gcc/tree-ssa-phiprop.c

   1 /* Backward propagation of indirect loads through PHIs.
   2    Copyright (C) 2007, 2008 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 "tm.h"
  25 #include "ggc.h"
  26 #include "tree.h"
  27 #include "rtl.h"
  28 #include "tm_p.h"
  29 #include "basic-block.h"
  30 #include "timevar.h"
  31 #include "diagnostic.h"
  32 #include "tree-flow.h"
  33 #include "tree-pass.h"
  34 #include "tree-dump.h"
  35 #include "langhooks.h"
  36 #include "flags.h"
  37
  38 /* This pass propagates indirect loads through the PHI node for its
  39    address to make the load source possibly non-addressable and to
  40    allow for PHI optimization to trigger.
  41
  42    For example the pass changes
  43
  44      # addr_1 = PHI <&a, &b>
  45      tmp_1 = *addr_1;
  46
  47    to
  48
  49      # tmp_1 = PHI <a, b>
  50
  51    but also handles more complex scenarios like
  52
  53      D.2077_2 = &this_1(D)->a1;
  54      ...
  55
  56      # b_12 = PHI <&c(2), D.2077_2(3)>
  57      D.2114_13 = *b_12;
  58      ...
  59
  60      # b_15 = PHI <b_12(4), &b(5)>
  61      D.2080_5 = &this_1(D)->a0;
  62      ...
  63
  64      # b_18 = PHI <D.2080_5(6), &c(7)>
  65      ...
  66
  67      # b_21 = PHI <b_15(8), b_18(9)>
  68      D.2076_8 = *b_21;
  69
  70    where the addresses loaded are defined by PHIs itself.
  71    The above happens for
  72
  73      std::max(std::min(a0, c), std::min(std::max(a1, c), b))
  74
  75    where this pass transforms it to a form later PHI optimization
  76    recognizes and transforms it to the simple
  77
  78      D.2109_10 = this_1(D)->a1;
  79      D.2110_11 = c;
  80      D.2114_31 = MAX_EXPR <D.2109_10, D.2110_11>;
  81      D.2115_14 = b;
  82      D.2125_17 = MIN_EXPR <D.2115_14, D.2114_31>;
  83      D.2119_16 = this_1(D)->a0;
  84      D.2124_32 = MIN_EXPR <D.2110_11, D.2119_16>;
  85      D.2076_33 = MAX_EXPR <D.2125_17, D.2124_32>;
  86
  87    The pass does a dominator walk processing loads using a basic-block
  88    local analysis and stores the result for use by transformations on
  89    dominated basic-blocks.  */
  90
  91
  92 /* Structure to keep track of the value of a dereferenced PHI result
  93    and the set of virtual operands used for that dereference.  */
  94
  95 struct phiprop_d
  96 {
  97   tree value;
  98   gimple vop_stmt;
  99 };
 100
 101 /* Verify if the value recorded for NAME in PHIVN is still valid at
 102    the start of basic block BB.  */
 103
 104 static bool
 105 phivn_valid_p (struct phiprop_d *phivn, tree name, basic_block bb)
 106 {
 107   gimple vop_stmt = phivn[SSA_NAME_VERSION (name)].vop_stmt;
 108   tree vuse;
 109
 110   /* The def stmts of all virtual uses need to be post-dominated
 111      by bb.  */
 112   if ((vuse = gimple_vuse (vop_stmt)))
 113     {
 114       gimple use_stmt;
 115       imm_use_iterator ui2;
 116       bool ok = true;
 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 (((is_gimple_assign (use_stmt)
 122                 && gimple_vdef (use_stmt))
 123                || gimple_code (use_stmt) == GIMPLE_PHI)
 124               && !dominated_by_p (CDI_DOMINATORS, gimple_bb (use_stmt), bb))
 125             {
 126               ok = false;
 127               BREAK_FROM_IMM_USE_STMT (ui2);
 128             }
 129         }
 130       if (!ok)
 131         return false;
 132     }
 133
 134   return true;
 135 }
 136
 137 /* Insert a new phi node for the dereference of PHI at basic_block
 138    BB with the virtual operands from USE_STMT.  */
 139
 140 static tree
 141 phiprop_insert_phi (basic_block bb, gimple phi, gimple use_stmt,
 142                     struct phiprop_d *phivn, size_t n)
 143 {
 144   tree res;
 145   gimple new_phi;
 146   edge_iterator ei;
 147   edge e;
 148
 149   gcc_assert (is_gimple_assign (use_stmt)
 150               && gimple_assign_rhs_code (use_stmt) == INDIRECT_REF);
 151
 152   /* Build a new PHI node to replace the definition of
 153      the indirect reference lhs.  */
 154   res = gimple_assign_lhs (use_stmt);
 155   SSA_NAME_DEF_STMT (res) = new_phi = create_phi_node (res, bb);
 156
 157   /* Add PHI arguments for each edge inserting loads of the
 158      addressable operands.  */
 159   FOR_EACH_EDGE (e, ei, bb->preds)
 160     {
 161       tree old_arg, new_var;
 162       gimple tmp;
 163
 164       old_arg = PHI_ARG_DEF_FROM_EDGE (phi, e);
 165       while (TREE_CODE (old_arg) == SSA_NAME
 166              && (SSA_NAME_VERSION (old_arg) >= n
 167                  || phivn[SSA_NAME_VERSION (old_arg)].value == NULL_TREE))
 168         {
 169           gimple def_stmt = SSA_NAME_DEF_STMT (old_arg);
 170           old_arg = gimple_assign_rhs1 (def_stmt);
 171         }
 172
 173       if (TREE_CODE (old_arg) == SSA_NAME)
 174         /* Reuse a formerly created dereference.  */
 175         new_var = phivn[SSA_NAME_VERSION (old_arg)].value;
 176       else
 177         {
 178           gcc_assert (TREE_CODE (old_arg) == ADDR_EXPR);
 179           old_arg = TREE_OPERAND (old_arg, 0);
 180           new_var = create_tmp_var (TREE_TYPE (old_arg), NULL);
 181           tmp = gimple_build_assign (new_var, unshare_expr (old_arg));
 182           if (TREE_CODE (TREE_TYPE (old_arg)) == COMPLEX_TYPE
 183               || TREE_CODE (TREE_TYPE (old_arg)) == VECTOR_TYPE)
 184             DECL_GIMPLE_REG_P (new_var) = 1;
 185           gcc_assert (is_gimple_reg (new_var));
 186           add_referenced_var (new_var);
 187           new_var = make_ssa_name (new_var, tmp);
 188           gimple_assign_set_lhs (tmp, new_var);
 189
 190           gsi_insert_on_edge (e, tmp);
 191
 192           update_stmt (tmp);
 193           mark_symbols_for_renaming (tmp);
 194         }
 195
 196       add_phi_arg (new_phi, new_var, e);
 197     }
 198
 199   update_stmt (new_phi);
 200
 201   return res;
 202 }
 203
 204 /* Propagate between the phi node arguments of PHI in BB and phi result
 205    users.  For now this matches
 206         # p_2 = PHI <&x, &y>
 207       <Lx>:;
 208         p_3 = p_2;
 209         z_2 = *p_3;
 210    and converts it to
 211         # z_2 = PHI <x, y>
 212       <Lx>:;
 213    Returns true if a transformation was done and edge insertions
 214    need to be committed.  Global data PHIVN and N is used to track
 215    past transformation results.  We need to be especially careful here
 216    with aliasing issues as we are moving memory reads.  */
 217
 218 static bool
 219 propagate_with_phi (basic_block bb, gimple phi, struct phiprop_d *phivn,
 220                     size_t n)
 221 {
 222   tree ptr = PHI_RESULT (phi);
 223   gimple use_stmt;
 224   tree res = NULL_TREE;
 225   gimple_stmt_iterator gsi;
 226   imm_use_iterator ui;
 227   use_operand_p arg_p, use;
 228   ssa_op_iter i;
 229   bool phi_inserted;
 230
 231   if (!POINTER_TYPE_P (TREE_TYPE (ptr))
 232       || !is_gimple_reg_type (TREE_TYPE (TREE_TYPE (ptr))))
 233     return false;
 234
 235   /* Check if we can "cheaply" dereference all phi arguments.  */
 236   FOR_EACH_PHI_ARG (arg_p, phi, i, SSA_OP_USE)
 237     {
 238       tree arg = USE_FROM_PTR (arg_p);
 239       /* Walk the ssa chain until we reach a ssa name we already
 240          created a value for or we reach a definition of the form
 241          ssa_name_n = &var;  */
 242       while (TREE_CODE (arg) == SSA_NAME
 243              && !SSA_NAME_IS_DEFAULT_DEF (arg)
 244              && (SSA_NAME_VERSION (arg) >= n
 245                  || phivn[SSA_NAME_VERSION (arg)].value == NULL_TREE))
 246         {
 247           gimple def_stmt = SSA_NAME_DEF_STMT (arg);
 248           if (!gimple_assign_single_p (def_stmt))
 249             return false;
 250           arg = gimple_assign_rhs1 (def_stmt);
 251         }
 252       if ((TREE_CODE (arg) != ADDR_EXPR
 253            /* Avoid to have to decay *&a to a[0] later.  */
 254            || !is_gimple_reg_type (TREE_TYPE (TREE_OPERAND (arg, 0))))
 255           && !(TREE_CODE (arg) == SSA_NAME
 256                && SSA_NAME_VERSION (arg) < n
 257                && phivn[SSA_NAME_VERSION (arg)].value != NULL_TREE
 258                && phivn_valid_p (phivn, arg, bb)))
 259         return false;
 260     }
 261
 262   /* Find a dereferencing use.  First follow (single use) ssa
 263      copy chains for ptr.  */
 264   while (single_imm_use (ptr, &use, &use_stmt)
 265          && gimple_assign_ssa_name_copy_p (use_stmt))
 266     ptr = gimple_assign_lhs (use_stmt);
 267
 268   /* Replace the first dereference of *ptr if there is one and if we
 269      can move the loads to the place of the ptr phi node.  */
 270   phi_inserted = false;
 271   FOR_EACH_IMM_USE_STMT (use_stmt, ui, ptr)
 272     {
 273       tree vuse;
 274
 275       /* Check whether this is a load of *ptr.  */
 276       if (!(is_gimple_assign (use_stmt)
 277             && TREE_CODE (gimple_assign_lhs (use_stmt)) == SSA_NAME
 278             && gimple_assign_rhs_code (use_stmt) == INDIRECT_REF
 279             && TREE_OPERAND (gimple_assign_rhs1 (use_stmt), 0) == ptr
 280             /* We cannot replace a load that may throw or is volatile.  */
 281             && !stmt_can_throw_internal (use_stmt)))
 282         continue;
 283
 284       /* Check if we can move the loads.  The def stmts of all virtual uses
 285          need to be post-dominated by bb.  */
 286       if ((vuse = gimple_vuse (use_stmt)) != NULL_TREE)
 287         {
 288           gimple def_stmt = SSA_NAME_DEF_STMT (vuse);
 289           if (!SSA_NAME_IS_DEFAULT_DEF (vuse)
 290               && (gimple_bb (def_stmt) == bb
 291                   || !dominated_by_p (CDI_DOMINATORS,
 292                                       bb, gimple_bb (def_stmt))))
 293             goto next;
 294         }
 295
 296       /* Found a proper dereference.  Insert a phi node if this
 297          is the first load transformation.  */
 298       if (!phi_inserted)
 299         {
 300           res = phiprop_insert_phi (bb, phi, use_stmt, phivn, n);
 301
 302           /* Remember the value we created for *ptr.  */
 303           phivn[SSA_NAME_VERSION (ptr)].value = res;
 304           phivn[SSA_NAME_VERSION (ptr)].vop_stmt = use_stmt;
 305
 306           /* Remove old stmt.  The phi is taken care of by DCE, if we
 307              want to delete it here we also have to delete all intermediate
 308              copies.  */
 309           gsi = gsi_for_stmt (use_stmt);
 310           gsi_remove (&gsi, false);
 311
 312           phi_inserted = true;
 313         }
 314       else
 315         {
 316           /* Further replacements are easy, just make a copy out of the
 317              load.  */
 318           gimple_assign_set_rhs1 (use_stmt, res);
 319           update_stmt (use_stmt);
 320         }
 321
 322 next:;
 323       /* Continue searching for a proper dereference.  */
 324     }
 325
 326   return phi_inserted;
 327 }
 328
 329 /* Main entry for phiprop pass.  */
 330
 331 static unsigned int
 332 tree_ssa_phiprop (void)
 333 {
 334   VEC(basic_block, heap) *bbs;
 335   struct phiprop_d *phivn;
 336   bool did_something = false;
 337   basic_block bb;
 338   gimple_stmt_iterator gsi;
 339   unsigned i;
 340   size_t n;
 341
 342   calculate_dominance_info (CDI_DOMINATORS);
 343
 344   n = num_ssa_names;
 345   phivn = XCNEWVEC (struct phiprop_d, n);
 346
 347   /* Walk the dominator tree in preorder.  */
 348   bbs = get_all_dominated_blocks (CDI_DOMINATORS,
 349                                   single_succ (ENTRY_BLOCK_PTR));
 350   for (i = 0; VEC_iterate (basic_block, bbs, i, bb); ++i)
 351     for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 352       did_something |= propagate_with_phi (bb, gsi_stmt (gsi), phivn, n);
 353
 354   if (did_something)
 355     gsi_commit_edge_inserts ();
 356
 357   VEC_free (basic_block, heap, bbs);
 358   free (phivn);
 359
 360   return 0;
 361 }
 362
 363 static bool
 364 gate_phiprop (void)
 365 {
 366   return 1;
 367 }
 368
 369 struct gimple_opt_pass pass_phiprop =
 370 {
 371  {
 372   GIMPLE_PASS,
 373   "phiprop",                    /* name */
 374   gate_phiprop,                 /* gate */
 375   tree_ssa_phiprop,             /* execute */
 376   NULL,                         /* sub */
 377   NULL,                         /* next */
 378   0,                            /* static_pass_number */
 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_dump_func
 385   | TODO_ggc_collect
 386   | TODO_update_ssa
 387   | TODO_verify_ssa             /* todo_flags_finish */
 388  }
 389 };