gcc/tracer.c

   1 /* The tracer pass for the GNU compiler.
   2    Contributed by Jan Hubicka, SuSE Labs.
   3    Adapted to work on GIMPLE instead of RTL by Robert Kidd, UIUC.
   4    Copyright (C) 2001-2020 Free Software Foundation, Inc.
   5
   6    This file is part of GCC.
   7
   8    GCC is free software; you can redistribute it and/or modify it
   9    under the terms of the GNU General Public License as published by
  10    the Free Software Foundation; either version 3, or (at your option)
  11    any later version.
  12
  13    GCC is distributed in the hope that it will be useful, but WITHOUT
  14    ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  15    or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public
  16    License for more details.
  17
  18    You should have received a copy of the GNU General Public License
  19    along with GCC; see the file COPYING3.  If not see
  20    <http://www.gnu.org/licenses/>.  */
  21
  22 /* This pass performs the tail duplication needed for superblock formation.
  23    For more information see:
  24
  25      Design and Analysis of Profile-Based Optimization in Compaq's
  26      Compilation Tools for Alpha; Journal of Instruction-Level
  27      Parallelism 3 (2000) 1-25
  28
  29    Unlike Compaq's implementation we don't do the loop peeling as most
  30    probably a better job can be done by a special pass and we don't
  31    need to worry too much about the code size implications as the tail
  32    duplicates are crossjumped again if optimizations are not
  33    performed.  */
  34
  35
  36 #include "config.h"
  37 #include "system.h"
  38 #include "coretypes.h"
  39 #include "backend.h"
  40 #include "rtl.h"
  41 #include "tree.h"
  42 #include "gimple.h"
  43 #include "cfghooks.h"
  44 #include "tree-pass.h"
  45 #include "profile.h"
  46 #include "cfganal.h"
  47 #include "gimple-iterator.h"
  48 #include "tree-cfg.h"
  49 #include "tree-ssa.h"
  50 #include "tree-inline.h"
  51 #include "cfgloop.h"
  52 #include "alloc-pool.h"
  53 #include "fibonacci_heap.h"
  54 #include "tracer.h"
  55
  56 static int count_insns (basic_block);
  57 static bool better_p (const_edge, const_edge);
  58 static edge find_best_successor (basic_block);
  59 static edge find_best_predecessor (basic_block);
  60 static int find_trace (basic_block, basic_block *);
  61
  62 /* Minimal outgoing edge probability considered for superblock formation.  */
  63 static int probability_cutoff;
  64 static int branch_ratio_cutoff;
  65
  66 /* A bit BB->index is set if BB has already been seen, i.e. it is
  67    connected to some trace already.  */
  68 static sbitmap bb_seen;
  69
  70 static inline void
  71 mark_bb_seen (basic_block bb)
  72 {
  73   unsigned int size = SBITMAP_SIZE (bb_seen);
  74
  75   if ((unsigned int)bb->index >= size)
  76     bb_seen = sbitmap_resize (bb_seen, size * 2, 0);
  77
  78   bitmap_set_bit (bb_seen, bb->index);
  79 }
  80
  81 static inline bool
  82 bb_seen_p (basic_block bb)
  83 {
  84   return bitmap_bit_p (bb_seen, bb->index);
  85 }
  86
  87 /* Return true if we should ignore the basic block for purposes of tracing.  */
  88 bool
  89 ignore_bb_p (const_basic_block bb)
  90 {
  91   if (bb->index < NUM_FIXED_BLOCKS)
  92     return true;
  93   if (optimize_bb_for_size_p (bb))
  94     return true;
  95
  96   if (gimple *g = last_stmt (CONST_CAST_BB (bb)))
  97     {
  98       /* A transaction is a single entry multiple exit region.  It
  99          must be duplicated in its entirety or not at all.  */
 100       if (gimple_code (g) == GIMPLE_TRANSACTION)
 101         return true;
 102
 103       /* An IFN_UNIQUE call must be duplicated as part of its group,
 104          or not at all.  */
 105       if (is_gimple_call (g)
 106           && gimple_call_internal_p (g)
 107           && gimple_call_internal_unique_p (g))
 108         return true;
 109     }
 110
 111   return false;
 112 }
 113
 114 /* Return number of instructions in the block.  */
 115
 116 static int
 117 count_insns (basic_block bb)
 118 {
 119   gimple_stmt_iterator gsi;
 120   gimple *stmt;
 121   int n = 0;
 122
 123   for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 124     {
 125       stmt = gsi_stmt (gsi);
 126       n += estimate_num_insns (stmt, &eni_size_weights);
 127     }
 128   return n;
 129 }
 130
 131 /* Return true if E1 is more frequent than E2.  */
 132 static bool
 133 better_p (const_edge e1, const_edge e2)
 134 {
 135   if ((e1->count () > e2->count ()) || (e1->count () < e2->count ()))
 136     return e1->count () > e2->count ();
 137   /* This is needed to avoid changes in the decision after
 138      CFG is modified.  */
 139   if (e1->src != e2->src)
 140     return e1->src->index > e2->src->index;
 141   return e1->dest->index > e2->dest->index;
 142 }
 143
 144 /* Return most frequent successor of basic block BB.  */
 145
 146 static edge
 147 find_best_successor (basic_block bb)
 148 {
 149   edge e;
 150   edge best = NULL;
 151   edge_iterator ei;
 152
 153   FOR_EACH_EDGE (e, ei, bb->succs)
 154     {
 155       if (!e->count ().initialized_p ())
 156         return NULL;
 157       if (!best || better_p (e, best))
 158         best = e;
 159     }
 160   if (!best || ignore_bb_p (best->dest))
 161     return NULL;
 162   if (!best->probability.initialized_p ()
 163       || best->probability.to_reg_br_prob_base () <= probability_cutoff)
 164     return NULL;
 165   return best;
 166 }
 167
 168 /* Return most frequent predecessor of basic block BB.  */
 169
 170 static edge
 171 find_best_predecessor (basic_block bb)
 172 {
 173   edge e;
 174   edge best = NULL;
 175   edge_iterator ei;
 176
 177   FOR_EACH_EDGE (e, ei, bb->preds)
 178     {
 179       if (!e->count ().initialized_p ())
 180         return NULL;
 181       if (!best || better_p (e, best))
 182         best = e;
 183     }
 184   if (!best || ignore_bb_p (best->src))
 185     return NULL;
 186   if (bb->count.initialized_p ()
 187       && (best->count ().to_frequency (cfun) * REG_BR_PROB_BASE
 188           < bb->count.to_frequency (cfun) * branch_ratio_cutoff))
 189     return NULL;
 190   return best;
 191 }
 192
 193 /* Find the trace using bb and record it in the TRACE array.
 194    Return number of basic blocks recorded.  */
 195
 196 static int
 197 find_trace (basic_block bb, basic_block *trace)
 198 {
 199   int i = 0;
 200   edge e;
 201
 202   if (dump_file)
 203     fprintf (dump_file, "Trace seed %i [%i]", bb->index, bb->count.to_frequency (cfun));
 204
 205   while ((e = find_best_predecessor (bb)) != NULL)
 206     {
 207       basic_block bb2 = e->src;
 208       if (bb_seen_p (bb2) || (e->flags & (EDGE_DFS_BACK | EDGE_COMPLEX))
 209           || find_best_successor (bb2) != e)
 210         break;
 211       if (dump_file)
 212         fprintf (dump_file, ",%i [%i]", bb->index, bb->count.to_frequency (cfun));
 213       bb = bb2;
 214     }
 215   if (dump_file)
 216     fprintf (dump_file, " forward %i [%i]", bb->index, bb->count.to_frequency (cfun));
 217   trace[i++] = bb;
 218
 219   /* Follow the trace in forward direction.  */
 220   while ((e = find_best_successor (bb)) != NULL)
 221     {
 222       bb = e->dest;
 223       if (bb_seen_p (bb) || (e->flags & (EDGE_DFS_BACK | EDGE_COMPLEX))
 224           || find_best_predecessor (bb) != e)
 225         break;
 226       if (dump_file)
 227         fprintf (dump_file, ",%i [%i]", bb->index, bb->count.to_frequency (cfun));
 228       trace[i++] = bb;
 229     }
 230   if (dump_file)
 231     fprintf (dump_file, "\n");
 232   return i;
 233 }
 234
 235 /* Duplicate block BB2, placing it after BB in the CFG.  Return the
 236    newly created block.  */
 237 basic_block
 238 transform_duplicate (basic_block bb, basic_block bb2)
 239 {
 240   edge e;
 241   basic_block copy;
 242
 243   e = find_edge (bb, bb2);
 244
 245   copy = duplicate_block (bb2, e, bb);
 246   flush_pending_stmts (e);
 247
 248   add_phi_args_after_copy (&copy, 1, NULL);
 249
 250   return (copy);
 251 }
 252
 253 /* Look for basic blocks in frequency order, construct traces and tail duplicate
 254    if profitable.  */
 255
 256 static bool
 257 tail_duplicate (void)
 258 {
 259   auto_vec<fibonacci_node<long, basic_block_def>*> blocks;
 260   blocks.safe_grow_cleared (last_basic_block_for_fn (cfun), true);
 261
 262   basic_block *trace = XNEWVEC (basic_block, n_basic_blocks_for_fn (cfun));
 263   int *counts = XNEWVEC (int, last_basic_block_for_fn (cfun));
 264   int ninsns = 0, nduplicated = 0;
 265   gcov_type weighted_insns = 0, traced_insns = 0;
 266   fibonacci_heap<long, basic_block_def> heap (LONG_MIN);
 267   gcov_type cover_insns;
 268   int max_dup_insns;
 269   basic_block bb;
 270   bool changed = false;
 271
 272   /* Create an oversized sbitmap to reduce the chance that we need to
 273      resize it.  */
 274   bb_seen = sbitmap_alloc (last_basic_block_for_fn (cfun) * 2);
 275   bitmap_clear (bb_seen);
 276   initialize_original_copy_tables ();
 277
 278   if (profile_info && profile_status_for_fn (cfun) == PROFILE_READ)
 279     probability_cutoff = param_tracer_min_branch_probability_feedback;
 280   else
 281     probability_cutoff = param_tracer_min_branch_probability;
 282   probability_cutoff = REG_BR_PROB_BASE / 100 * probability_cutoff;
 283
 284   branch_ratio_cutoff =
 285     (REG_BR_PROB_BASE / 100 * param_tracer_min_branch_ratio);
 286
 287   FOR_EACH_BB_FN (bb, cfun)
 288     {
 289       int n = count_insns (bb);
 290       if (!ignore_bb_p (bb))
 291         blocks[bb->index] = heap.insert (-bb->count.to_frequency (cfun), bb);
 292
 293       counts [bb->index] = n;
 294       ninsns += n;
 295       weighted_insns += n * bb->count.to_frequency (cfun);
 296     }
 297
 298   if (profile_info && profile_status_for_fn (cfun) == PROFILE_READ)
 299     cover_insns = param_tracer_dynamic_coverage_feedback;
 300   else
 301     cover_insns = param_tracer_dynamic_coverage;
 302   cover_insns = (weighted_insns * cover_insns + 50) / 100;
 303   max_dup_insns = (ninsns * param_tracer_max_code_growth + 50) / 100;
 304
 305   while (traced_insns < cover_insns && nduplicated < max_dup_insns
 306          && !heap.empty ())
 307     {
 308       basic_block bb = heap.extract_min ();
 309       int n, pos;
 310
 311       if (!bb)
 312         break;
 313
 314       blocks[bb->index] = NULL;
 315
 316       if (ignore_bb_p (bb))
 317         continue;
 318       gcc_assert (!bb_seen_p (bb));
 319
 320       n = find_trace (bb, trace);
 321
 322       bb = trace[0];
 323       traced_insns += bb->count.to_frequency (cfun) * counts [bb->index];
 324       if (blocks[bb->index])
 325         {
 326           heap.delete_node (blocks[bb->index]);
 327           blocks[bb->index] = NULL;
 328         }
 329
 330       for (pos = 1; pos < n; pos++)
 331         {
 332           basic_block bb2 = trace[pos];
 333
 334           if (blocks[bb2->index])
 335             {
 336               heap.delete_node (blocks[bb2->index]);
 337               blocks[bb2->index] = NULL;
 338             }
 339           traced_insns += bb2->count.to_frequency (cfun) * counts [bb2->index];
 340           if (EDGE_COUNT (bb2->preds) > 1
 341               && can_duplicate_block_p (bb2)
 342               /* We have the tendency to duplicate the loop header
 343                  of all do { } while loops.  Do not do that - it is
 344                  not profitable and it might create a loop with multiple
 345                  entries or at least rotate the loop.  */
 346               && bb2->loop_father->header != bb2)
 347             {
 348               nduplicated += counts [bb2->index];
 349               basic_block copy = transform_duplicate (bb, bb2);
 350
 351               /* Reconsider the original copy of block we've duplicated.
 352                  Removing the most common predecessor may make it to be
 353                  head.  */
 354               blocks[bb2->index] = heap.insert (-bb2->count.to_frequency (cfun), bb2);
 355
 356               if (dump_file)
 357                 fprintf (dump_file, "Duplicated %i as %i [%i]\n",
 358                          bb2->index, copy->index, copy->count.to_frequency (cfun));
 359
 360               bb2 = copy;
 361               changed = true;
 362             }
 363           mark_bb_seen (bb2);
 364           bb = bb2;
 365           /* In case the trace became infrequent, stop duplicating.  */
 366           if (ignore_bb_p (bb))
 367             break;
 368         }
 369       if (dump_file)
 370         fprintf (dump_file, " covered now %.1f\n\n",
 371                  traced_insns * 100.0 / weighted_insns);
 372     }
 373   if (dump_file)
 374     fprintf (dump_file, "Duplicated %i insns (%i%%)\n", nduplicated,
 375              nduplicated * 100 / ninsns);
 376
 377   free_original_copy_tables ();
 378   sbitmap_free (bb_seen);
 379   free (trace);
 380   free (counts);
 381
 382   return changed;
 383 }
 384 \f
 385 namespace {
 386
 387 const pass_data pass_data_tracer =
 388 {
 389   GIMPLE_PASS, /* type */
 390   "tracer", /* name */
 391   OPTGROUP_NONE, /* optinfo_flags */
 392   TV_TRACER, /* tv_id */
 393   0, /* properties_required */
 394   0, /* properties_provided */
 395   0, /* properties_destroyed */
 396   0, /* todo_flags_start */
 397   TODO_update_ssa, /* todo_flags_finish */
 398 };
 399
 400 class pass_tracer : public gimple_opt_pass
 401 {
 402 public:
 403   pass_tracer (gcc::context *ctxt)
 404     : gimple_opt_pass (pass_data_tracer, ctxt)
 405   {}
 406
 407   /* opt_pass methods: */
 408   virtual bool gate (function *)
 409     {
 410       return (optimize > 0 && flag_tracer && flag_reorder_blocks);
 411     }
 412
 413   virtual unsigned int execute (function *);
 414
 415 }; // class pass_tracer
 416
 417 unsigned int
 418 pass_tracer::execute (function *fun)
 419 {
 420   bool changed;
 421
 422   if (n_basic_blocks_for_fn (fun) <= NUM_FIXED_BLOCKS + 1)
 423     return 0;
 424
 425   mark_dfs_back_edges ();
 426   if (dump_file)
 427     brief_dump_cfg (dump_file, dump_flags);
 428
 429   /* Trace formation is done on the fly inside tail_duplicate */
 430   changed = tail_duplicate ();
 431   if (changed)
 432     {
 433       free_dominance_info (CDI_DOMINATORS);
 434       /* If we changed the CFG schedule loops for fixup by cleanup_cfg.  */
 435       loops_state_set (LOOPS_NEED_FIXUP);
 436     }
 437
 438   if (dump_file)
 439     brief_dump_cfg (dump_file, dump_flags);
 440
 441   return changed ? TODO_cleanup_cfg : 0;
 442 }
 443 } // anon namespace
 444
 445 gimple_opt_pass *
 446 make_pass_tracer (gcc::context *ctxt)
 447 {
 448   return new pass_tracer (ctxt);
 449 }