gcc/loop-unswitch.c

   1 /* Loop unswitching for GNU compiler.
   2    Copyright (C) 2002, 2003, 2004, 2005, 2007 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 "cfglayout.h"
  30 #include "params.h"
  31 #include "output.h"
  32 #include "expr.h"
  33
  34 /* This pass moves constant conditions out of loops, duplicating the loop
  35    in progress, i.e. this code:
  36
  37    while (loop_cond)
  38      {
  39        A;
  40        if (cond)
  41          branch1;
  42        else
  43          branch2;
  44        B;
  45        if (cond)
  46          branch3;
  47        C;
  48      }
  49    where nothing inside the loop alters cond is transformed
  50    into
  51
  52    if (cond)
  53      {
  54        while (loop_cond)
  55          {
  56            A;
  57            branch1;
  58            B;
  59            branch3;
  60            C;
  61          }
  62      }
  63    else
  64      {
  65        while (loop_cond)
  66          {
  67            A;
  68            branch2;
  69            B;
  70            C;
  71          }
  72      }
  73
  74   Duplicating the loop might lead to code growth exponential in number of
  75   branches inside loop, so we limit the number of unswitchings performed
  76   in a single loop to PARAM_MAX_UNSWITCH_LEVEL.  We only perform the
  77   transformation on innermost loops, as the benefit of doing it on loops
  78   containing subloops would not be very large compared to complications
  79   with handling this case.  */
  80
  81 static struct loop *unswitch_loop (struct loops *, struct loop *,
  82                                    basic_block, rtx, rtx);
  83 static void unswitch_single_loop (struct loops *, struct loop *, rtx, int);
  84 static rtx may_unswitch_on (basic_block, struct loop *, rtx *);
  85
  86 /* Prepare a sequence comparing OP0 with OP1 using COMP and jumping to LABEL if
  87    true, with probability PROB.  If CINSN is not NULL, it is the insn to copy
  88    in order to create a jump.  */
  89
  90 rtx
  91 compare_and_jump_seq (rtx op0, rtx op1, enum rtx_code comp, rtx label, int prob,
  92                       rtx cinsn)
  93 {
  94   rtx seq, jump, cond;
  95   enum machine_mode mode;
  96
  97   mode = GET_MODE (op0);
  98   if (mode == VOIDmode)
  99     mode = GET_MODE (op1);
 100
 101   start_sequence ();
 102   if (GET_MODE_CLASS (mode) == MODE_CC)
 103     {
 104       /* A hack -- there seems to be no easy generic way how to make a
 105          conditional jump from a ccmode comparison.  */
 106       gcc_assert (cinsn);
 107       cond = XEXP (SET_SRC (pc_set (cinsn)), 0);
 108       gcc_assert (GET_CODE (cond) == comp);
 109       gcc_assert (rtx_equal_p (op0, XEXP (cond, 0)));
 110       gcc_assert (rtx_equal_p (op1, XEXP (cond, 1)));
 111       emit_jump_insn (copy_insn (PATTERN (cinsn)));
 112       jump = get_last_insn ();
 113       JUMP_LABEL (jump) = JUMP_LABEL (cinsn);
 114       LABEL_NUSES (JUMP_LABEL (jump))++;
 115       redirect_jump (jump, label, 0);
 116     }
 117   else
 118     {
 119       gcc_assert (!cinsn);
 120
 121       op0 = force_operand (op0, NULL_RTX);
 122       op1 = force_operand (op1, NULL_RTX);
 123       do_compare_rtx_and_jump (op0, op1, comp, 0,
 124                                mode, NULL_RTX, NULL_RTX, label);
 125       jump = get_last_insn ();
 126       JUMP_LABEL (jump) = label;
 127       LABEL_NUSES (label)++;
 128     }
 129   REG_NOTES (jump) = gen_rtx_EXPR_LIST (REG_BR_PROB, GEN_INT (prob),
 130                                         REG_NOTES (jump));
 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 (struct loops *loops)
 140 {
 141   int i, num;
 142   struct loop *loop;
 143
 144   /* Go through inner loops (only original ones).  */
 145   num = loops->num;
 146
 147   for (i = 1; i < num; i++)
 148     {
 149       /* Removed loop?  */
 150       loop = loops->parray[i];
 151       if (!loop)
 152         continue;
 153
 154       if (loop->inner)
 155         continue;
 156
 157       unswitch_single_loop (loops, loop, NULL_RTX, 0);
 158 #ifdef ENABLE_CHECKING
 159       verify_dominators (CDI_DOMINATORS);
 160       verify_loop_structure (loops);
 161 #endif
 162     }
 163
 164   iv_analysis_done ();
 165 }
 166
 167 /* Checks whether we can unswitch LOOP on condition at end of BB -- one of its
 168    basic blocks (for what it means see comments below).  In case condition
 169    compares loop invariant cc mode register, return the jump in CINSN.  */
 170
 171 static rtx
 172 may_unswitch_on (basic_block bb, struct loop *loop, rtx *cinsn)
 173 {
 174   rtx test, at, op[2], stest;
 175   struct rtx_iv iv;
 176   unsigned i;
 177   enum machine_mode mode;
 178
 179   /* BB must end in a simple conditional jump.  */
 180   if (EDGE_COUNT (bb->succs) != 2)
 181     return NULL_RTX;
 182   if (!any_condjump_p (BB_END (bb)))
 183     return NULL_RTX;
 184
 185   /* With branches inside loop.  */
 186   if (!flow_bb_inside_loop_p (loop, EDGE_SUCC (bb, 0)->dest)
 187       || !flow_bb_inside_loop_p (loop, EDGE_SUCC (bb, 1)->dest))
 188     return NULL_RTX;
 189
 190   /* It must be executed just once each iteration (because otherwise we
 191      are unable to update dominator/irreducible loop information correctly).  */
 192   if (!just_once_each_iteration_p (loop, bb))
 193     return NULL_RTX;
 194
 195   /* Condition must be invariant.  */
 196   test = get_condition (BB_END (bb), &at, true, false);
 197   if (!test)
 198     return NULL_RTX;
 199
 200   for (i = 0; i < 2; i++)
 201     {
 202       op[i] = XEXP (test, i);
 203
 204       if (CONSTANT_P (op[i]))
 205         continue;
 206
 207       if (!iv_analyze (at, op[i], &iv))
 208         return NULL_RTX;
 209       if (iv.step != const0_rtx
 210           || iv.first_special)
 211         return NULL_RTX;
 212
 213       op[i] = get_iv_value (&iv, const0_rtx);
 214     }
 215
 216   mode = GET_MODE (op[0]);
 217   if (mode == VOIDmode)
 218     mode = GET_MODE (op[1]);
 219   if (GET_MODE_CLASS (mode) == MODE_CC)
 220     {
 221       if (at != BB_END (bb))
 222         return NULL_RTX;
 223
 224       if (!rtx_equal_p (op[0], XEXP (test, 0))
 225           || !rtx_equal_p (op[1], XEXP (test, 1)))
 226         return NULL_RTX;
 227
 228       *cinsn = BB_END (bb);
 229       return test;
 230     }
 231
 232   stest = simplify_gen_relational (GET_CODE (test), SImode,
 233                                    mode, op[0], op[1]);
 234   if (stest == const0_rtx
 235       || stest == const_true_rtx)
 236     return stest;
 237
 238   return canon_condition (gen_rtx_fmt_ee (GET_CODE (test), SImode,
 239                                           op[0], op[1]));
 240 }
 241
 242 /* Reverses CONDition; returns NULL if we cannot.  */
 243 rtx
 244 reversed_condition (rtx cond)
 245 {
 246   enum rtx_code reversed;
 247   reversed = reversed_comparison_code (cond, NULL);
 248   if (reversed == UNKNOWN)
 249     return NULL_RTX;
 250   else
 251     return gen_rtx_fmt_ee (reversed,
 252                            GET_MODE (cond), XEXP (cond, 0),
 253                            XEXP (cond, 1));
 254 }
 255
 256 /* Unswitch single LOOP.  COND_CHECKED holds list of conditions we already
 257    unswitched on and are therefore known to be true in this LOOP.  NUM is
 258    number of unswitchings done; do not allow it to grow too much, it is too
 259    easy to create example on that the code would grow exponentially.  */
 260 static void
 261 unswitch_single_loop (struct loops *loops, struct loop *loop,
 262                       rtx cond_checked, int num)
 263 {
 264   basic_block *bbs;
 265   struct loop *nloop;
 266   unsigned i;
 267   rtx cond, rcond = NULL_RTX, conds, rconds, acond, cinsn;
 268   int repeat;
 269   edge e;
 270
 271   /* Do not unswitch too much.  */
 272   if (num > PARAM_VALUE (PARAM_MAX_UNSWITCH_LEVEL))
 273     {
 274       if (dump_file)
 275         fprintf (dump_file, ";; Not unswitching anymore, hit max level\n");
 276       return;
 277     }
 278
 279   /* Only unswitch innermost loops.  */
 280   if (loop->inner)
 281     {
 282       if (dump_file)
 283         fprintf (dump_file, ";; Not unswitching, not innermost loop\n");
 284       return;
 285     }
 286
 287   /* We must be able to duplicate loop body.  */
 288   if (!can_duplicate_loop_p (loop))
 289     {
 290       if (dump_file)
 291         fprintf (dump_file, ";; Not unswitching, can't duplicate loop\n");
 292       return;
 293     }
 294
 295   /* The loop should not be too large, to limit code growth.  */
 296   if (num_loop_insns (loop) > PARAM_VALUE (PARAM_MAX_UNSWITCH_INSNS))
 297     {
 298       if (dump_file)
 299         fprintf (dump_file, ";; Not unswitching, loop too big\n");
 300       return;
 301     }
 302
 303   /* Do not unswitch in cold areas.  */
 304   if (!maybe_hot_bb_p (loop->header))
 305     {
 306       if (dump_file)
 307         fprintf (dump_file, ";; Not unswitching, not hot area\n");
 308       return;
 309     }
 310
 311   /* Nor if the loop usually does not roll.  */
 312   if (expected_loop_iterations (loop) < 1)
 313     {
 314       if (dump_file)
 315         fprintf (dump_file, ";; Not unswitching, loop iterations < 1\n");
 316       return;
 317     }
 318
 319   do
 320     {
 321       repeat = 0;
 322       cinsn = NULL_RTX;
 323
 324       /* Find a bb to unswitch on.  */
 325       bbs = get_loop_body (loop);
 326       iv_analysis_loop_init (loop);
 327       for (i = 0; i < loop->num_nodes; i++)
 328         if ((cond = may_unswitch_on (bbs[i], loop, &cinsn)))
 329           break;
 330
 331       if (i == loop->num_nodes)
 332         {
 333           free (bbs);
 334           return;
 335         }
 336
 337       if (cond != const0_rtx
 338           && cond != const_true_rtx)
 339         {
 340           rcond = reversed_condition (cond);
 341           if (rcond)
 342             rcond = canon_condition (rcond);
 343
 344           /* Check whether the result can be predicted.  */
 345           for (acond = cond_checked; acond; acond = XEXP (acond, 1))
 346             simplify_using_condition (XEXP (acond, 0), &cond, NULL);
 347         }
 348
 349       if (cond == const_true_rtx)
 350         {
 351           /* Remove false path.  */
 352           e = FALLTHRU_EDGE (bbs[i]);
 353           remove_path (loops, e);
 354           free (bbs);
 355           repeat = 1;
 356         }
 357       else if (cond == const0_rtx)
 358         {
 359           /* Remove true path.  */
 360           e = BRANCH_EDGE (bbs[i]);
 361           remove_path (loops, e);
 362           free (bbs);
 363           repeat = 1;
 364         }
 365     } while (repeat);
 366
 367   /* We found the condition we can unswitch on.  */
 368   conds = alloc_EXPR_LIST (0, cond, cond_checked);
 369   if (rcond)
 370     rconds = alloc_EXPR_LIST (0, rcond, cond_checked);
 371   else
 372     rconds = cond_checked;
 373
 374   if (dump_file)
 375     fprintf (dump_file, ";; Unswitching loop\n");
 376
 377   /* Unswitch the loop on this condition.  */
 378   nloop = unswitch_loop (loops, loop, bbs[i], cond, cinsn);
 379   gcc_assert (nloop);
 380
 381   /* Invoke itself on modified loops.  */
 382   unswitch_single_loop (loops, nloop, rconds, num + 1);
 383   unswitch_single_loop (loops, loop, conds, num + 1);
 384
 385   free_EXPR_LIST_node (conds);
 386   if (rcond)
 387     free_EXPR_LIST_node (rconds);
 388
 389   free (bbs);
 390 }
 391
 392 /* Unswitch a LOOP w.r. to given basic block UNSWITCH_ON.  We only support
 393    unswitching of innermost loops.  UNSWITCH_ON must be executed in every
 394    iteration, i.e. it must dominate LOOP latch.  COND is the condition
 395    determining which loop is entered.  Returns NULL if impossible, new loop
 396    otherwise.  The new loop is entered if COND is true.  If CINSN is not
 397    NULL, it is the insn in that COND is compared.  */
 398
 399 static struct loop *
 400 unswitch_loop (struct loops *loops, struct loop *loop, basic_block unswitch_on,
 401                rtx cond, rtx cinsn)
 402 {
 403   edge entry, latch_edge, true_edge, false_edge, e;
 404   basic_block switch_bb, unswitch_on_alt;
 405   struct loop *nloop;
 406   sbitmap zero_bitmap;
 407   int irred_flag, prob;
 408   rtx seq;
 409
 410   /* Some sanity checking.  */
 411   gcc_assert (flow_bb_inside_loop_p (loop, unswitch_on));
 412   gcc_assert (EDGE_COUNT (unswitch_on->succs) == 2);
 413   gcc_assert (just_once_each_iteration_p (loop, unswitch_on));
 414   gcc_assert (!loop->inner);
 415   gcc_assert (flow_bb_inside_loop_p (loop, EDGE_SUCC (unswitch_on, 0)->dest));
 416   gcc_assert (flow_bb_inside_loop_p (loop, EDGE_SUCC (unswitch_on, 1)->dest));
 417
 418   entry = loop_preheader_edge (loop);
 419
 420   /* Make a copy.  */
 421   irred_flag = entry->flags & EDGE_IRREDUCIBLE_LOOP;
 422   entry->flags &= ~EDGE_IRREDUCIBLE_LOOP;
 423   zero_bitmap = sbitmap_alloc (2);
 424   sbitmap_zero (zero_bitmap);
 425   if (!duplicate_loop_to_header_edge (loop, entry, loops, 1,
 426         zero_bitmap, NULL, NULL, NULL, 0))
 427     {
 428       sbitmap_free (zero_bitmap);
 429       return NULL;
 430     }
 431   sbitmap_free (zero_bitmap);
 432   entry->flags |= irred_flag;
 433
 434   /* Record the block with condition we unswitch on.  */
 435   unswitch_on_alt = get_bb_copy (unswitch_on);
 436   true_edge = BRANCH_EDGE (unswitch_on_alt);
 437   false_edge = FALLTHRU_EDGE (unswitch_on);
 438   latch_edge = single_succ_edge (get_bb_copy (loop->latch));
 439
 440   /* Create a block with the condition.  */
 441   prob = true_edge->probability;
 442   switch_bb = create_empty_bb (EXIT_BLOCK_PTR->prev_bb);
 443   seq = compare_and_jump_seq (XEXP (cond, 0), XEXP (cond, 1), GET_CODE (cond),
 444                               block_label (true_edge->dest),
 445                               prob, cinsn);
 446   emit_insn_after (seq, BB_END (switch_bb));
 447   e = make_edge (switch_bb, true_edge->dest, 0);
 448   e->probability = prob;
 449   e->count = latch_edge->count * prob / REG_BR_PROB_BASE;
 450   e = make_edge (switch_bb, FALLTHRU_EDGE (unswitch_on)->dest, EDGE_FALLTHRU);
 451   e->probability = false_edge->probability;
 452   e->count = latch_edge->count * (false_edge->probability) / REG_BR_PROB_BASE;
 453
 454   if (irred_flag)
 455     {
 456       switch_bb->flags |= BB_IRREDUCIBLE_LOOP;
 457       EDGE_SUCC (switch_bb, 0)->flags |= EDGE_IRREDUCIBLE_LOOP;
 458       EDGE_SUCC (switch_bb, 1)->flags |= EDGE_IRREDUCIBLE_LOOP;
 459     }
 460   else
 461     {
 462       switch_bb->flags &= ~BB_IRREDUCIBLE_LOOP;
 463       EDGE_SUCC (switch_bb, 0)->flags &= ~EDGE_IRREDUCIBLE_LOOP;
 464       EDGE_SUCC (switch_bb, 1)->flags &= ~EDGE_IRREDUCIBLE_LOOP;
 465     }
 466
 467   /* Loopify from the copy of LOOP body, constructing the new loop.  */
 468   nloop = loopify (loops, latch_edge,
 469                    single_pred_edge (get_bb_copy (loop->header)), switch_bb,
 470                    BRANCH_EDGE (switch_bb), FALLTHRU_EDGE (switch_bb), true);
 471
 472   /* Remove branches that are now unreachable in new loops.  */
 473   remove_path (loops, true_edge);
 474   remove_path (loops, false_edge);
 475
 476   /* One of created loops do not have to be subloop of the outer loop now,
 477      so fix its placement in loop data structure.  */
 478   fix_loop_placement (loop);
 479   fix_loop_placement (nloop);
 480
 481   /* Preserve the simple loop preheaders.  */
 482   loop_split_edge_with (loop_preheader_edge (loop), NULL_RTX);
 483   loop_split_edge_with (loop_preheader_edge (nloop), NULL_RTX);
 484
 485   return nloop;
 486 }