gcc/loop-unswitch.c

   1 /* Loop unswitching for GNU compiler.
   2    Copyright (C) 2002, 2003, 2004 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 2, 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 COPYING.  If not, write to the Free
  18 Software Foundation, 59 Temple Place - Suite 330, Boston, MA
  19 02111-1307, USA.  */
  20
  21 #include "config.h"
  22 #include "system.h"
  23 #include "coretypes.h"
  24 #include "tm.h"
  25 #include "rtl.h"
  26 #include "hard-reg-set.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       if (!cinsn)
 107         abort ();
 108       cond = XEXP (SET_SRC (pc_set (cinsn)), 0);
 109       if (GET_CODE (cond) != comp
 110           || !rtx_equal_p (op0, XEXP (cond, 0))
 111           || !rtx_equal_p (op1, XEXP (cond, 1)))
 112         abort ();
 113       emit_jump_insn (copy_insn (PATTERN (cinsn)));
 114       jump = get_last_insn ();
 115       JUMP_LABEL (jump) = JUMP_LABEL (cinsn);
 116       LABEL_NUSES (JUMP_LABEL (jump))++;
 117       redirect_jump (jump, label, 0);
 118     }
 119   else
 120     {
 121       if (cinsn)
 122         abort ();
 123
 124       op0 = force_operand (op0, NULL_RTX);
 125       op1 = force_operand (op1, NULL_RTX);
 126       do_compare_rtx_and_jump (op0, op1, comp, 0,
 127                                mode, NULL_RTX, NULL_RTX, label);
 128       jump = get_last_insn ();
 129       JUMP_LABEL (jump) = label;
 130       LABEL_NUSES (label)++;
 131     }
 132   REG_NOTES (jump) = gen_rtx_EXPR_LIST (REG_BR_PROB, GEN_INT (prob),
 133                                         REG_NOTES (jump));
 134   seq = get_insns ();
 135   end_sequence ();
 136
 137   return seq;
 138 }
 139
 140 /* Main entry point.  Perform loop unswitching on all suitable LOOPS.  */
 141 void
 142 unswitch_loops (struct loops *loops)
 143 {
 144   int i, num;
 145   struct loop *loop;
 146
 147   /* Go through inner loops (only original ones).  */
 148   num = loops->num;
 149
 150   for (i = 1; i < num; i++)
 151     {
 152       /* Removed loop?  */
 153       loop = loops->parray[i];
 154       if (!loop)
 155         continue;
 156
 157       if (loop->inner)
 158         continue;
 159
 160       unswitch_single_loop (loops, loop, NULL_RTX, 0);
 161 #ifdef ENABLE_CHECKING
 162       verify_dominators (CDI_DOMINATORS);
 163       verify_loop_structure (loops);
 164 #endif
 165     }
 166
 167   iv_analysis_done ();
 168 }
 169
 170 /* Checks whether we can unswitch LOOP on condition at end of BB -- one of its
 171    basic blocks (for what it means see comments below).  In case condition
 172    compares loop invariant cc mode register, return the jump in CINSN.  */
 173
 174 static rtx
 175 may_unswitch_on (basic_block bb, struct loop *loop, rtx *cinsn)
 176 {
 177   rtx test, at, insn, op[2], stest;
 178   struct rtx_iv iv;
 179   unsigned i;
 180   enum machine_mode mode;
 181
 182   /* BB must end in a simple conditional jump.  */
 183   if (EDGE_COUNT (bb->succs) != 2)
 184     return NULL_RTX;
 185   if (!any_condjump_p (BB_END (bb)))
 186     return NULL_RTX;
 187
 188   /* With branches inside loop.  */
 189   if (!flow_bb_inside_loop_p (loop, EDGE_SUCC (bb, 0)->dest)
 190       || !flow_bb_inside_loop_p (loop, EDGE_SUCC (bb, 1)->dest))
 191     return NULL_RTX;
 192
 193   /* It must be executed just once each iteration (because otherwise we
 194      are unable to update dominator/irreducible loop information correctly).  */
 195   if (!just_once_each_iteration_p (loop, bb))
 196     return NULL_RTX;
 197
 198   /* Condition must be invariant.  */
 199   test = get_condition (BB_END (bb), &at, true, false);
 200   if (!test)
 201     return NULL_RTX;
 202
 203   for (i = 0; i < 2; i++)
 204     {
 205       op[i] = XEXP (test, i);
 206
 207       if (CONSTANT_P (op[i]))
 208         continue;
 209
 210       insn = iv_get_reaching_def (at, op[i]);
 211       if (!iv_analyze (insn, op[i], &iv))
 212         return NULL_RTX;
 213       if (iv.step != const0_rtx
 214           || iv.first_special)
 215         return NULL_RTX;
 216
 217       op[i] = get_iv_value (&iv, const0_rtx);
 218     }
 219
 220   mode = GET_MODE (op[0]);
 221   if (mode == VOIDmode)
 222     mode = GET_MODE (op[1]);
 223   if (GET_MODE_CLASS (mode) == MODE_CC)
 224     {
 225       if (at != BB_END (bb))
 226         return NULL_RTX;
 227
 228       *cinsn = BB_END (bb);
 229       if (!rtx_equal_p (op[0], XEXP (test, 0))
 230           || !rtx_equal_p (op[1], XEXP (test, 1)))
 231         return NULL_RTX;
 232
 233       return test;
 234     }
 235
 236   stest = simplify_gen_relational (GET_CODE (test), SImode,
 237                                    mode, op[0], op[1]);
 238   if (stest == const0_rtx
 239       || stest == const_true_rtx)
 240     return stest;
 241
 242   return canon_condition (gen_rtx_fmt_ee (GET_CODE (test), SImode,
 243                                           op[0], op[1]));
 244 }
 245
 246 /* Reverses CONDition; returns NULL if we cannot.  */
 247 rtx
 248 reversed_condition (rtx cond)
 249 {
 250   enum rtx_code reversed;
 251   reversed = reversed_comparison_code (cond, NULL);
 252   if (reversed == UNKNOWN)
 253     return NULL_RTX;
 254   else
 255     return gen_rtx_fmt_ee (reversed,
 256                            GET_MODE (cond), XEXP (cond, 0),
 257                            XEXP (cond, 1));
 258 }
 259
 260 /* Unswitch single LOOP.  COND_CHECKED holds list of conditions we already
 261    unswitched on and are therefore known to be true in this LOOP.  NUM is
 262    number of unswitchings done; do not allow it to grow too much, it is too
 263    easy to create example on that the code would grow exponentially.  */
 264 static void
 265 unswitch_single_loop (struct loops *loops, struct loop *loop,
 266                       rtx cond_checked, int num)
 267 {
 268   basic_block *bbs;
 269   struct loop *nloop;
 270   unsigned i;
 271   rtx cond, rcond = NULL_RTX, conds, rconds, acond, cinsn = NULL_RTX;
 272   int repeat;
 273   edge e;
 274
 275   /* Do not unswitch too much.  */
 276   if (num > PARAM_VALUE (PARAM_MAX_UNSWITCH_LEVEL))
 277     {
 278       if (dump_file)
 279         fprintf (dump_file, ";; Not unswitching anymore, hit max level\n");
 280       return;
 281     }
 282
 283   /* Only unswitch innermost loops.  */
 284   if (loop->inner)
 285     {
 286       if (dump_file)
 287         fprintf (dump_file, ";; Not unswitching, not innermost loop\n");
 288       return;
 289     }
 290
 291   /* We must be able to duplicate loop body.  */
 292   if (!can_duplicate_loop_p (loop))
 293     {
 294       if (dump_file)
 295         fprintf (dump_file, ";; Not unswitching, can't duplicate loop\n");
 296       return;
 297     }
 298
 299   /* The loop should not be too large, to limit code growth.  */
 300   if (num_loop_insns (loop) > PARAM_VALUE (PARAM_MAX_UNSWITCH_INSNS))
 301     {
 302       if (dump_file)
 303         fprintf (dump_file, ";; Not unswitching, loop too big\n");
 304       return;
 305     }
 306
 307   /* Do not unswitch in cold areas.  */
 308   if (!maybe_hot_bb_p (loop->header))
 309     {
 310       if (dump_file)
 311         fprintf (dump_file, ";; Not unswitching, not hot area\n");
 312       return;
 313     }
 314
 315   /* Nor if the loop usually does not roll.  */
 316   if (expected_loop_iterations (loop) < 1)
 317     {
 318       if (dump_file)
 319         fprintf (dump_file, ";; Not unswitching, loop iterations < 1\n");
 320       return;
 321     }
 322
 323   do
 324     {
 325       repeat = 0;
 326
 327       /* Find a bb to unswitch on.  */
 328       bbs = get_loop_body (loop);
 329       iv_analysis_loop_init (loop);
 330       for (i = 0; i < loop->num_nodes; i++)
 331         if ((cond = may_unswitch_on (bbs[i], loop, &cinsn)))
 332           break;
 333
 334       if (i == loop->num_nodes)
 335         {
 336           free (bbs);
 337           return;
 338         }
 339
 340       if (cond != const0_rtx
 341           && cond != const_true_rtx)
 342         {
 343           rcond = reversed_condition (cond);
 344           if (rcond)
 345             rcond = canon_condition (rcond);
 346
 347           /* Check whether the result can be predicted.  */
 348           for (acond = cond_checked; acond; acond = XEXP (acond, 1))
 349             simplify_using_condition (XEXP (acond, 0), &cond, NULL);
 350         }
 351
 352       if (cond == const_true_rtx)
 353         {
 354           /* Remove false path.  */
 355           e = FALLTHRU_EDGE (bbs[i]);
 356           remove_path (loops, e);
 357           free (bbs);
 358           repeat = 1;
 359         }
 360       else if (cond == const0_rtx)
 361         {
 362           /* Remove true path.  */
 363           e = BRANCH_EDGE (bbs[i]);
 364           remove_path (loops, e);
 365           free (bbs);
 366           repeat = 1;
 367         }
 368     } while (repeat);
 369
 370   /* We found the condition we can unswitch on.  */
 371   conds = alloc_EXPR_LIST (0, cond, cond_checked);
 372   if (rcond)
 373     rconds = alloc_EXPR_LIST (0, rcond, cond_checked);
 374   else
 375     rconds = cond_checked;
 376
 377   if (dump_file)
 378     fprintf (dump_file, ";; Unswitching loop\n");
 379
 380   /* Unswitch the loop on this condition.  */
 381   nloop = unswitch_loop (loops, loop, bbs[i], cond, cinsn);
 382   if (!nloop)
 383   abort ();
 384
 385   /* Invoke itself on modified loops.  */
 386   unswitch_single_loop (loops, nloop, rconds, num + 1);
 387   unswitch_single_loop (loops, loop, conds, num + 1);
 388
 389   free_EXPR_LIST_node (conds);
 390   if (rcond)
 391     free_EXPR_LIST_node (rconds);
 392
 393   free (bbs);
 394 }
 395
 396 /* Unswitch a LOOP w.r. to given basic block UNSWITCH_ON.  We only support
 397    unswitching of innermost loops.  UNSWITCH_ON must be executed in every
 398    iteration, i.e. it must dominate LOOP latch.  COND is the condition
 399    determining which loop is entered.  Returns NULL if impossible, new loop
 400    otherwise.  The new loop is entered if COND is true.  If CINSN is not
 401    NULL, it is the insn in that COND is compared.  */
 402
 403 static struct loop *
 404 unswitch_loop (struct loops *loops, struct loop *loop, basic_block unswitch_on,
 405                rtx cond, rtx cinsn)
 406 {
 407   edge entry, latch_edge, true_edge, false_edge, e;
 408   basic_block switch_bb, unswitch_on_alt, src;
 409   struct loop *nloop;
 410   sbitmap zero_bitmap;
 411   int irred_flag, prob;
 412   rtx seq;
 413
 414   /* Some sanity checking.  */
 415   if (!flow_bb_inside_loop_p (loop, unswitch_on))
 416     abort ();
 417   if (EDGE_COUNT (unswitch_on->succs) != 2)
 418     abort ();
 419   if (!just_once_each_iteration_p (loop, unswitch_on))
 420     abort ();
 421   if (loop->inner)
 422     abort ();
 423   if (!flow_bb_inside_loop_p (loop, EDGE_SUCC (unswitch_on, 0)->dest))
 424     abort ();
 425   if (!flow_bb_inside_loop_p (loop, EDGE_SUCC (unswitch_on, 1)->dest))
 426     abort ();
 427
 428   entry = loop_preheader_edge (loop);
 429
 430   /* Make a copy.  */
 431   src = entry->src;
 432   irred_flag = entry->flags & EDGE_IRREDUCIBLE_LOOP;
 433   entry->flags &= ~EDGE_IRREDUCIBLE_LOOP;
 434   zero_bitmap = sbitmap_alloc (2);
 435   sbitmap_zero (zero_bitmap);
 436   if (!duplicate_loop_to_header_edge (loop, entry, loops, 1,
 437         zero_bitmap, NULL, NULL, NULL, 0))
 438     return NULL;
 439   free (zero_bitmap);
 440   entry->flags |= irred_flag;
 441
 442   /* Record the block with condition we unswitch on.  */
 443   unswitch_on_alt = unswitch_on->rbi->copy;
 444   true_edge = BRANCH_EDGE (unswitch_on_alt);
 445   false_edge = FALLTHRU_EDGE (unswitch_on);
 446   latch_edge = EDGE_SUCC (loop->latch->rbi->copy, 0);
 447
 448   /* Create a block with the condition.  */
 449   prob = true_edge->probability;
 450   switch_bb = create_empty_bb (EXIT_BLOCK_PTR->prev_bb);
 451   seq = compare_and_jump_seq (XEXP (cond, 0), XEXP (cond, 1), GET_CODE (cond),
 452                               block_label (true_edge->dest),
 453                               prob, cinsn);
 454   emit_insn_after (seq, BB_END (switch_bb));
 455   e = make_edge (switch_bb, true_edge->dest, 0);
 456   e->probability = prob;
 457   e->count = latch_edge->count * prob / REG_BR_PROB_BASE;
 458   e = make_edge (switch_bb, FALLTHRU_EDGE (unswitch_on)->dest, EDGE_FALLTHRU);
 459   e->probability = false_edge->probability;
 460   e->count = latch_edge->count * (false_edge->probability) / REG_BR_PROB_BASE;
 461
 462   if (irred_flag)
 463     {
 464       switch_bb->flags |= BB_IRREDUCIBLE_LOOP;
 465       EDGE_SUCC (switch_bb, 0)->flags |= EDGE_IRREDUCIBLE_LOOP;
 466       EDGE_SUCC (switch_bb, 1)->flags |= EDGE_IRREDUCIBLE_LOOP;
 467     }
 468   else
 469     {
 470       switch_bb->flags &= ~BB_IRREDUCIBLE_LOOP;
 471       EDGE_SUCC (switch_bb, 0)->flags &= ~EDGE_IRREDUCIBLE_LOOP;
 472       EDGE_SUCC (switch_bb, 1)->flags &= ~EDGE_IRREDUCIBLE_LOOP;
 473     }
 474
 475   /* Loopify from the copy of LOOP body, constructing the new loop.  */
 476   nloop = loopify (loops, latch_edge,
 477                    EDGE_PRED (loop->header->rbi->copy, 0), switch_bb,
 478                    BRANCH_EDGE (switch_bb), FALLTHRU_EDGE (switch_bb), true);
 479
 480   /* Remove branches that are now unreachable in new loops.  */
 481   remove_path (loops, true_edge);
 482   remove_path (loops, false_edge);
 483
 484   /* One of created loops do not have to be subloop of the outer loop now,
 485      so fix its placement in loop data structure.  */
 486   fix_loop_placement (loop);
 487   fix_loop_placement (nloop);
 488
 489   /* Preserve the simple loop preheaders.  */
 490   loop_split_edge_with (loop_preheader_edge (loop), NULL_RTX);
 491   loop_split_edge_with (loop_preheader_edge (nloop), NULL_RTX);
 492
 493   return nloop;
 494 }