summaryrefslogtreecommitdiff
path: root/gcc/compare-elim.c
blob: c66f13152ceeb79010c8deb7d99b4aa9797e8308 (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
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
/* Post-reload compare elimination.
   Copyright (C) 2010-2016 Free Software Foundation, Inc.

This file is part of GCC.

GCC is free software; you can redistribute it and/or modify it under
the terms of the GNU General Public License as published by the Free
Software Foundation; either version 3, or (at your option) any later
version.

GCC is distributed in the hope that it will be useful, but WITHOUT ANY
WARRANTY; without even the implied warranty of MERCHANTABILITY or
FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
for more details.

You should have received a copy of the GNU General Public License
along with GCC; see the file COPYING3.  If not see
<http://www.gnu.org/licenses/>.  */

/* There is a set of targets whose general-purpose move or addition
   instructions clobber the flags.  These targets cannot split their
   CBRANCH/CSTORE etc patterns before reload is complete, lest reload
   itself insert these instructions in between the flags setter and user.
   Because these targets cannot split the compare from the use, they
   cannot make use of the comparison elimination offered by the combine pass.

   This is a small pass intended to provide comparison elimination similar to
   what is available via NOTICE_UPDATE_CC for cc0 targets.  This should help
   encourage cc0 targets to convert to an explicit post-reload representation
   of the flags.

   This pass assumes:

   (0) CBRANCH/CSTORE etc have been split in pass_split_after_reload.

   (1) All comparison patterns are represented as

	[(set (reg:CC) (compare:CC (reg) (reg_or_immediate)))]

   (2) All insn patterns that modify the flags are represented as

	[(set (reg) (operation)
	 (clobber (reg:CC))]

   (3) If an insn of form (2) can usefully set the flags, there is
       another pattern of the form

	[(set (reg) (operation)
	 (set (reg:CCM) (compare:CCM (operation) (immediate)))]

       The mode CCM will be chosen as if by SELECT_CC_MODE.

   Note that unlike NOTICE_UPDATE_CC, we do not handle memory operands.
   This could be handled as a future enhancement.
*/

#include "config.h"
#include "system.h"
#include "coretypes.h"
#include "backend.h"
#include "target.h"
#include "rtl.h"
#include "df.h"
#include "memmodel.h"
#include "tm_p.h"
#include "insn-config.h"
#include "recog.h"
#include "cfgrtl.h"
#include "tree-pass.h"
#include "domwalk.h"


/* These structures describe a comparison and how it is used.  */

/* The choice of maximum 3 uses comes from wanting to eliminate the two
   duplicate compares from a three-way branch on the sign of a value.
   This is also sufficient to eliminate the duplicate compare against the
   high-part of a double-word comparison.  */
#define MAX_CMP_USE 3

struct comparison_use
{
  /* The instruction in which the result of the compare is used.  */
  rtx_insn *insn;
  /* The location of the flags register within the use.  */
  rtx *loc;
  /* The comparison code applied against the flags register.  */
  enum rtx_code code;
};

struct comparison
{
  /* The comparison instruction.  */
  rtx_insn *insn;

  /* The insn prior to the comparison insn that clobbers the flags.  */
  rtx_insn *prev_clobber;

  /* The two values being compared.  These will be either REGs or
     constants.  */
  rtx in_a, in_b;

  /* The REG_EH_REGION of the comparison.  */
  rtx eh_note;

  /* Information about how this comparison is used.  */
  struct comparison_use uses[MAX_CMP_USE];

  /* The original CC_MODE for this comparison.  */
  machine_mode orig_mode;

  /* The number of uses identified for this comparison.  */
  unsigned short n_uses;

  /* True if not all uses of this comparison have been identified.
     This can happen either for overflowing the array above, or if
     the flags register is used in some unusual context.  */
  bool missing_uses;

  /* True if its inputs are still valid at the end of the block.  */
  bool inputs_valid;
};
  
static vec<comparison *> all_compares;

/* Look for a "conforming" comparison, as defined above.  If valid, return
   the rtx for the COMPARE itself.  */

static rtx
conforming_compare (rtx_insn *insn)
{
  rtx set, src, dest;

  set = single_set (insn);
  if (set == NULL)
    return NULL;

  src = SET_SRC (set);
  if (GET_CODE (src) != COMPARE)
    return NULL;

  dest = SET_DEST (set);
  if (!REG_P (dest) || REGNO (dest) != targetm.flags_regnum)
    return NULL;

  if (!REG_P (XEXP (src, 0)))
    return NULL;

  if (CONSTANT_P (XEXP (src, 1)) || REG_P (XEXP (src, 1)))
    return src;

  if (GET_CODE (XEXP (src, 1)) == UNSPEC)
    {
      for (int i = 0; i < XVECLEN (XEXP (src, 1), 0); i++)
	if (!REG_P (XVECEXP (XEXP (src, 1), 0, i)))
	  return NULL;
      return src;
    }

  return NULL;
}

/* Look for a pattern of the "correct" form for an insn with a flags clobber
   for which we may be able to eliminate a compare later.  We're not looking
   to validate any inputs at this time, merely see that the basic shape is
   correct.  The term "arithmetic" may be somewhat misleading...  */

static bool
arithmetic_flags_clobber_p (rtx_insn *insn)
{
  rtx pat, x;

  if (!NONJUMP_INSN_P (insn))
    return false;
  pat = PATTERN (insn);
  if (asm_noperands (pat) >= 0)
    return false;

  if (GET_CODE (pat) == PARALLEL && XVECLEN (pat, 0) == 2)
    {
      x = XVECEXP (pat, 0, 0);
      if (GET_CODE (x) != SET)
	return false;
      x = SET_DEST (x);
      if (!REG_P (x))
	return false;

      x = XVECEXP (pat, 0, 1);
      if (GET_CODE (x) == CLOBBER)
	{
	  x = XEXP (x, 0);
	  if (REG_P (x) && REGNO (x) == targetm.flags_regnum)
	    return true;
	}
    }

  return false;
}

/* Look for uses of FLAGS in INSN.  If we find one we can analyze, record
   it in CMP; otherwise indicate that we've missed a use.  */

static void
find_flags_uses_in_insn (struct comparison *cmp, rtx_insn *insn)
{
  df_ref use;

  /* If we've already lost track of uses, don't bother collecting more.  */
  if (cmp->missing_uses)
    return;

  /* Find a USE of the flags register.  */
  FOR_EACH_INSN_USE (use, insn)
    if (DF_REF_REGNO (use) == targetm.flags_regnum)
      {
	rtx x, *loc;

	/* If this is an unusual use, quit.  */
	if (DF_REF_TYPE (use) != DF_REF_REG_USE)
	  goto fail;

	/* If we've run out of slots to record uses, quit.  */
	if (cmp->n_uses == MAX_CMP_USE)
	  goto fail;

	/* Unfortunately the location of the flags register, while present
	   in the reference structure, doesn't help.  We need to find the
	   comparison code that is outer to the actual flags use.  */
	loc = DF_REF_LOC (use);
	x = PATTERN (insn);
	if (GET_CODE (x) == PARALLEL)
	  x = XVECEXP (x, 0, 0);
	x = SET_SRC (x);
	if (GET_CODE (x) == IF_THEN_ELSE)
	  x = XEXP (x, 0);
	if (COMPARISON_P (x)
	    && loc == &XEXP (x, 0)
	    && XEXP (x, 1) == const0_rtx)
	  {
	    /* We've found a use of the flags that we understand.  */
	    struct comparison_use *cuse = &cmp->uses[cmp->n_uses++];
	    cuse->insn = insn;
	    cuse->loc = loc;
	    cuse->code = GET_CODE (x);
	  }
	else
	  goto fail;
      }
  return;

 fail:
  /* We failed to recognize this use of the flags register.  */
  cmp->missing_uses = true;
}

class find_comparison_dom_walker : public dom_walker
{
public:
  find_comparison_dom_walker (cdi_direction direction)
    : dom_walker (direction) {}

  virtual edge before_dom_children (basic_block);
};

/* Return true if conforming COMPARE with EH_NOTE is redundant with comparison
   CMP and can thus be eliminated.  */

static bool
can_eliminate_compare (rtx compare, rtx eh_note, struct comparison *cmp)
{
  /* Take care that it's in the same EH region.  */
  if (cfun->can_throw_non_call_exceptions
      && !rtx_equal_p (eh_note, cmp->eh_note))
    return false;

  /* Make sure the compare is redundant with the previous.  */
  if (!rtx_equal_p (XEXP (compare, 0), cmp->in_a)
      || !rtx_equal_p (XEXP (compare, 1), cmp->in_b))
    return false;

  /* New mode must be compatible with the previous compare mode.  */
  enum machine_mode new_mode
    = targetm.cc_modes_compatible (GET_MODE (compare), cmp->orig_mode);

  if (new_mode == VOIDmode)
    return false;

  if (cmp->orig_mode != new_mode)
    {
      /* Generate new comparison for substitution.  */
      rtx flags = gen_rtx_REG (new_mode, targetm.flags_regnum);
      rtx x = gen_rtx_COMPARE (new_mode, cmp->in_a, cmp->in_b);
      x = gen_rtx_SET (flags, x);

      if (!validate_change (cmp->insn, &PATTERN (cmp->insn), x, false))
	return false;

      cmp->orig_mode = new_mode;
    }

  return true;
}

/* Identify comparison instructions within BB.  If the flags from the last
   compare in the BB is live at the end of the block, install the compare
   in BB->AUX.  Called via dom_walker.walk ().  */

edge
find_comparison_dom_walker::before_dom_children (basic_block bb)
{
  struct comparison *last_cmp;
  rtx_insn *insn, *next, *last_clobber;
  bool last_cmp_valid;
  bool need_purge = false;
  bitmap killed;

  killed = BITMAP_ALLOC (NULL);

  /* The last comparison that was made.  Will be reset to NULL
     once the flags are clobbered.  */
  last_cmp = NULL;

  /* True iff the last comparison has not been clobbered, nor
     have its inputs.  Used to eliminate duplicate compares.  */
  last_cmp_valid = false;

  /* The last insn that clobbered the flags, if that insn is of
     a form that may be valid for eliminating a following compare.
     To be reset to NULL once the flags are set otherwise.  */
  last_clobber = NULL;

  /* Propagate the last live comparison throughout the extended basic block. */
  if (single_pred_p (bb))
    {
      last_cmp = (struct comparison *) single_pred (bb)->aux;
      if (last_cmp)
	last_cmp_valid = last_cmp->inputs_valid;
    }

  for (insn = BB_HEAD (bb); insn; insn = next)
    {
      rtx src;

      next = (insn == BB_END (bb) ? NULL : NEXT_INSN (insn));
      if (!NONDEBUG_INSN_P (insn))
	continue;

      /* Compute the set of registers modified by this instruction.  */
      bitmap_clear (killed);
      df_simulate_find_defs (insn, killed);

      src = conforming_compare (insn);
      if (src)
	{
	  rtx eh_note = NULL;

	  if (cfun->can_throw_non_call_exceptions)
	    eh_note = find_reg_note (insn, REG_EH_REGION, NULL);

	  if (last_cmp_valid && can_eliminate_compare (src, eh_note, last_cmp))
	    {
	      if (eh_note)
		need_purge = true;
	      delete_insn (insn);
	      continue;
	    }

	  last_cmp = XCNEW (struct comparison);
	  last_cmp->insn = insn;
	  last_cmp->prev_clobber = last_clobber;
	  last_cmp->in_a = XEXP (src, 0);
	  last_cmp->in_b = XEXP (src, 1);
	  last_cmp->eh_note = eh_note;
	  last_cmp->orig_mode = GET_MODE (src);
	  all_compares.safe_push (last_cmp);

	  /* It's unusual, but be prepared for comparison patterns that
	     also clobber an input, or perhaps a scratch.  */
	  last_clobber = NULL;
	  last_cmp_valid = true;
	}

      else
	{
	  /* Notice if this instruction uses the flags register.  */
	  if (last_cmp)
	    find_flags_uses_in_insn (last_cmp, insn);

	  /* Notice if this instruction kills the flags register.  */
	  if (bitmap_bit_p (killed, targetm.flags_regnum))
	    {
	      /* See if this insn could be the "clobber" that eliminates
		 a future comparison.   */
	      last_clobber = (arithmetic_flags_clobber_p (insn) ? insn : NULL);

	      /* In either case, the previous compare is no longer valid.  */
	      last_cmp = NULL;
	      last_cmp_valid = false;
	    }
	}

      /* Notice if any of the inputs to the comparison have changed.  */
      if (last_cmp_valid
	  && (bitmap_bit_p (killed, REGNO (last_cmp->in_a))
	      || (REG_P (last_cmp->in_b)
		  && bitmap_bit_p (killed, REGNO (last_cmp->in_b)))))
	last_cmp_valid = false;
    }

  BITMAP_FREE (killed);

  /* Remember the live comparison for subsequent members of
     the extended basic block.  */
  if (last_cmp)
    {
      bb->aux = last_cmp;
      last_cmp->inputs_valid = last_cmp_valid;

      /* Look to see if the flags register is live outgoing here, and
	 incoming to any successor not part of the extended basic block.  */
      if (bitmap_bit_p (df_get_live_out (bb), targetm.flags_regnum))
	{
	  edge e;
	  edge_iterator ei;

	  FOR_EACH_EDGE (e, ei, bb->succs)
	    {
	      basic_block dest = e->dest;
	      if (bitmap_bit_p (df_get_live_in (bb), targetm.flags_regnum)
		  && !single_pred_p (dest))
		{
		  last_cmp->missing_uses = true;
		  break;
		}
	    }
	}
    }

  /* If we deleted a compare with a REG_EH_REGION note, we may need to
     remove EH edges.  */
  if (need_purge)
    purge_dead_edges (bb);

  return NULL;
}

/* Find all comparisons in the function.  */

static void
find_comparisons (void)
{
  calculate_dominance_info (CDI_DOMINATORS);

  find_comparison_dom_walker (CDI_DOMINATORS)
    .walk (cfun->cfg->x_entry_block_ptr);

  clear_aux_for_blocks ();
  free_dominance_info (CDI_DOMINATORS);
}

/* Select an alternate CC_MODE for a comparison insn comparing A and B.
   Note that inputs are almost certainly different than the IN_A and IN_B
   stored in CMP -- we're called while attempting to eliminate the compare
   after all.  Return the new FLAGS rtx if successful, else return NULL.
   Note that this function may start a change group.  */

static rtx
maybe_select_cc_mode (struct comparison *cmp, rtx a ATTRIBUTE_UNUSED,
		      rtx b ATTRIBUTE_UNUSED)
{
  machine_mode sel_mode;
  const int n = cmp->n_uses;
  rtx flags = NULL;

#ifndef SELECT_CC_MODE
  /* Minimize code differences when this target macro is undefined.  */
  return NULL;
#define SELECT_CC_MODE(A,B,C) (gcc_unreachable (), VOIDmode)
#endif

  /* If we don't have access to all of the uses, we can't validate.  */
  if (cmp->missing_uses || n == 0)
    return NULL;

  /* Find a new mode that works for all of the uses.  Special case the
     common case of exactly one use.  */
  if (n == 1)
    {
      sel_mode = SELECT_CC_MODE (cmp->uses[0].code, a, b);
      if (sel_mode != cmp->orig_mode)
	{
	  flags = gen_rtx_REG (sel_mode, targetm.flags_regnum);
	  validate_change (cmp->uses[0].insn, cmp->uses[0].loc, flags, true);
	}
    }
  else
    {
      int i;

      sel_mode = SELECT_CC_MODE (cmp->uses[0].code, a, b);
      for (i = 1; i < n; ++i)
	{
	  machine_mode new_mode = SELECT_CC_MODE (cmp->uses[i].code, a, b);
	  if (new_mode != sel_mode)
	    {
	      sel_mode = targetm.cc_modes_compatible (sel_mode, new_mode);
	      if (sel_mode == VOIDmode)
		return NULL;
	    }
	}

      if (sel_mode != cmp->orig_mode)
	{
	  flags = gen_rtx_REG (sel_mode, targetm.flags_regnum);
	  for (i = 0; i < n; ++i)
	    validate_change (cmp->uses[i].insn, cmp->uses[i].loc, flags, true);
	}
    }

  return flags;
}

/* Return a register RTX holding the same value at START as REG at END, or
   NULL_RTX if there is none.  */

static rtx
equivalent_reg_at_start (rtx reg, rtx_insn *end, rtx_insn *start)
{
  rtx_insn *bb_head = BB_HEAD (BLOCK_FOR_INSN (end));

  for (rtx_insn *insn = PREV_INSN (end);
       insn != start;
       insn = PREV_INSN (insn))
    {
      const int abnormal_flags
	= (DF_REF_CONDITIONAL | DF_REF_PARTIAL | DF_REF_MAY_CLOBBER
	   | DF_REF_MUST_CLOBBER | DF_REF_SIGN_EXTRACT
	   | DF_REF_ZERO_EXTRACT | DF_REF_STRICT_LOW_PART
	   | DF_REF_PRE_POST_MODIFY);
      df_ref def;

      /* Note that the BB_HEAD is always either a note or a label, but in
	 any case it means that IN_A is defined outside the block.  */
      if (insn == bb_head)
	return NULL_RTX;
      if (NOTE_P (insn) || DEBUG_INSN_P (insn))
	continue;

      /* Find a possible def of IN_A in INSN.  */
      FOR_EACH_INSN_DEF (def, insn)
	if (DF_REF_REGNO (def) == REGNO (reg))
	  break;

      /* No definitions of IN_A; continue searching.  */
      if (def == NULL)
	continue;

      /* Bail if this is not a totally normal set of IN_A.  */
      if (DF_REF_IS_ARTIFICIAL (def))
	return NULL_RTX;
      if (DF_REF_FLAGS (def) & abnormal_flags)
	return NULL_RTX;

      /* We've found an insn between the compare and the clobber that sets
	 IN_A.  Given that pass_cprop_hardreg has not yet run, we still find
	 situations in which we can usefully look through a copy insn.  */
      rtx x = single_set (insn);
      if (x == NULL_RTX)
	return NULL_RTX;
      reg = SET_SRC (x);
      if (!REG_P (reg))
	return NULL_RTX;
    }

  return reg;
}

/* Attempt to replace a comparison with a prior arithmetic insn that can
   compute the same flags value as the comparison itself.  Return true if
   successful, having made all rtl modifications necessary.  */

static bool
try_eliminate_compare (struct comparison *cmp)
{
  rtx x, flags, in_a, in_b, cmp_src;

  /* We must have found an interesting "clobber" preceding the compare.  */
  if (cmp->prev_clobber == NULL)
    return false;

  /* Verify that IN_A is not clobbered in between CMP and PREV_CLOBBER.
     Given that this target requires this pass, we can assume that most
     insns do clobber the flags, and so the distance between the compare
     and the clobber is likely to be small.  */
  /* ??? This is one point at which one could argue that DF_REF_CHAIN would
     be useful, but it is thought to be too heavy-weight a solution here.  */
  in_a = equivalent_reg_at_start (cmp->in_a, cmp->insn, cmp->prev_clobber);
  if (!in_a)
    return false;

  /* Likewise for IN_B if need be.  */
  if (CONSTANT_P (cmp->in_b))
    in_b = cmp->in_b;
  else if (REG_P (cmp->in_b))
    {
      in_b = equivalent_reg_at_start (cmp->in_b, cmp->insn, cmp->prev_clobber);
      if (!in_b)
	return false;
    }
  else if (GET_CODE (cmp->in_b) == UNSPEC)
    {
      const int len = XVECLEN (cmp->in_b, 0);
      rtvec v = rtvec_alloc (len);
      for (int i = 0; i < len; i++)
	{
	  rtx r = equivalent_reg_at_start (XVECEXP (cmp->in_b, 0, i),
					   cmp->insn, cmp->prev_clobber);
	  if (!r)
	    return false;
	  RTVEC_ELT (v, i) = r;
	}
      in_b = gen_rtx_UNSPEC (GET_MODE (cmp->in_b), v, XINT (cmp->in_b, 1));
    }
  else
    gcc_unreachable ();

  /* We've reached PREV_CLOBBER without finding a modification of IN_A.
     Validate that PREV_CLOBBER itself does in fact refer to IN_A.  Do
     recall that we've already validated the shape of PREV_CLOBBER.  */
  rtx_insn *insn = cmp->prev_clobber;
  x = XVECEXP (PATTERN (insn), 0, 0);
  if (rtx_equal_p (SET_DEST (x), in_a))
    cmp_src = SET_SRC (x);

  /* Also check operations with implicit extensions, e.g.:
     [(set (reg:DI)
	   (zero_extend:DI (plus:SI (reg:SI) (reg:SI))))
      (set (reg:CCZ flags)
	   (compare:CCZ (plus:SI (reg:SI) (reg:SI))
			(const_int 0)))] */
  else if (REG_P (SET_DEST (x))
	   && REG_P (in_a)
	   && REGNO (SET_DEST (x)) == REGNO (in_a)
	   && (GET_CODE (SET_SRC (x)) == ZERO_EXTEND
	       || GET_CODE (SET_SRC (x)) == SIGN_EXTEND)
	   && GET_MODE (XEXP (SET_SRC (x), 0)) == GET_MODE (in_a))
    cmp_src = XEXP (SET_SRC (x), 0);

  /* Also check fully redundant comparisons, e.g.:
     [(set (reg:SI)
	   (minus:SI (reg:SI) (reg:SI))))
      (set (reg:CC flags)
	   (compare:CC (reg:SI) (reg:SI)))] */
  else if (REG_P (in_b)
	   && GET_CODE (SET_SRC (x)) == MINUS
	   && rtx_equal_p (XEXP (SET_SRC (x), 0), in_a)
	   && rtx_equal_p (XEXP (SET_SRC (x), 1), in_b))
    cmp_src = in_a;

  else
    return false;

  /* Determine if we ought to use a different CC_MODE here.  */
  flags = maybe_select_cc_mode (cmp, cmp_src, in_b);
  if (flags == NULL)
    flags = gen_rtx_REG (cmp->orig_mode, targetm.flags_regnum);

  /* Generate a new comparison for installation in the setter.  */
  x = copy_rtx (cmp_src);
  x = gen_rtx_COMPARE (GET_MODE (flags), x, in_b);
  x = gen_rtx_SET (flags, x);

  /* Succeed if the new instruction is valid.  Note that we may have started
     a change group within maybe_select_cc_mode, therefore we must continue. */
  validate_change (insn, &XVECEXP (PATTERN (insn), 0, 1), x, true);
  if (!apply_change_group ())
    return false;

  /* Success.  Delete the compare insn...  */
  delete_insn (cmp->insn);

  /* ... and any notes that are now invalid due to multiple sets.  */
  x = find_regno_note (insn, REG_UNUSED, targetm.flags_regnum);
  if (x)
    remove_note (insn, x);
  x = find_reg_note (insn, REG_EQUAL, NULL);
  if (x)
    remove_note (insn, x);
  x = find_reg_note (insn, REG_EQUIV, NULL);
  if (x)
    remove_note (insn, x);

  return true;
}

/* Main entry point to the pass.  */

static unsigned int
execute_compare_elim_after_reload (void)
{
  df_analyze ();

  gcc_checking_assert (!all_compares.exists ());

  /* Locate all comparisons and their uses, and eliminate duplicates.  */
  find_comparisons ();
  if (all_compares.exists ())
    {
      struct comparison *cmp;
      size_t i;

      /* Eliminate comparisons that are redundant with flags computation.  */
      FOR_EACH_VEC_ELT (all_compares, i, cmp)
	{
	  try_eliminate_compare (cmp);
	  XDELETE (cmp);
	}

      all_compares.release ();
    }

  return 0;
}

namespace {

const pass_data pass_data_compare_elim_after_reload =
{
  RTL_PASS, /* type */
  "cmpelim", /* name */
  OPTGROUP_NONE, /* optinfo_flags */
  TV_NONE, /* tv_id */
  0, /* properties_required */
  0, /* properties_provided */
  0, /* properties_destroyed */
  0, /* todo_flags_start */
  ( TODO_df_finish | TODO_df_verify ), /* todo_flags_finish */
};

class pass_compare_elim_after_reload : public rtl_opt_pass
{
public:
  pass_compare_elim_after_reload (gcc::context *ctxt)
    : rtl_opt_pass (pass_data_compare_elim_after_reload, ctxt)
  {}

  /* opt_pass methods: */
  virtual bool gate (function *)
    {
      /* Setting this target hook value is how a backend indicates the need.  */
      if (targetm.flags_regnum == INVALID_REGNUM)
	return false;
      return flag_compare_elim_after_reload;
    }

  virtual unsigned int execute (function *)
    {
      return execute_compare_elim_after_reload ();
    }

}; // class pass_compare_elim_after_reload

} // anon namespace

rtl_opt_pass *
make_pass_compare_elim_after_reload (gcc::context *ctxt)
{
  return new pass_compare_elim_after_reload (ctxt);
}