gcc/loop-doloop.c

   1 /* Perform doloop optimizations
   2    Copyright (C) 2004, 2005, 2006 Free Software Foundation, Inc.
   3    Based on code by Michael P. Hayes (m.hayes@elec.canterbury.ac.nz)
   4
   5 This file is part of GCC.
   6
   7 GCC is free software; you can redistribute it and/or modify it under
   8 the terms of the GNU General Public License as published by the Free
   9 Software Foundation; either version 2, or (at your option) any later
  10 version.
  11
  12 GCC is distributed in the hope that it will be useful, but WITHOUT ANY
  13 WARRANTY; without even the implied warranty of MERCHANTABILITY or
  14 FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  15 for more details.
  16
  17 You should have received a copy of the GNU General Public License
  18 along with GCC; see the file COPYING.  If not, write to the Free
  19 Software Foundation, 51 Franklin Street, Fifth Floor, Boston, MA
  20 02110-1301, USA.  */
  21
  22 #include "config.h"
  23 #include "system.h"
  24 #include "coretypes.h"
  25 #include "tm.h"
  26 #include "rtl.h"
  27 #include "flags.h"
  28 #include "expr.h"
  29 #include "hard-reg-set.h"
  30 #include "basic-block.h"
  31 #include "toplev.h"
  32 #include "tm_p.h"
  33 #include "cfgloop.h"
  34 #include "output.h"
  35 #include "params.h"
  36 #include "target.h"
  37
  38 /* This module is used to modify loops with a determinable number of
  39    iterations to use special low-overhead looping instructions.
  40
  41    It first validates whether the loop is well behaved and has a
  42    determinable number of iterations (either at compile or run-time).
  43    It then modifies the loop to use a low-overhead looping pattern as
  44    follows:
  45
  46    1. A pseudo register is allocated as the loop iteration counter.
  47
  48    2. The number of loop iterations is calculated and is stored
  49       in the loop counter.
  50
  51    3. At the end of the loop, the jump insn is replaced by the
  52       doloop_end pattern.  The compare must remain because it might be
  53       used elsewhere.  If the loop-variable or condition register are
  54       used elsewhere, they will be eliminated by flow.
  55
  56    4. An optional doloop_begin pattern is inserted at the top of the
  57       loop.
  58
  59    TODO The optimization should only performed when either the biv used for exit
  60    condition is unused at all except for the exit test, or if we do not have to
  61    change its value, since otherwise we have to add a new induction variable,
  62    which usually will not pay up (unless the cost of the doloop pattern is
  63    somehow extremely lower than the cost of compare & jump, or unless the bct
  64    register cannot be used for anything else but doloop -- ??? detect these
  65    cases).  */
  66
  67 #ifdef HAVE_doloop_end
  68
  69 /* Return the loop termination condition for PATTERN or zero
  70    if it is not a decrement and branch jump insn.  */
  71
  72 rtx
  73 doloop_condition_get (rtx pattern)
  74 {
  75   rtx cmp;
  76   rtx inc;
  77   rtx reg;
  78   rtx inc_src;
  79   rtx condition;
  80
  81   /* The canonical doloop pattern we expect is:
  82
  83      (parallel [(set (pc) (if_then_else (condition)
  84                                         (label_ref (label))
  85                                         (pc)))
  86                 (set (reg) (plus (reg) (const_int -1)))
  87                 (additional clobbers and uses)])
  88
  89      Some targets (IA-64) wrap the set of the loop counter in
  90      an if_then_else too.
  91
  92      In summary, the branch must be the first entry of the
  93      parallel (also required by jump.c), and the second
  94      entry of the parallel must be a set of the loop counter
  95      register.  */
  96
  97   if (GET_CODE (pattern) != PARALLEL)
  98     return 0;
  99
 100   cmp = XVECEXP (pattern, 0, 0);
 101   inc = XVECEXP (pattern, 0, 1);
 102
 103   /* Check for (set (reg) (something)).  */
 104   if (GET_CODE (inc) != SET)
 105     return 0;
 106   reg = SET_DEST (inc);
 107   if (! REG_P (reg))
 108     return 0;
 109
 110   /* Check if something = (plus (reg) (const_int -1)).
 111      On IA-64, this decrement is wrapped in an if_then_else.  */
 112   inc_src = SET_SRC (inc);
 113   if (GET_CODE (inc_src) == IF_THEN_ELSE)
 114     inc_src = XEXP (inc_src, 1);
 115   if (GET_CODE (inc_src) != PLUS
 116       || XEXP (inc_src, 0) != reg
 117       || XEXP (inc_src, 1) != constm1_rtx)
 118     return 0;
 119
 120   /* Check for (set (pc) (if_then_else (condition)
 121                                        (label_ref (label))
 122                                        (pc))).  */
 123   if (GET_CODE (cmp) != SET
 124       || SET_DEST (cmp) != pc_rtx
 125       || GET_CODE (SET_SRC (cmp)) != IF_THEN_ELSE
 126       || GET_CODE (XEXP (SET_SRC (cmp), 1)) != LABEL_REF
 127       || XEXP (SET_SRC (cmp), 2) != pc_rtx)
 128     return 0;
 129
 130   /* Extract loop termination condition.  */
 131   condition = XEXP (SET_SRC (cmp), 0);
 132
 133   /* We expect a GE or NE comparison with 0 or 1.  */
 134   if ((GET_CODE (condition) != GE
 135        && GET_CODE (condition) != NE)
 136       || (XEXP (condition, 1) != const0_rtx
 137           && XEXP (condition, 1) != const1_rtx))
 138     return 0;
 139
 140   if ((XEXP (condition, 0) == reg)
 141       || (GET_CODE (XEXP (condition, 0)) == PLUS
 142                    && XEXP (XEXP (condition, 0), 0) == reg))
 143     return condition;
 144
 145   /* ??? If a machine uses a funny comparison, we could return a
 146      canonicalized form here.  */
 147
 148   return 0;
 149 }
 150
 151 /* Return nonzero if the loop specified by LOOP is suitable for
 152    the use of special low-overhead looping instructions.  DESC
 153    describes the number of iterations of the loop.  */
 154
 155 static bool
 156 doloop_valid_p (struct loop *loop, struct niter_desc *desc)
 157 {
 158   basic_block *body = get_loop_body (loop), bb;
 159   rtx insn;
 160   unsigned i;
 161   bool result = true;
 162
 163   /* Check for loops that may not terminate under special conditions.  */
 164   if (!desc->simple_p
 165       || desc->assumptions
 166       || desc->infinite)
 167     {
 168       /* There are some cases that would require a special attention.
 169          For example if the comparison is LEU and the comparison value
 170          is UINT_MAX then the loop will not terminate.  Similarly, if the
 171          comparison code is GEU and the comparison value is 0, the
 172          loop will not terminate.
 173
 174          If the absolute increment is not 1, the loop can be infinite
 175          even with LTU/GTU, e.g. for (i = 3; i > 0; i -= 2)
 176
 177          ??? We could compute these conditions at run-time and have a
 178          additional jump around the loop to ensure an infinite loop.
 179          However, it is very unlikely that this is the intended
 180          behavior of the loop and checking for these rare boundary
 181          conditions would pessimize all other code.
 182
 183          If the loop is executed only a few times an extra check to
 184          restart the loop could use up most of the benefits of using a
 185          count register loop.  Note however, that normally, this
 186          restart branch would never execute, so it could be predicted
 187          well by the CPU.  We should generate the pessimistic code by
 188          default, and have an option, e.g. -funsafe-loops that would
 189          enable count-register loops in this case.  */
 190       if (dump_file)
 191         fprintf (dump_file, "Doloop: Possible infinite iteration case.\n");
 192       result = false;
 193       goto cleanup;
 194     }
 195
 196   for (i = 0; i < loop->num_nodes; i++)
 197     {
 198       bb = body[i];
 199
 200       for (insn = BB_HEAD (bb);
 201            insn != NEXT_INSN (BB_END (bb));
 202            insn = NEXT_INSN (insn))
 203         {
 204           /* Different targets have different necessities for low-overhead
 205              looping.  Call the back end for each instruction within the loop
 206              to let it decide whether the insn prohibits a low-overhead loop.
 207              It will then return the cause for it to emit to the dump file.  */
 208           const char * invalid = targetm.invalid_within_doloop (insn);
 209           if (invalid)
 210             {
 211               if (dump_file)
 212                 fprintf (dump_file, "Doloop: %s\n", invalid);
 213               result = false;
 214               goto cleanup;
 215             }
 216         }
 217     }
 218   result = true;
 219
 220 cleanup:
 221   free (body);
 222
 223   return result;
 224 }
 225
 226 /* Adds test of COND jumping to DEST on edge *E and set *E to the new fallthru
 227    edge.  If the condition is always false, do not do anything.  If it is always
 228    true, redirect E to DEST and return false.  In all other cases, true is
 229    returned.  */
 230
 231 static bool
 232 add_test (rtx cond, edge *e, basic_block dest)
 233 {
 234   rtx seq, jump, label;
 235   enum machine_mode mode;
 236   rtx op0 = XEXP (cond, 0), op1 = XEXP (cond, 1);
 237   enum rtx_code code = GET_CODE (cond);
 238   basic_block bb;
 239
 240   mode = GET_MODE (XEXP (cond, 0));
 241   if (mode == VOIDmode)
 242     mode = GET_MODE (XEXP (cond, 1));
 243
 244   start_sequence ();
 245   op0 = force_operand (op0, NULL_RTX);
 246   op1 = force_operand (op1, NULL_RTX);
 247   label = block_label (dest);
 248   do_compare_rtx_and_jump (op0, op1, code, 0, mode, NULL_RTX, NULL_RTX, label);
 249
 250   jump = get_last_insn ();
 251   if (!jump || !JUMP_P (jump))
 252     {
 253       /* The condition is always false and the jump was optimized out.  */
 254       end_sequence ();
 255       return true;
 256     }
 257
 258   seq = get_insns ();
 259   end_sequence ();
 260
 261   /* There always is at least the jump insn in the sequence.  */
 262   gcc_assert (seq != NULL_RTX);
 263
 264   bb = split_edge_and_insert (*e, seq);
 265   *e = single_succ_edge (bb);
 266
 267   if (any_uncondjump_p (jump))
 268     {
 269       /* The condition is always true.  */
 270       delete_insn (jump);
 271       redirect_edge_and_branch_force (*e, dest);
 272       return false;
 273     }
 274
 275   JUMP_LABEL (jump) = label;
 276
 277   /* The jump is supposed to handle an unlikely special case.  */
 278   REG_NOTES (jump)
 279           = gen_rtx_EXPR_LIST (REG_BR_PROB,
 280                                const0_rtx, REG_NOTES (jump));
 281   LABEL_NUSES (label)++;
 282
 283   make_edge (bb, dest, (*e)->flags & ~EDGE_FALLTHRU);
 284   return true;
 285 }
 286
 287 /* Modify the loop to use the low-overhead looping insn where LOOP
 288    describes the loop, DESC describes the number of iterations of the
 289    loop, and DOLOOP_INSN is the low-overhead looping insn to emit at the
 290    end of the loop.  CONDITION is the condition separated from the
 291    DOLOOP_SEQ.  COUNT is the number of iterations of the LOOP.  */
 292
 293 static void
 294 doloop_modify (struct loop *loop, struct niter_desc *desc,
 295                rtx doloop_seq, rtx condition, rtx count)
 296 {
 297   rtx counter_reg;
 298   rtx tmp, noloop = NULL_RTX;
 299   rtx sequence;
 300   rtx jump_insn;
 301   rtx jump_label;
 302   int nonneg = 0;
 303   bool increment_count;
 304   basic_block loop_end = desc->out_edge->src;
 305   enum machine_mode mode;
 306
 307   jump_insn = BB_END (loop_end);
 308
 309   if (dump_file)
 310     {
 311       fprintf (dump_file, "Doloop: Inserting doloop pattern (");
 312       if (desc->const_iter)
 313         fprintf (dump_file, HOST_WIDEST_INT_PRINT_DEC, desc->niter);
 314       else
 315         fputs ("runtime", dump_file);
 316       fputs (" iterations).\n", dump_file);
 317     }
 318
 319   /* Discard original jump to continue loop.  The original compare
 320      result may still be live, so it cannot be discarded explicitly.  */
 321   delete_insn (jump_insn);
 322
 323   counter_reg = XEXP (condition, 0);
 324   if (GET_CODE (counter_reg) == PLUS)
 325     counter_reg = XEXP (counter_reg, 0);
 326   mode = GET_MODE (counter_reg);
 327
 328   increment_count = false;
 329   switch (GET_CODE (condition))
 330     {
 331     case NE:
 332       /* Currently only NE tests against zero and one are supported.  */
 333       noloop = XEXP (condition, 1);
 334       if (noloop != const0_rtx)
 335         {
 336           gcc_assert (noloop == const1_rtx);
 337           increment_count = true;
 338         }
 339       break;
 340
 341     case GE:
 342       /* Currently only GE tests against zero are supported.  */
 343       gcc_assert (XEXP (condition, 1) == const0_rtx);
 344
 345       noloop = constm1_rtx;
 346
 347       /* The iteration count does not need incrementing for a GE test.  */
 348       increment_count = false;
 349
 350       /* Determine if the iteration counter will be non-negative.
 351          Note that the maximum value loaded is iterations_max - 1.  */
 352       if (desc->niter_max
 353           <= ((unsigned HOST_WIDEST_INT) 1
 354               << (GET_MODE_BITSIZE (mode) - 1)))
 355         nonneg = 1;
 356       break;
 357
 358       /* Abort if an invalid doloop pattern has been generated.  */
 359     default:
 360       gcc_unreachable ();
 361     }
 362
 363   if (increment_count)
 364     count = simplify_gen_binary (PLUS, mode, count, const1_rtx);
 365
 366   /* Insert initialization of the count register into the loop header.  */
 367   start_sequence ();
 368   tmp = force_operand (count, counter_reg);
 369   convert_move (counter_reg, tmp, 1);
 370   sequence = get_insns ();
 371   end_sequence ();
 372   emit_insn_after (sequence, BB_END (loop_preheader_edge (loop)->src));
 373
 374   if (desc->noloop_assumptions)
 375     {
 376       rtx ass = copy_rtx (desc->noloop_assumptions);
 377       basic_block preheader = loop_preheader_edge (loop)->src;
 378       basic_block set_zero
 379               = split_edge (loop_preheader_edge (loop));
 380       basic_block new_preheader
 381               = split_edge (loop_preheader_edge (loop));
 382       edge te;
 383
 384       /* Expand the condition testing the assumptions and if it does not pass,
 385          reset the count register to 0.  */
 386       redirect_edge_and_branch_force (single_succ_edge (preheader), new_preheader);
 387       set_immediate_dominator (CDI_DOMINATORS, new_preheader, preheader);
 388
 389       set_zero->count = 0;
 390       set_zero->frequency = 0;
 391
 392       te = single_succ_edge (preheader);
 393       for (; ass; ass = XEXP (ass, 1))
 394         if (!add_test (XEXP (ass, 0), &te, set_zero))
 395           break;
 396
 397       if (ass)
 398         {
 399           /* We reached a condition that is always true.  This is very hard to
 400              reproduce (such a loop does not roll, and thus it would most
 401              likely get optimized out by some of the preceding optimizations).
 402              In fact, I do not have any testcase for it.  However, it would
 403              also be very hard to show that it is impossible, so we must
 404              handle this case.  */
 405           set_zero->count = preheader->count;
 406           set_zero->frequency = preheader->frequency;
 407         }
 408
 409       if (EDGE_COUNT (set_zero->preds) == 0)
 410         {
 411           /* All the conditions were simplified to false, remove the
 412              unreachable set_zero block.  */
 413           delete_basic_block (set_zero);
 414         }
 415       else
 416         {
 417           /* Reset the counter to zero in the set_zero block.  */
 418           start_sequence ();
 419           convert_move (counter_reg, noloop, 0);
 420           sequence = get_insns ();
 421           end_sequence ();
 422           emit_insn_after (sequence, BB_END (set_zero));
 423
 424           set_immediate_dominator (CDI_DOMINATORS, set_zero,
 425                                    recompute_dominator (CDI_DOMINATORS,
 426                                                         set_zero));
 427         }
 428
 429       set_immediate_dominator (CDI_DOMINATORS, new_preheader,
 430                                recompute_dominator (CDI_DOMINATORS,
 431                                                     new_preheader));
 432     }
 433
 434   /* Some targets (eg, C4x) need to initialize special looping
 435      registers.  */
 436 #ifdef HAVE_doloop_begin
 437   {
 438     rtx init;
 439     unsigned level = get_loop_level (loop) + 1;
 440     init = gen_doloop_begin (counter_reg,
 441                              desc->const_iter ? desc->niter_expr : const0_rtx,
 442                              GEN_INT (desc->niter_max),
 443                              GEN_INT (level));
 444     if (init)
 445       {
 446         start_sequence ();
 447         emit_insn (init);
 448         sequence = get_insns ();
 449         end_sequence ();
 450         emit_insn_after (sequence, BB_END (loop_preheader_edge (loop)->src));
 451       }
 452   }
 453 #endif
 454
 455   /* Insert the new low-overhead looping insn.  */
 456   emit_jump_insn_after (doloop_seq, BB_END (loop_end));
 457   jump_insn = BB_END (loop_end);
 458   jump_label = block_label (desc->in_edge->dest);
 459   JUMP_LABEL (jump_insn) = jump_label;
 460   LABEL_NUSES (jump_label)++;
 461
 462   /* Ensure the right fallthru edge is marked, for case we have reversed
 463      the condition.  */
 464   desc->in_edge->flags &= ~EDGE_FALLTHRU;
 465   desc->out_edge->flags |= EDGE_FALLTHRU;
 466
 467   /* Add a REG_NONNEG note if the actual or estimated maximum number
 468      of iterations is non-negative.  */
 469   if (nonneg)
 470     {
 471       REG_NOTES (jump_insn)
 472         = gen_rtx_EXPR_LIST (REG_NONNEG, NULL_RTX, REG_NOTES (jump_insn));
 473     }
 474 }
 475
 476 /* Process loop described by LOOP validating that the loop is suitable for
 477    conversion to use a low overhead looping instruction, replacing the jump
 478    insn where suitable.  Returns true if the loop was successfully
 479    modified.  */
 480
 481 static bool
 482 doloop_optimize (struct loop *loop)
 483 {
 484   enum machine_mode mode;
 485   rtx doloop_seq, doloop_pat, doloop_reg;
 486   rtx iterations, count;
 487   rtx iterations_max;
 488   rtx start_label;
 489   rtx condition;
 490   unsigned level, est_niter;
 491   int max_cost;
 492   struct niter_desc *desc;
 493   unsigned word_mode_size;
 494   unsigned HOST_WIDE_INT word_mode_max;
 495
 496   if (dump_file)
 497     fprintf (dump_file, "Doloop: Processing loop %d.\n", loop->num);
 498
 499   iv_analysis_loop_init (loop);
 500
 501   /* Find the simple exit of a LOOP.  */
 502   desc = get_simple_loop_desc (loop);
 503
 504   /* Check that loop is a candidate for a low-overhead looping insn.  */
 505   if (!doloop_valid_p (loop, desc))
 506     {
 507       if (dump_file)
 508         fprintf (dump_file,
 509                  "Doloop: The loop is not suitable.\n");
 510       return false;
 511     }
 512   mode = desc->mode;
 513
 514   est_niter = 3;
 515   if (desc->const_iter)
 516     est_niter = desc->niter;
 517   /* If the estimate on number of iterations is reliable (comes from profile
 518      feedback), use it.  Do not use it normally, since the expected number
 519      of iterations of an unrolled loop is 2.  */
 520   if (loop->header->count)
 521     est_niter = expected_loop_iterations (loop);
 522
 523   if (est_niter < 3)
 524     {
 525       if (dump_file)
 526         fprintf (dump_file,
 527                  "Doloop: Too few iterations (%u) to be profitable.\n",
 528                  est_niter);
 529       return false;
 530     }
 531
 532   max_cost
 533     = COSTS_N_INSNS (PARAM_VALUE (PARAM_MAX_ITERATIONS_COMPUTATION_COST));
 534   if (rtx_cost (desc->niter_expr, SET) > max_cost)
 535     {
 536       if (dump_file)
 537         fprintf (dump_file,
 538                  "Doloop: number of iterations too costly to compute.\n");
 539       return false;
 540     }
 541
 542   count = copy_rtx (desc->niter_expr);
 543   iterations = desc->const_iter ? desc->niter_expr : const0_rtx;
 544   iterations_max = GEN_INT (desc->niter_max);
 545   level = get_loop_level (loop) + 1;
 546
 547   /* Generate looping insn.  If the pattern FAILs then give up trying
 548      to modify the loop since there is some aspect the back-end does
 549      not like.  */
 550   start_label = block_label (desc->in_edge->dest);
 551   doloop_reg = gen_reg_rtx (mode);
 552   doloop_seq = gen_doloop_end (doloop_reg, iterations, iterations_max,
 553                                GEN_INT (level), start_label);
 554
 555   word_mode_size = GET_MODE_BITSIZE (word_mode);
 556   word_mode_max
 557           = ((unsigned HOST_WIDE_INT) 1 << (word_mode_size - 1) << 1) - 1;
 558   if (! doloop_seq
 559       && mode != word_mode
 560       /* Before trying mode different from the one in that # of iterations is
 561          computed, we must be sure that the number of iterations fits into
 562          the new mode.  */
 563       && (word_mode_size >= GET_MODE_BITSIZE (mode)
 564           || desc->niter_max <= word_mode_max))
 565     {
 566       if (word_mode_size > GET_MODE_BITSIZE (mode))
 567         {
 568           count = simplify_gen_unary (ZERO_EXTEND, word_mode,
 569                                       count, mode);
 570           iterations = simplify_gen_unary (ZERO_EXTEND, word_mode,
 571                                            iterations, mode);
 572           iterations_max = simplify_gen_unary (ZERO_EXTEND, word_mode,
 573                                                iterations_max, mode);
 574         }
 575       else
 576         {
 577           count = lowpart_subreg (word_mode, count, mode);
 578           iterations = lowpart_subreg (word_mode, iterations, mode);
 579           iterations_max = lowpart_subreg (word_mode, iterations_max, mode);
 580         }
 581       PUT_MODE (doloop_reg, word_mode);
 582       doloop_seq = gen_doloop_end (doloop_reg, iterations, iterations_max,
 583                                    GEN_INT (level), start_label);
 584     }
 585   if (! doloop_seq)
 586     {
 587       if (dump_file)
 588         fprintf (dump_file,
 589                  "Doloop: Target unwilling to use doloop pattern!\n");
 590       return false;
 591     }
 592
 593   /* If multiple instructions were created, the last must be the
 594      jump instruction.  Also, a raw define_insn may yield a plain
 595      pattern.  */
 596   doloop_pat = doloop_seq;
 597   if (INSN_P (doloop_pat))
 598     {
 599       while (NEXT_INSN (doloop_pat) != NULL_RTX)
 600         doloop_pat = NEXT_INSN (doloop_pat);
 601       if (JUMP_P (doloop_pat))
 602         doloop_pat = PATTERN (doloop_pat);
 603       else
 604         doloop_pat = NULL_RTX;
 605     }
 606
 607   if (! doloop_pat
 608       || ! (condition = doloop_condition_get (doloop_pat)))
 609     {
 610       if (dump_file)
 611         fprintf (dump_file, "Doloop: Unrecognizable doloop pattern!\n");
 612       return false;
 613     }
 614
 615   doloop_modify (loop, desc, doloop_seq, condition, count);
 616   return true;
 617 }
 618
 619 /* This is the main entry point.  Process all loops using doloop_optimize.  */
 620
 621 void
 622 doloop_optimize_loops (void)
 623 {
 624   loop_iterator li;
 625   struct loop *loop;
 626
 627   FOR_EACH_LOOP (li, loop, 0)
 628     {
 629       doloop_optimize (loop);
 630     }
 631
 632   iv_analysis_done ();
 633
 634 #ifdef ENABLE_CHECKING
 635   verify_dominators (CDI_DOMINATORS);
 636   verify_loop_structure ();
 637 #endif
 638 }
 639 #endif /* HAVE_doloop_end */
 640