gcc/tree-ssa-phiprop.c

   1 /* Backward propagation of indirect loads through PHIs.
   2    Copyright (C) 2007-2015 Free Software Foundation, Inc.
   3    Contributed by Richard Guenther <rguenther@suse.de>
   4
   5 This file is part of GCC.
   6
   7 GCC is free software; you can redistribute it and/or modify
   8 it under the terms of the GNU General Public License as published by
   9 the Free Software Foundation; either version 3, or (at your option)
  10 any later version.
  11
  12 GCC is distributed in the hope that it will be useful,
  13 but WITHOUT ANY WARRANTY; without even the implied warranty of
  14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15 GNU General Public License for more details.
  16
  17 You should have received a copy of the GNU General Public License
  18 along with GCC; see the file COPYING3.  If not see
  19 <http://www.gnu.org/licenses/>.  */
  20
  21 #include "config.h"
  22 #include "system.h"
  23 #include "coretypes.h"
  24 #include "backend.h"
  25 #include "tree.h"
  26 #include "gimple.h"
  27 #include "hard-reg-set.h"
  28 #include "ssa.h"
  29 #include "alias.h"
  30 #include "fold-const.h"
  31 #include "tm_p.h"
  32 #include "gimple-pretty-print.h"
  33 #include "internal-fn.h"
  34 #include "tree-eh.h"
  35 #include "gimplify.h"
  36 #include "gimple-iterator.h"
  37 #include "tree-pass.h"
  38 #include "langhooks.h"
  39 #include "flags.h"
  40
  41 /* This pass propagates indirect loads through the PHI node for its
  42    address to make the load source possibly non-addressable and to
  43    allow for PHI optimization to trigger.
  44
  45    For example the pass changes
  46
  47      # addr_1 = PHI <&a, &b>
  48      tmp_1 = *addr_1;
  49
  50    to
  51
  52      # tmp_1 = PHI <a, b>
  53
  54    but also handles more complex scenarios like
  55
  56      D.2077_2 = &this_1(D)->a1;
  57      ...
  58
  59      # b_12 = PHI <&c(2), D.2077_2(3)>
  60      D.2114_13 = *b_12;
  61      ...
  62
  63      # b_15 = PHI <b_12(4), &b(5)>
  64      D.2080_5 = &this_1(D)->a0;
  65      ...
  66
  67      # b_18 = PHI <D.2080_5(6), &c(7)>
  68      ...
  69
  70      # b_21 = PHI <b_15(8), b_18(9)>
  71      D.2076_8 = *b_21;
  72
  73    where the addresses loaded are defined by PHIs itself.
  74    The above happens for
  75
  76      std::max(std::min(a0, c), std::min(std::max(a1, c), b))
  77
  78    where this pass transforms it to a form later PHI optimization
  79    recognizes and transforms it to the simple
  80
  81      D.2109_10 = this_1(D)->a1;
  82      D.2110_11 = c;
  83      D.2114_31 = MAX_EXPR <D.2109_10, D.2110_11>;
  84      D.2115_14 = b;
  85      D.2125_17 = MIN_EXPR <D.2115_14, D.2114_31>;
  86      D.2119_16 = this_1(D)->a0;
  87      D.2124_32 = MIN_EXPR <D.2110_11, D.2119_16>;
  88      D.2076_33 = MAX_EXPR <D.2125_17, D.2124_32>;
  89
  90    The pass does a dominator walk processing loads using a basic-block
  91    local analysis and stores the result for use by transformations on
  92    dominated basic-blocks.  */
  93
  94
  95 /* Structure to keep track of the value of a dereferenced PHI result
  96    and the virtual operand used for that dereference.  */
  97
  98 struct phiprop_d
  99 {
 100   tree value;
 101   tree vuse;
 102 };
 103
 104 /* Verify if the value recorded for NAME in PHIVN is still valid at
 105    the start of basic block BB.  */
 106
 107 static bool
 108 phivn_valid_p (struct phiprop_d *phivn, tree name, basic_block bb)
 109 {
 110   tree vuse = phivn[SSA_NAME_VERSION (name)].vuse;
 111   gimple *use_stmt;
 112   imm_use_iterator ui2;
 113   bool ok = true;
 114
 115   /* The def stmts of the virtual uses need to be dominated by bb.  */
 116   gcc_assert (vuse != NULL_TREE);
 117
 118   FOR_EACH_IMM_USE_STMT (use_stmt, ui2, vuse)
 119     {
 120       /* If BB does not dominate a VDEF, the value is invalid.  */
 121       if ((gimple_vdef (use_stmt) != NULL_TREE
 122            || gimple_code (use_stmt) == GIMPLE_PHI)
 123           && !dominated_by_p (CDI_DOMINATORS, gimple_bb (use_stmt), bb))
 124         {
 125           ok = false;
 126           BREAK_FROM_IMM_USE_STMT (ui2);
 127         }
 128     }
 129
 130   return ok;
 131 }
 132
 133 /* Insert a new phi node for the dereference of PHI at basic_block
 134    BB with the virtual operands from USE_STMT.  */
 135
 136 static tree
 137 phiprop_insert_phi (basic_block bb, gphi *phi, gimple *use_stmt,
 138                     struct phiprop_d *phivn, size_t n)
 139 {
 140   tree res;
 141   gphi *new_phi;
 142   edge_iterator ei;
 143   edge e;
 144
 145   gcc_assert (is_gimple_assign (use_stmt)
 146               && gimple_assign_rhs_code (use_stmt) == MEM_REF);
 147
 148   /* Build a new PHI node to replace the definition of
 149      the indirect reference lhs.  */
 150   res = gimple_assign_lhs (use_stmt);
 151   new_phi = create_phi_node (res, bb);
 152
 153   if (dump_file && (dump_flags & TDF_DETAILS))
 154     {
 155       fprintf (dump_file, "Inserting PHI for result of load ");
 156       print_gimple_stmt (dump_file, use_stmt, 0, 0);
 157     }
 158
 159   /* Add PHI arguments for each edge inserting loads of the
 160      addressable operands.  */
 161   FOR_EACH_EDGE (e, ei, bb->preds)
 162     {
 163       tree old_arg, new_var;
 164       gassign *tmp;
 165       source_location locus;
 166
 167       old_arg = PHI_ARG_DEF_FROM_EDGE (phi, e);
 168       locus = gimple_phi_arg_location_from_edge (phi, e);
 169       while (TREE_CODE (old_arg) == SSA_NAME
 170              && (SSA_NAME_VERSION (old_arg) >= n
 171                  || phivn[SSA_NAME_VERSION (old_arg)].value == NULL_TREE))
 172         {
 173           gimple *def_stmt = SSA_NAME_DEF_STMT (old_arg);
 174           old_arg = gimple_assign_rhs1 (def_stmt);
 175           locus = gimple_location (def_stmt);
 176         }
 177
 178       if (TREE_CODE (old_arg) == SSA_NAME)
 179         {
 180           if (dump_file && (dump_flags & TDF_DETAILS))
 181             {
 182               fprintf (dump_file, "  for edge defining ");
 183               print_generic_expr (dump_file, PHI_ARG_DEF_FROM_EDGE (phi, e), 0);
 184               fprintf (dump_file, " reusing PHI result ");
 185               print_generic_expr (dump_file,
 186                                   phivn[SSA_NAME_VERSION (old_arg)].value, 0);
 187               fprintf (dump_file, "\n");
 188             }
 189           /* Reuse a formerly created dereference.  */
 190           new_var = phivn[SSA_NAME_VERSION (old_arg)].value;
 191         }
 192       else
 193         {
 194           tree rhs = gimple_assign_rhs1 (use_stmt);
 195           gcc_assert (TREE_CODE (old_arg) == ADDR_EXPR);
 196           new_var = make_ssa_name (TREE_TYPE (rhs));
 197           if (!is_gimple_min_invariant (old_arg))
 198             old_arg = PHI_ARG_DEF_FROM_EDGE (phi, e);
 199           else
 200             old_arg = unshare_expr (old_arg);
 201           tmp = gimple_build_assign (new_var,
 202                                      fold_build2 (MEM_REF, TREE_TYPE (rhs),
 203                                                   old_arg,
 204                                                   TREE_OPERAND (rhs, 1)));
 205           gimple_set_location (tmp, locus);
 206
 207           gsi_insert_on_edge (e, tmp);
 208           update_stmt (tmp);
 209
 210           if (dump_file && (dump_flags & TDF_DETAILS))
 211             {
 212               fprintf (dump_file, "  for edge defining ");
 213               print_generic_expr (dump_file, PHI_ARG_DEF_FROM_EDGE (phi, e), 0);
 214               fprintf (dump_file, " inserting load ");
 215               print_gimple_stmt (dump_file, tmp, 0, 0);
 216             }
 217         }
 218
 219       add_phi_arg (new_phi, new_var, e, locus);
 220     }
 221
 222   update_stmt (new_phi);
 223
 224   if (dump_file && (dump_flags & TDF_DETAILS))
 225     print_gimple_stmt (dump_file, new_phi, 0, 0);
 226
 227   return res;
 228 }
 229
 230 /* Propagate between the phi node arguments of PHI in BB and phi result
 231    users.  For now this matches
 232         # p_2 = PHI <&x, &y>
 233       <Lx>:;
 234         p_3 = p_2;
 235         z_2 = *p_3;
 236    and converts it to
 237         # z_2 = PHI <x, y>
 238       <Lx>:;
 239    Returns true if a transformation was done and edge insertions
 240    need to be committed.  Global data PHIVN and N is used to track
 241    past transformation results.  We need to be especially careful here
 242    with aliasing issues as we are moving memory reads.  */
 243
 244 static bool
 245 propagate_with_phi (basic_block bb, gphi *phi, struct phiprop_d *phivn,
 246                     size_t n)
 247 {
 248   tree ptr = PHI_RESULT (phi);
 249   gimple *use_stmt;
 250   tree res = NULL_TREE;
 251   gimple_stmt_iterator gsi;
 252   imm_use_iterator ui;
 253   use_operand_p arg_p, use;
 254   ssa_op_iter i;
 255   bool phi_inserted;
 256   tree type = NULL_TREE;
 257
 258   if (!POINTER_TYPE_P (TREE_TYPE (ptr))
 259       || !is_gimple_reg_type (TREE_TYPE (TREE_TYPE (ptr))))
 260     return false;
 261
 262   /* Check if we can "cheaply" dereference all phi arguments.  */
 263   FOR_EACH_PHI_ARG (arg_p, phi, i, SSA_OP_USE)
 264     {
 265       tree arg = USE_FROM_PTR (arg_p);
 266       /* Walk the ssa chain until we reach a ssa name we already
 267          created a value for or we reach a definition of the form
 268          ssa_name_n = &var;  */
 269       while (TREE_CODE (arg) == SSA_NAME
 270              && !SSA_NAME_IS_DEFAULT_DEF (arg)
 271              && (SSA_NAME_VERSION (arg) >= n
 272                  || phivn[SSA_NAME_VERSION (arg)].value == NULL_TREE))
 273         {
 274           gimple *def_stmt = SSA_NAME_DEF_STMT (arg);
 275           if (!gimple_assign_single_p (def_stmt))
 276             return false;
 277           arg = gimple_assign_rhs1 (def_stmt);
 278         }
 279       if (TREE_CODE (arg) != ADDR_EXPR
 280           && !(TREE_CODE (arg) == SSA_NAME
 281                && SSA_NAME_VERSION (arg) < n
 282                && phivn[SSA_NAME_VERSION (arg)].value != NULL_TREE
 283                && (!type
 284                    || types_compatible_p
 285                        (type, TREE_TYPE (phivn[SSA_NAME_VERSION (arg)].value)))
 286                && phivn_valid_p (phivn, arg, bb)))
 287         return false;
 288       if (!type
 289           && TREE_CODE (arg) == SSA_NAME)
 290         type = TREE_TYPE (phivn[SSA_NAME_VERSION (arg)].value);
 291     }
 292
 293   /* Find a dereferencing use.  First follow (single use) ssa
 294      copy chains for ptr.  */
 295   while (single_imm_use (ptr, &use, &use_stmt)
 296          && gimple_assign_ssa_name_copy_p (use_stmt))
 297     ptr = gimple_assign_lhs (use_stmt);
 298
 299   /* Replace the first dereference of *ptr if there is one and if we
 300      can move the loads to the place of the ptr phi node.  */
 301   phi_inserted = false;
 302   FOR_EACH_IMM_USE_STMT (use_stmt, ui, ptr)
 303     {
 304       gimple *def_stmt;
 305       tree vuse;
 306
 307       /* Only replace loads in blocks that post-dominate the PHI node.  That
 308          makes sure we don't end up speculating loads.  */
 309       if (!dominated_by_p (CDI_POST_DOMINATORS,
 310                            bb, gimple_bb (use_stmt)))
 311         continue;
 312
 313       /* Check whether this is a load of *ptr.  */
 314       if (!(is_gimple_assign (use_stmt)
 315             && TREE_CODE (gimple_assign_lhs (use_stmt)) == SSA_NAME
 316             && gimple_assign_rhs_code (use_stmt) == MEM_REF
 317             && TREE_OPERAND (gimple_assign_rhs1 (use_stmt), 0) == ptr
 318             && integer_zerop (TREE_OPERAND (gimple_assign_rhs1 (use_stmt), 1))
 319             && (!type
 320                 || types_compatible_p
 321                      (TREE_TYPE (gimple_assign_lhs (use_stmt)), type))
 322             /* We cannot replace a load that may throw or is volatile.  */
 323             && !stmt_can_throw_internal (use_stmt)))
 324         continue;
 325
 326       /* Check if we can move the loads.  The def stmt of the virtual use
 327          needs to be in a different basic block dominating bb.  */
 328       vuse = gimple_vuse (use_stmt);
 329       def_stmt = SSA_NAME_DEF_STMT (vuse);
 330       if (!SSA_NAME_IS_DEFAULT_DEF (vuse)
 331           && (gimple_bb (def_stmt) == bb
 332               || !dominated_by_p (CDI_DOMINATORS,
 333                                   bb, gimple_bb (def_stmt))))
 334         goto next;
 335
 336       /* Found a proper dereference.  Insert a phi node if this
 337          is the first load transformation.  */
 338       if (!phi_inserted)
 339         {
 340           res = phiprop_insert_phi (bb, phi, use_stmt, phivn, n);
 341           type = TREE_TYPE (res);
 342
 343           /* Remember the value we created for *ptr.  */
 344           phivn[SSA_NAME_VERSION (ptr)].value = res;
 345           phivn[SSA_NAME_VERSION (ptr)].vuse = vuse;
 346
 347           /* Remove old stmt.  The phi is taken care of by DCE, if we
 348              want to delete it here we also have to delete all intermediate
 349              copies.  */
 350           gsi = gsi_for_stmt (use_stmt);
 351           gsi_remove (&gsi, true);
 352
 353           phi_inserted = true;
 354         }
 355       else
 356         {
 357           /* Further replacements are easy, just make a copy out of the
 358              load.  */
 359           gimple_assign_set_rhs1 (use_stmt, res);
 360           update_stmt (use_stmt);
 361         }
 362
 363 next:;
 364       /* Continue searching for a proper dereference.  */
 365     }
 366
 367   return phi_inserted;
 368 }
 369
 370 /* Main entry for phiprop pass.  */
 371
 372 namespace {
 373
 374 const pass_data pass_data_phiprop =
 375 {
 376   GIMPLE_PASS, /* type */
 377   "phiprop", /* name */
 378   OPTGROUP_NONE, /* optinfo_flags */
 379   TV_TREE_PHIPROP, /* tv_id */
 380   ( PROP_cfg | PROP_ssa ), /* properties_required */
 381   0, /* properties_provided */
 382   0, /* properties_destroyed */
 383   0, /* todo_flags_start */
 384   TODO_update_ssa, /* todo_flags_finish */
 385 };
 386
 387 class pass_phiprop : public gimple_opt_pass
 388 {
 389 public:
 390   pass_phiprop (gcc::context *ctxt)
 391     : gimple_opt_pass (pass_data_phiprop, ctxt)
 392   {}
 393
 394   /* opt_pass methods: */
 395   virtual bool gate (function *) { return flag_tree_phiprop; }
 396   virtual unsigned int execute (function *);
 397
 398 }; // class pass_phiprop
 399
 400 unsigned int
 401 pass_phiprop::execute (function *fun)
 402 {
 403   vec<basic_block> bbs;
 404   struct phiprop_d *phivn;
 405   bool did_something = false;
 406   basic_block bb;
 407   gphi_iterator gsi;
 408   unsigned i;
 409   size_t n;
 410
 411   calculate_dominance_info (CDI_DOMINATORS);
 412   calculate_dominance_info (CDI_POST_DOMINATORS);
 413
 414   n = num_ssa_names;
 415   phivn = XCNEWVEC (struct phiprop_d, n);
 416
 417   /* Walk the dominator tree in preorder.  */
 418   bbs = get_all_dominated_blocks (CDI_DOMINATORS,
 419                                   single_succ (ENTRY_BLOCK_PTR_FOR_FN (fun)));
 420   FOR_EACH_VEC_ELT (bbs, i, bb)
 421     for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 422       did_something |= propagate_with_phi (bb, gsi.phi (), phivn, n);
 423
 424   if (did_something)
 425     gsi_commit_edge_inserts ();
 426
 427   bbs.release ();
 428   free (phivn);
 429
 430   free_dominance_info (CDI_POST_DOMINATORS);
 431
 432   return 0;
 433 }
 434
 435 } // anon namespace
 436
 437 gimple_opt_pass *
 438 make_pass_phiprop (gcc::context *ctxt)
 439 {
 440   return new pass_phiprop (ctxt);
 441 }