsaBasicBlock succ = insn.getBlock().getPrimarySuccessor();
                result = succ.getInsns().get(0).getResult();
            } else {
                return;
            }
        }

        int resultReg = result.getReg();
        int resultValue = VARYING;
        Constant resultConstant = null;

        switch (opcode) {
            case RegOps.CONST: {
                CstInsn cstInsn = (CstInsn)ropInsn;
                resultValue = CONSTANT;
                resultConstant = cstInsn.getConstant();
                break;
            }
            case RegOps.MOVE: {
                if (insn.getSources().size() == 1) {
                    int sourceReg = insn.getSources().get(0).getReg();
                    resultValue = latticeValues[sourceReg];
                    resultConstant = latticeConstants[sourceReg];
                }
                break;
            }
            case RegOps.ADD:
            case RegOps.SUB:
            case RegOps.MUL:
            case RegOps.DIV:
            case RegOps.AND:
            case RegOps.OR:
            case RegOps.XOR:
            case RegOps.SHL:
            case RegOps.SHR:
            case RegOps.USHR:
            case RegOps.REM: {
                resultConstant = simulateMath(insn, result.getBasicType());
                if (resultConstant != null) {
                    resultValue = CONSTANT;
                }
                break;
            }
            case RegOps.MOVE_RESULT_PSEUDO: {
                if (latticeValues[resultReg] == CONSTANT) {
                    resultValue = latticeValues[resultReg];
                    resultConstant = latticeConstants[resultReg];
                }
                break;
            }
            // TODO: Handle non-int arithmetic.
            // TODO: Eliminate check casts that we can prove the type of.
            default: {}
        }
        if (setLatticeValueTo(resultReg, resultValue, resultConstant)) {
            addUsersToWorklist(resultReg, resultValue);
        }
    }

    private void run() {
        SsaBasicBlock firstBlock = ssaMeth.getEntryBlock();
        addBlockToWorklist(firstBlock);

        /* Empty all the worklists by propagating our values */
        while (!cfgWorklist.isEmpty()
                || !cfgPhiWorklist.isEmpty()
                || !ssaWorklist.isEmpty()
                || !varyingWorklist.isEmpty()) {
            while (!cfgWorklist.isEmpty()) {
                int listSize = cfgWorklist.size() - 1;
                SsaBasicBlock block = cfgWorklist.remove(listSize);
                simulateBlock(block);
            }

            while (!cfgPhiWorklist.isEmpty()) {
                int listSize = cfgPhiWorklist.size() - 1;
                SsaBasicBlock block = cfgPhiWorklist.remove(listSize);
                simulatePhiBlock(block);
            }

            while (!varyingWorklist.isEmpty()) {
                int listSize = varyingWorklist.size() - 1;
                SsaInsn insn = varyingWorklist.remove(listSize);

                if (!executableBlocks.get(insn.getBlock().getIndex())) {
                    continue;
                }

                if (insn instanceof PhiInsn) {
                    simulatePhi((PhiInsn)insn);
                } else {
                    simulateStmt(insn);
                }
            }
            while (!ssaWorklist.isEmpty()) {
                int listSize = ssaWorklist.size() - 1;
                SsaInsn insn = ssaWorklist.remove(listSize);

                if (!executableBlocks.get(insn.getBlock().getIndex())) {
                    continue;
                }

                if (insn instanceof PhiInsn) {
                    simulatePhi((PhiInsn)insn);
                } else {
                    simulateStmt(insn);
                }
            }
        }

        replaceConstants();
        replaceBranches();
    }

    /**
     * Replaces TypeBearers in source register specs with constant type
     * bearers if possible. These are then referenced in later optimization
     * steps.
     */
    private void replaceConstants() {
        for (int reg = 0; reg < regCount; reg++) {
            if (latticeValues[reg] != CONSTANT) {
                continue;
            }
            if (!(latticeConstants[reg] instanceof TypedConstant)) {
                // We can't do much with these
                continue;
            }

            SsaInsn defn = ssaMeth.getDefinitionForRegister(reg);
            TypeBearer typeBearer = defn.getResult().getTypeBearer();

            if (typeBearer.isConstant()) {
                /*
                 * The definition was a constant already.
                 * The uses should be as well.
                 */
                continue;
            }

            // Update the destination RegisterSpec with the constant value
            RegisterSpec dest = defn.getResult();
            RegisterSpec newDest
                    = dest.withType((TypedConstant)latticeConstants[reg]);
            defn.setResult(newDest);

            /*
             * Update the sources RegisterSpec's of all non-move uses.
             * These will be used in later steps.
             */
            for (SsaInsn insn : ssaMeth.getUseListForRegister(reg)) {
                if (insn.isPhiOrMove()) {
                    continue;
                }

                NormalSsaInsn nInsn = (NormalSsaInsn) insn;
                RegisterSpecList sources = insn.getSources();

                int index = sources.indexOfRegister(reg);

                RegisterSpec spec = sources.get(index);
                RegisterSpec newSpec
                        = spec.withType((TypedConstant)latticeConstants[reg]);

                nInsn.changeOneSource(index, newSpec);
            }
        }
    }

    /**
     * Replaces branches that have constant conditions with gotos
     */
    private void replaceBranches() {
        for (SsaInsn insn : branchWorklist) {
            // Find if a successor block is never executed
            int oldSuccessor = -1;
            SsaBasicBlock block = insn.getBlock();
            int successorSize = block.getSuccessorList().size();
            for (int i = 0; i < successorSize; i++) {
                int successorBlock = block.getSuccessorList().get(i);
                if (!executableBlocks.get(successorBlock)) {
                    oldSuccessor = successorBlock;
                }
            }

            /*
             * Prune branches that have already been handled and ones that no
             * longer have constant conditions (no nonexecutable successors)
             */
            if (successorSize != 2 || oldSuccessor == -1) continue;

            // Replace branch with goto
            Insn originalRopInsn = insn.getOriginalRopInsn();
            block.replaceLastInsn(new PlainInsn(Rops.GOTO,
                originalRopInsn.getPosition(), null, RegisterSpecList.EMPTY));
            block.removeSuccessor(oldSuccessor);
        }
    }
}