gcc/tree-loop-linear.c

   1 /* Linear Loop transforms
   2    Copyright (C) 2003, 2004, 2005 Free Software Foundation, Inc.
   3    Contributed by Daniel Berlin <dberlin@dberlin.org>.
   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
  23 #include "config.h"
  24 #include "system.h"
  25 #include "coretypes.h"
  26 #include "tm.h"
  27 #include "ggc.h"
  28 #include "tree.h"
  29 #include "target.h"
  30
  31 #include "rtl.h"
  32 #include "basic-block.h"
  33 #include "diagnostic.h"
  34 #include "tree-flow.h"
  35 #include "tree-dump.h"
  36 #include "timevar.h"
  37 #include "cfgloop.h"
  38 #include "expr.h"
  39 #include "optabs.h"
  40 #include "tree-chrec.h"
  41 #include "tree-data-ref.h"
  42 #include "tree-scalar-evolution.h"
  43 #include "tree-pass.h"
  44 #include "lambda.h"
  45
  46 /* Linear loop transforms include any composition of interchange,
  47    scaling, skewing, and reversal.  They are used to change the
  48    iteration order of loop nests in order to optimize data locality of
  49    traversals, or remove dependences that prevent
  50    parallelization/vectorization/etc.
  51
  52    TODO: Determine reuse vectors/matrix and use it to determine optimal
  53    transform matrix for locality purposes.
  54    TODO: Completion of partial transforms.  */
  55
  56 /* Gather statistics for loop interchange.  LOOP is the loop being
  57    considered. The first loop in the considered loop nest is
  58    FIRST_LOOP, and consequently, the index of the considered loop is
  59    obtained by LOOP->DEPTH - FIRST_LOOP->DEPTH
  60
  61    Initializes:
  62    - DEPENDENCE_STEPS the sum of all the data dependence distances
  63    carried by loop LOOP,
  64
  65    - NB_DEPS_NOT_CARRIED_BY_LOOP the number of dependence relations
  66    for which the loop LOOP is not carrying any dependence,
  67
  68    - ACCESS_STRIDES the sum of all the strides in LOOP.
  69
  70    Example: for the following loop,
  71
  72    | loop_1 runs 1335 times
  73    |   loop_2 runs 1335 times
  74    |     A[{{0, +, 1}_1, +, 1335}_2]
  75    |     B[{{0, +, 1}_1, +, 1335}_2]
  76    |   endloop_2
  77    |   A[{0, +, 1336}_1]
  78    | endloop_1
  79
  80    gather_interchange_stats (in loop_1) will return
  81    DEPENDENCE_STEPS = 3002
  82    NB_DEPS_NOT_CARRIED_BY_LOOP = 5
  83    ACCESS_STRIDES = 10694
  84
  85    gather_interchange_stats (in loop_2) will return
  86    DEPENDENCE_STEPS = 3000
  87    NB_DEPS_NOT_CARRIED_BY_LOOP = 7
  88    ACCESS_STRIDES = 8010
  89 */
  90
  91 static void
  92 gather_interchange_stats (VEC (ddr_p, heap) *dependence_relations,
  93                           VEC (data_reference_p, heap) *datarefs,
  94                           struct loop *loop,
  95                           struct loop *first_loop,
  96                           unsigned int *dependence_steps,
  97                           unsigned int *nb_deps_not_carried_by_loop,
  98                           unsigned int *access_strides)
  99 {
 100   unsigned int i, j;
 101   struct data_dependence_relation *ddr;
 102   struct data_reference *dr;
 103
 104   *dependence_steps = 0;
 105   *nb_deps_not_carried_by_loop = 0;
 106   *access_strides = 0;
 107
 108   for (i = 0; VEC_iterate (ddr_p, dependence_relations, i, ddr); i++)
 109     {
 110       /* If we don't know anything about this dependence, or the distance
 111          vector is NULL, or there is no dependence, then there is no reuse of
 112          data.  */
 113       if (DDR_ARE_DEPENDENT (ddr) == chrec_dont_know
 114           || DDR_ARE_DEPENDENT (ddr) == chrec_known
 115           || DDR_NUM_DIST_VECTS (ddr) == 0)
 116         continue;
 117
 118       for (j = 0; j < DDR_NUM_DIST_VECTS (ddr); j++)
 119         {
 120           int dist = DDR_DIST_VECT (ddr, j)[loop->depth - first_loop->depth];
 121
 122           if (dist == 0)
 123             (*nb_deps_not_carried_by_loop) += 1;
 124
 125           else if (dist < 0)
 126             (*dependence_steps) += -dist;
 127
 128           else
 129             (*dependence_steps) += dist;
 130         }
 131     }
 132
 133   /* Compute the access strides.  */
 134   for (i = 0; VEC_iterate (data_reference_p, datarefs, i, dr); i++)
 135     {
 136       unsigned int it;
 137       tree stmt = DR_STMT (dr);
 138       struct loop *stmt_loop = loop_containing_stmt (stmt);
 139       struct loop *inner_loop = first_loop->inner;
 140
 141       if (inner_loop != stmt_loop
 142           && !flow_loop_nested_p (inner_loop, stmt_loop))
 143         continue;
 144       for (it = 0; it < DR_NUM_DIMENSIONS (dr); it++)
 145         {
 146           tree chrec = DR_ACCESS_FN (dr, it);
 147           tree tstride = evolution_part_in_loop_num
 148             (chrec, loop->num);
 149
 150           if (tstride == NULL_TREE
 151               || TREE_CODE (tstride) != INTEGER_CST)
 152             continue;
 153
 154           (*access_strides) += int_cst_value (tstride);
 155         }
 156     }
 157 }
 158
 159 /* Attempt to apply interchange transformations to TRANS to maximize the
 160    spatial and temporal locality of the loop.
 161    Returns the new transform matrix.  The smaller the reuse vector
 162    distances in the inner loops, the fewer the cache misses.
 163    FIRST_LOOP is the loop->num of the first loop in the analyzed loop
 164    nest.  */
 165
 166
 167 static lambda_trans_matrix
 168 try_interchange_loops (lambda_trans_matrix trans,
 169                        unsigned int depth,
 170                        VEC (ddr_p, heap) *dependence_relations,
 171                        VEC (data_reference_p, heap) *datarefs,
 172                        struct loop *first_loop)
 173 {
 174   struct loop *loop_i;
 175   struct loop *loop_j;
 176   unsigned int dependence_steps_i, dependence_steps_j;
 177   unsigned int access_strides_i, access_strides_j;
 178   unsigned int nb_deps_not_carried_by_i, nb_deps_not_carried_by_j;
 179   struct data_dependence_relation *ddr;
 180
 181   /* When there is an unknown relation in the dependence_relations, we
 182      know that it is no worth looking at this loop nest: give up.  */
 183   ddr = VEC_index (ddr_p, dependence_relations, 0);
 184   if (ddr == NULL || DDR_ARE_DEPENDENT (ddr) == chrec_dont_know)
 185     return trans;
 186
 187   /* LOOP_I is always the outer loop.  */
 188   for (loop_j = first_loop->inner;
 189        loop_j;
 190        loop_j = loop_j->inner)
 191     for (loop_i = first_loop;
 192          loop_i->depth < loop_j->depth;
 193          loop_i = loop_i->inner)
 194       {
 195         gather_interchange_stats (dependence_relations, datarefs,
 196                                   loop_i, first_loop,
 197                                   &dependence_steps_i,
 198                                   &nb_deps_not_carried_by_i,
 199                                   &access_strides_i);
 200         gather_interchange_stats (dependence_relations, datarefs,
 201                                   loop_j, first_loop,
 202                                   &dependence_steps_j,
 203                                   &nb_deps_not_carried_by_j,
 204                                   &access_strides_j);
 205
 206         /* Heuristics for loop interchange profitability:
 207
 208            1. (spatial locality) Inner loops should have smallest
 209               dependence steps.
 210
 211            2. (spatial locality) Inner loops should contain more
 212            dependence relations not carried by the loop.
 213
 214            3. (temporal locality) Inner loops should have smallest
 215               array access strides.
 216         */
 217         if (dependence_steps_i < dependence_steps_j
 218             || nb_deps_not_carried_by_i > nb_deps_not_carried_by_j
 219             || access_strides_i < access_strides_j)
 220           {
 221             lambda_matrix_row_exchange (LTM_MATRIX (trans),
 222                                         loop_i->depth - first_loop->depth,
 223                                         loop_j->depth - first_loop->depth);
 224             /* Validate the resulting matrix.  When the transformation
 225                is not valid, reverse to the previous transformation.  */
 226             if (!lambda_transform_legal_p (trans, depth, dependence_relations))
 227               lambda_matrix_row_exchange (LTM_MATRIX (trans),
 228                                           loop_i->depth - first_loop->depth,
 229                                           loop_j->depth - first_loop->depth);
 230           }
 231       }
 232
 233   return trans;
 234 }
 235
 236 /* Perform a set of linear transforms on LOOPS.  */
 237
 238 void
 239 linear_transform_loops (struct loops *loops)
 240 {
 241   unsigned int i;
 242   VEC(tree,heap) *oldivs = NULL;
 243   VEC(tree,heap) *invariants = NULL;
 244
 245   for (i = 1; i < loops->num; i++)
 246     {
 247       unsigned int depth = 0;
 248       VEC (ddr_p, heap) *dependence_relations;
 249       VEC (data_reference_p, heap) *datarefs;
 250       struct loop *loop_nest = loops->parray[i];
 251       struct loop *temp;
 252       lambda_loopnest before, after;
 253       lambda_trans_matrix trans;
 254       bool problem = false;
 255       bool need_perfect_nest = false;
 256       /* If it's not a loop nest, we don't want it.
 257          We also don't handle sibling loops properly,
 258          which are loops of the following form:
 259          for (i = 0; i < 50; i++)
 260            {
 261              for (j = 0; j < 50; j++)
 262                {
 263                 ...
 264                }
 265            for (j = 0; j < 50; j++)
 266                {
 267                 ...
 268                }
 269            } */
 270       if (!loop_nest || !loop_nest->inner)
 271         continue;
 272       VEC_truncate (tree, oldivs, 0);
 273       VEC_truncate (tree, invariants, 0);
 274       depth = 1;
 275       for (temp = loop_nest->inner; temp; temp = temp->inner)
 276         {
 277           /* If we have a sibling loop or multiple exit edges, jump ship.  */
 278           if (temp->next || !temp->single_exit)
 279             {
 280               problem = true;
 281               break;
 282             }
 283           depth ++;
 284         }
 285       if (problem)
 286         continue;
 287
 288       /* Analyze data references and dependence relations using scev.  */
 289
 290       datarefs = VEC_alloc (data_reference_p, heap, 10);
 291       dependence_relations = VEC_alloc (ddr_p, heap, 10 * 10);
 292       compute_data_dependences_for_loop (loop_nest, true, &datarefs,
 293                                          &dependence_relations);
 294
 295       if (dump_file && (dump_flags & TDF_DETAILS))
 296         dump_ddrs (dump_file, dependence_relations);
 297
 298       /* Build the transformation matrix.  */
 299       trans = lambda_trans_matrix_new (depth, depth);
 300       lambda_matrix_id (LTM_MATRIX (trans), depth);
 301       trans = try_interchange_loops (trans, depth, dependence_relations,
 302                                      datarefs, loop_nest);
 303
 304       if (lambda_trans_matrix_id_p (trans))
 305         {
 306           if (dump_file)
 307            fprintf (dump_file, "Won't transform loop. Optimal transform is the identity transform\n");
 308           continue;
 309         }
 310
 311       /* Check whether the transformation is legal.  */
 312       if (!lambda_transform_legal_p (trans, depth, dependence_relations))
 313         {
 314           if (dump_file)
 315             fprintf (dump_file, "Can't transform loop, transform is illegal:\n");
 316           continue;
 317         }
 318
 319       if (!perfect_nest_p (loop_nest))
 320         need_perfect_nest = true;
 321
 322       before = gcc_loopnest_to_lambda_loopnest (loops,
 323                                                 loop_nest, &oldivs,
 324                                                 &invariants,
 325                                                 need_perfect_nest);
 326       if (!before)
 327         continue;
 328
 329       if (dump_file)
 330         {
 331           fprintf (dump_file, "Before:\n");
 332           print_lambda_loopnest (dump_file, before, 'i');
 333         }
 334
 335       after = lambda_loopnest_transform (before, trans);
 336
 337       if (dump_file)
 338         {
 339           fprintf (dump_file, "After:\n");
 340           print_lambda_loopnest (dump_file, after, 'u');
 341         }
 342
 343       lambda_loopnest_to_gcc_loopnest (loop_nest, oldivs, invariants,
 344                                        after, trans);
 345
 346       if (dump_file)
 347         fprintf (dump_file, "Successfully transformed loop.\n");
 348
 349       free_dependence_relations (dependence_relations);
 350       free_data_refs (datarefs);
 351     }
 352
 353   VEC_free (tree, heap, oldivs);
 354   VEC_free (tree, heap, invariants);
 355   scev_reset ();
 356   rewrite_into_loop_closed_ssa (NULL, TODO_update_ssa_full_phi);
 357 }