gcc/loop-unswitch.c

   1 /* Loop unswitching for GNU compiler.
   2    Copyright (C) 2002-2013 Free Software Foundation, Inc.
   3
   4 This file is part of GCC.
   5
   6 GCC is free software; you can redistribute it and/or modify it under
   7 the terms of the GNU General Public License as published by the Free
   8 Software Foundation; either version 3, or (at your option) any later
   9 version.
  10
  11 GCC is distributed in the hope that it will be useful, but WITHOUT ANY
  12 WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13 FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14 for more details.
  15
  16 You should have received a copy of the GNU General Public License
  17 along with GCC; see the file COPYING3.  If not see
  18 <http://www.gnu.org/licenses/>.  */
  19
  20 #include "config.h"
  21 #include "system.h"
  22 #include "coretypes.h"
  23 #include "tm.h"
  24 #include "rtl.h"
  25 #include "hard-reg-set.h"
  26 #include "obstack.h"
  27 #include "basic-block.h"
  28 #include "cfgloop.h"
  29 #include "params.h"
  30 #include "expr.h"
  31 #include "dumpfile.h"
  32
  33 /* This pass moves constant conditions out of loops, duplicating the loop
  34    in progress, i.e. this code:
  35
  36    while (loop_cond)
  37      {
  38        A;
  39        if (cond)
  40          branch1;
  41        else
  42          branch2;
  43        B;
  44        if (cond)
  45          branch3;
  46        C;
  47      }
  48    where nothing inside the loop alters cond is transformed
  49    into
  50
  51    if (cond)
  52      {
  53        while (loop_cond)
  54          {
  55            A;
  56            branch1;
  57            B;
  58            branch3;
  59            C;
  60          }
  61      }
  62    else
  63      {
  64        while (loop_cond)
  65          {
  66            A;
  67            branch2;
  68            B;
  69            C;
  70          }
  71      }
  72
  73   Duplicating the loop might lead to code growth exponential in number of
  74   branches inside loop, so we limit the number of unswitchings performed
  75   in a single loop to PARAM_MAX_UNSWITCH_LEVEL.  We only perform the
  76   transformation on innermost loops, as the benefit of doing it on loops
  77   containing subloops would not be very large compared to complications
  78   with handling this case.  */
  79
  80 static struct loop *unswitch_loop (struct loop *, basic_block, rtx, rtx);
  81 static bool unswitch_single_loop (struct loop *, rtx, int);
  82 static rtx may_unswitch_on (basic_block, struct loop *, rtx *);
  83
  84 /* Prepare a sequence comparing OP0 with OP1 using COMP and jumping to LABEL if
  85    true, with probability PROB.  If CINSN is not NULL, it is the insn to copy
  86    in order to create a jump.  */
  87
  88 rtx
  89 compare_and_jump_seq (rtx op0, rtx op1, enum rtx_code comp, rtx label, int prob,
  90                       rtx cinsn)
  91 {
  92   rtx seq, jump, cond;
  93   enum machine_mode mode;
  94
  95   mode = GET_MODE (op0);
  96   if (mode == VOIDmode)
  97     mode = GET_MODE (op1);
  98
  99   start_sequence ();
 100   if (GET_MODE_CLASS (mode) == MODE_CC)
 101     {
 102       /* A hack -- there seems to be no easy generic way how to make a
 103          conditional jump from a ccmode comparison.  */
 104       gcc_assert (cinsn);
 105       cond = XEXP (SET_SRC (pc_set (cinsn)), 0);
 106       gcc_assert (GET_CODE (cond) == comp);
 107       gcc_assert (rtx_equal_p (op0, XEXP (cond, 0)));
 108       gcc_assert (rtx_equal_p (op1, XEXP (cond, 1)));
 109       emit_jump_insn (copy_insn (PATTERN (cinsn)));
 110       jump = get_last_insn ();
 111       gcc_assert (JUMP_P (jump));
 112       JUMP_LABEL (jump) = JUMP_LABEL (cinsn);
 113       LABEL_NUSES (JUMP_LABEL (jump))++;
 114       redirect_jump (jump, label, 0);
 115     }
 116   else
 117     {
 118       gcc_assert (!cinsn);
 119
 120       op0 = force_operand (op0, NULL_RTX);
 121       op1 = force_operand (op1, NULL_RTX);
 122       do_compare_rtx_and_jump (op0, op1, comp, 0,
 123                                mode, NULL_RTX, NULL_RTX, label, -1);
 124       jump = get_last_insn ();
 125       gcc_assert (JUMP_P (jump));
 126       JUMP_LABEL (jump) = label;
 127       LABEL_NUSES (label)++;
 128     }
 129   add_reg_note (jump, REG_BR_PROB, GEN_INT (prob));
 130
 131   seq = get_insns ();
 132   end_sequence ();
 133
 134   return seq;
 135 }
 136
 137 /* Main entry point.  Perform loop unswitching on all suitable loops.  */
 138 void
 139 unswitch_loops (void)
 140 {
 141   loop_iterator li;
 142   struct loop *loop;
 143   bool changed = false;
 144
 145   /* Go through inner loops (only original ones).  */
 146
 147   FOR_EACH_LOOP (li, loop, LI_ONLY_INNERMOST)
 148     changed |= unswitch_single_loop (loop, NULL_RTX, 0);
 149
 150   iv_analysis_done ();
 151
 152   /* If we unswitched any loop discover new loops that are eventually
 153      exposed by making irreducible regions reducible.  */
 154   if (changed)
 155     {
 156       calculate_dominance_info (CDI_DOMINATORS);
 157       fix_loop_structure (NULL);
 158     }
 159 }
 160
 161 /* Checks whether we can unswitch LOOP on condition at end of BB -- one of its
 162    basic blocks (for what it means see comments below).  In case condition
 163    compares loop invariant cc mode register, return the jump in CINSN.  */
 164
 165 static rtx
 166 may_unswitch_on (basic_block bb, struct loop *loop, rtx *cinsn)
 167 {
 168   rtx test, at, op[2], stest;
 169   struct rtx_iv iv;
 170   unsigned i;
 171   enum machine_mode mode;
 172
 173   /* BB must end in a simple conditional jump.  */
 174   if (EDGE_COUNT (bb->succs) != 2)
 175     return NULL_RTX;
 176   if (!any_condjump_p (BB_END (bb)))
 177     return NULL_RTX;
 178
 179   /* With branches inside loop.  */
 180   if (!flow_bb_inside_loop_p (loop, EDGE_SUCC (bb, 0)->dest)
 181       || !flow_bb_inside_loop_p (loop, EDGE_SUCC (bb, 1)->dest))
 182     return NULL_RTX;
 183
 184   /* It must be executed just once each iteration (because otherwise we
 185      are unable to update dominator/irreducible loop information correctly).  */
 186   if (!just_once_each_iteration_p (loop, bb))
 187     return NULL_RTX;
 188
 189   /* Condition must be invariant.  */
 190   test = get_condition (BB_END (bb), &at, true, false);
 191   if (!test)
 192     return NULL_RTX;
 193
 194   for (i = 0; i < 2; i++)
 195     {
 196       op[i] = XEXP (test, i);
 197
 198       if (CONSTANT_P (op[i]))
 199         continue;
 200
 201       if (!iv_analyze (at, op[i], &iv))
 202         return NULL_RTX;
 203       if (iv.step != const0_rtx
 204           || iv.first_special)
 205         return NULL_RTX;
 206
 207       op[i] = get_iv_value (&iv, const0_rtx);
 208     }
 209
 210   mode = GET_MODE (op[0]);
 211   if (mode == VOIDmode)
 212     mode = GET_MODE (op[1]);
 213   if (GET_MODE_CLASS (mode) == MODE_CC)
 214     {
 215       if (at != BB_END (bb))
 216         return NULL_RTX;
 217
 218       if (!rtx_equal_p (op[0], XEXP (test, 0))
 219           || !rtx_equal_p (op[1], XEXP (test, 1)))
 220         return NULL_RTX;
 221
 222       *cinsn = BB_END (bb);
 223       return test;
 224     }
 225
 226   stest = simplify_gen_relational (GET_CODE (test), SImode,
 227                                    mode, op[0], op[1]);
 228   if (stest == const0_rtx
 229       || stest == const_true_rtx)
 230     return stest;
 231
 232   return canon_condition (gen_rtx_fmt_ee (GET_CODE (test), SImode,
 233                                           op[0], op[1]));
 234 }
 235
 236 /* Reverses CONDition; returns NULL if we cannot.  */
 237 rtx
 238 reversed_condition (rtx cond)
 239 {
 240   enum rtx_code reversed;
 241   reversed = reversed_comparison_code (cond, NULL);
 242   if (reversed == UNKNOWN)
 243     return NULL_RTX;
 244   else
 245     return gen_rtx_fmt_ee (reversed,
 246                            GET_MODE (cond), XEXP (cond, 0),
 247                            XEXP (cond, 1));
 248 }
 249
 250 /* Unswitch single LOOP.  COND_CHECKED holds list of conditions we already
 251    unswitched on and are therefore known to be true in this LOOP.  NUM is
 252    number of unswitchings done; do not allow it to grow too much, it is too
 253    easy to create example on that the code would grow exponentially.
 254    Returns true LOOP was unswitched.  */
 255 static bool
 256 unswitch_single_loop (struct loop *loop, rtx cond_checked, int num)
 257 {
 258   basic_block *bbs;
 259   struct loop *nloop;
 260   unsigned i;
 261   rtx cond, rcond = NULL_RTX, conds, rconds, acond, cinsn;
 262   int repeat;
 263   edge e;
 264   HOST_WIDE_INT iterations;
 265
 266   /* Do not unswitch too much.  */
 267   if (num > PARAM_VALUE (PARAM_MAX_UNSWITCH_LEVEL))
 268     {
 269       if (dump_file)
 270         fprintf (dump_file, ";; Not unswitching anymore, hit max level\n");
 271       return false;
 272     }
 273
 274   /* Only unswitch innermost loops.  */
 275   if (loop->inner)
 276     {
 277       if (dump_file)
 278         fprintf (dump_file, ";; Not unswitching, not innermost loop\n");
 279       return false;
 280     }
 281
 282   /* We must be able to duplicate loop body.  */
 283   if (!can_duplicate_loop_p (loop))
 284     {
 285       if (dump_file)
 286         fprintf (dump_file, ";; Not unswitching, can't duplicate loop\n");
 287       return false;
 288     }
 289
 290   /* The loop should not be too large, to limit code growth.  */
 291   if (num_loop_insns (loop) > PARAM_VALUE (PARAM_MAX_UNSWITCH_INSNS))
 292     {
 293       if (dump_file)
 294         fprintf (dump_file, ";; Not unswitching, loop too big\n");
 295       return false;
 296     }
 297
 298   /* Do not unswitch in cold areas.  */
 299   if (optimize_loop_for_size_p (loop))
 300     {
 301       if (dump_file)
 302         fprintf (dump_file, ";; Not unswitching, not hot area\n");
 303       return false;
 304     }
 305
 306   /* Nor if the loop usually does not roll.  */
 307   iterations = estimated_loop_iterations_int (loop);
 308   if (iterations >= 0 && iterations <= 1)
 309     {
 310       if (dump_file)
 311         fprintf (dump_file, ";; Not unswitching, loop iterations < 1\n");
 312       return false;
 313     }
 314
 315   do
 316     {
 317       repeat = 0;
 318       cinsn = NULL_RTX;
 319
 320       /* Find a bb to unswitch on.  */
 321       bbs = get_loop_body (loop);
 322       iv_analysis_loop_init (loop);
 323       for (i = 0; i < loop->num_nodes; i++)
 324         if ((cond = may_unswitch_on (bbs[i], loop, &cinsn)))
 325           break;
 326
 327       if (i == loop->num_nodes)
 328         {
 329           free (bbs);
 330           return false;
 331         }
 332
 333       if (cond != const0_rtx
 334           && cond != const_true_rtx)
 335         {
 336           rcond = reversed_condition (cond);
 337           if (rcond)
 338             rcond = canon_condition (rcond);
 339
 340           /* Check whether the result can be predicted.  */
 341           for (acond = cond_checked; acond; acond = XEXP (acond, 1))
 342             simplify_using_condition (XEXP (acond, 0), &cond, NULL);
 343         }
 344
 345       if (cond == const_true_rtx)
 346         {
 347           /* Remove false path.  */
 348           e = FALLTHRU_EDGE (bbs[i]);
 349           remove_path (e);
 350           free (bbs);
 351           repeat = 1;
 352         }
 353       else if (cond == const0_rtx)
 354         {
 355           /* Remove true path.  */
 356           e = BRANCH_EDGE (bbs[i]);
 357           remove_path (e);
 358           free (bbs);
 359           repeat = 1;
 360         }
 361     } while (repeat);
 362
 363   /* We found the condition we can unswitch on.  */
 364   conds = alloc_EXPR_LIST (0, cond, cond_checked);
 365   if (rcond)
 366     rconds = alloc_EXPR_LIST (0, rcond, cond_checked);
 367   else
 368     rconds = cond_checked;
 369
 370   if (dump_file)
 371     fprintf (dump_file, ";; Unswitching loop\n");
 372
 373   /* Unswitch the loop on this condition.  */
 374   nloop = unswitch_loop (loop, bbs[i], copy_rtx_if_shared (cond), cinsn);
 375   gcc_assert (nloop);
 376
 377   /* Invoke itself on modified loops.  */
 378   unswitch_single_loop (nloop, rconds, num + 1);
 379   unswitch_single_loop (loop, conds, num + 1);
 380
 381   free_EXPR_LIST_node (conds);
 382   if (rcond)
 383     free_EXPR_LIST_node (rconds);
 384
 385   free (bbs);
 386
 387   return true;
 388 }
 389
 390 /* Unswitch a LOOP w.r. to given basic block UNSWITCH_ON.  We only support
 391    unswitching of innermost loops.  UNSWITCH_ON must be executed in every
 392    iteration, i.e. it must dominate LOOP latch.  COND is the condition
 393    determining which loop is entered.  Returns NULL if impossible, new loop
 394    otherwise.  The new loop is entered if COND is true.  If CINSN is not
 395    NULL, it is the insn in that COND is compared.  */
 396
 397 static struct loop *
 398 unswitch_loop (struct loop *loop, basic_block unswitch_on, rtx cond, rtx cinsn)
 399 {
 400   edge entry, latch_edge, true_edge, false_edge, e;
 401   basic_block switch_bb, unswitch_on_alt;
 402   struct loop *nloop;
 403   int irred_flag, prob;
 404   rtx seq;
 405
 406   /* Some sanity checking.  */
 407   gcc_assert (flow_bb_inside_loop_p (loop, unswitch_on));
 408   gcc_assert (EDGE_COUNT (unswitch_on->succs) == 2);
 409   gcc_assert (just_once_each_iteration_p (loop, unswitch_on));
 410   gcc_assert (!loop->inner);
 411   gcc_assert (flow_bb_inside_loop_p (loop, EDGE_SUCC (unswitch_on, 0)->dest));
 412   gcc_assert (flow_bb_inside_loop_p (loop, EDGE_SUCC (unswitch_on, 1)->dest));
 413
 414   entry = loop_preheader_edge (loop);
 415
 416   /* Make a copy.  */
 417   irred_flag = entry->flags & EDGE_IRREDUCIBLE_LOOP;
 418   entry->flags &= ~EDGE_IRREDUCIBLE_LOOP;
 419   if (!duplicate_loop_to_header_edge (loop, entry, 1,
 420                                       NULL, NULL, NULL, 0))
 421     return NULL;
 422   entry->flags |= irred_flag;
 423
 424   /* Record the block with condition we unswitch on.  */
 425   unswitch_on_alt = get_bb_copy (unswitch_on);
 426   true_edge = BRANCH_EDGE (unswitch_on_alt);
 427   false_edge = FALLTHRU_EDGE (unswitch_on);
 428   latch_edge = single_succ_edge (get_bb_copy (loop->latch));
 429
 430   /* Create a block with the condition.  */
 431   prob = true_edge->probability;
 432   switch_bb = create_empty_bb (EXIT_BLOCK_PTR->prev_bb);
 433   seq = compare_and_jump_seq (XEXP (cond, 0), XEXP (cond, 1), GET_CODE (cond),
 434                               block_label (true_edge->dest),
 435                               prob, cinsn);
 436   emit_insn_after (seq, BB_END (switch_bb));
 437   e = make_edge (switch_bb, true_edge->dest, 0);
 438   e->probability = prob;
 439   /* Update to use apply_probability().  */
 440   e->count = latch_edge->count * prob / REG_BR_PROB_BASE;
 441   e = make_edge (switch_bb, FALLTHRU_EDGE (unswitch_on)->dest, EDGE_FALLTHRU);
 442   e->probability = false_edge->probability;
 443   /* Update to use apply_probability().  */
 444   e->count = latch_edge->count * (false_edge->probability) / REG_BR_PROB_BASE;
 445
 446   if (irred_flag)
 447     {
 448       switch_bb->flags |= BB_IRREDUCIBLE_LOOP;
 449       EDGE_SUCC (switch_bb, 0)->flags |= EDGE_IRREDUCIBLE_LOOP;
 450       EDGE_SUCC (switch_bb, 1)->flags |= EDGE_IRREDUCIBLE_LOOP;
 451     }
 452   else
 453     {
 454       switch_bb->flags &= ~BB_IRREDUCIBLE_LOOP;
 455       EDGE_SUCC (switch_bb, 0)->flags &= ~EDGE_IRREDUCIBLE_LOOP;
 456       EDGE_SUCC (switch_bb, 1)->flags &= ~EDGE_IRREDUCIBLE_LOOP;
 457     }
 458
 459   /* Loopify from the copy of LOOP body, constructing the new loop.  */
 460   nloop = loopify (latch_edge,
 461                    single_pred_edge (get_bb_copy (loop->header)), switch_bb,
 462                    BRANCH_EDGE (switch_bb), FALLTHRU_EDGE (switch_bb), true,
 463                    prob, REG_BR_PROB_BASE - prob);
 464
 465   copy_loop_info (loop, nloop);
 466   /* Remove branches that are now unreachable in new loops.  */
 467   remove_path (true_edge);
 468   remove_path (false_edge);
 469
 470   /* Preserve the simple loop preheaders.  */
 471   split_edge (loop_preheader_edge (loop));
 472   split_edge (loop_preheader_edge (nloop));
 473
 474   return nloop;
 475 }