gcc/tree-nrv.c

   1 /* Language independent return value optimizations
   2    Copyright (C) 2004-2013 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
   7 it under the terms of the GNU General Public License as published by
   8 the Free Software Foundation; either version 3, or (at your option)
   9 any later version.
  10
  11 GCC is distributed in the hope that it will be useful,
  12 but WITHOUT ANY WARRANTY; without even the implied warranty of
  13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14 GNU General Public License for more details.
  15
  16 You should have received a copy of the GNU General Public License
  17 along with GCC; see the file COPYING3.  If not see
  18 <http://www.gnu.org/licenses/>.  */
  19
  20 #include "config.h"
  21 #include "system.h"
  22 #include "coretypes.h"
  23 #include "tm.h"
  24 #include "tree.h"
  25 #include "function.h"
  26 #include "basic-block.h"
  27 #include "tree-pretty-print.h"
  28 #include "tree-flow.h"
  29 #include "tree-pass.h"
  30 #include "langhooks.h"
  31 #include "flags.h"      /* For "optimize" in gate_pass_return_slot.
  32                            FIXME: That should be up to the pass manager,
  33                            but pass_nrv is not in pass_all_optimizations.  */
  34
  35 /* This file implements return value optimizations for functions which
  36    return aggregate types.
  37
  38    Basically this pass searches the function for return statements which
  39    return a local aggregate.  When converted to RTL such statements will
  40    generate a copy from the local aggregate to final return value destination
  41    mandated by the target's ABI.
  42
  43    That copy can often be avoided by directly constructing the return value
  44    into the final destination mandated by the target's ABI.
  45
  46    This is basically a generic equivalent to the C++ front-end's
  47    Named Return Value optimization.  */
  48
  49 struct nrv_data
  50 {
  51   /* This is the temporary (a VAR_DECL) which appears in all of
  52      this function's RETURN_EXPR statements.  */
  53   tree var;
  54
  55   /* This is the function's RESULT_DECL.  We will replace all occurrences
  56      of VAR with RESULT_DECL when we apply this optimization.  */
  57   tree result;
  58   int modified;
  59 };
  60
  61 static tree finalize_nrv_r (tree *, int *, void *);
  62
  63 /* Callback for the tree walker.
  64
  65    If TP refers to a RETURN_EXPR, then set the expression being returned
  66    to nrv_data->result.
  67
  68    If TP refers to nrv_data->var, then replace nrv_data->var with
  69    nrv_data->result.
  70
  71    If we reach a node where we know all the subtrees are uninteresting,
  72    then set *WALK_SUBTREES to zero.  */
  73
  74 static tree
  75 finalize_nrv_r (tree *tp, int *walk_subtrees, void *data)
  76 {
  77   struct walk_stmt_info *wi = (struct walk_stmt_info *) data;
  78   struct nrv_data *dp = (struct nrv_data *) wi->info;
  79
  80   /* No need to walk into types.  */
  81   if (TYPE_P (*tp))
  82     *walk_subtrees = 0;
  83
  84   /* Otherwise replace all occurrences of VAR with RESULT.  */
  85   else if (*tp == dp->var)
  86     {
  87       *tp = dp->result;
  88       dp->modified = 1;
  89     }
  90
  91   /* Keep iterating.  */
  92   return NULL_TREE;
  93 }
  94
  95 /* Main entry point for return value optimizations.
  96
  97    If this function always returns the same local variable, and that
  98    local variable is an aggregate type, then replace the variable with
  99    the function's DECL_RESULT.
 100
 101    This is the equivalent of the C++ named return value optimization
 102    applied to optimized trees in a language independent form.  If we
 103    ever encounter languages which prevent this kind of optimization,
 104    then we could either have the languages register the optimization or
 105    we could change the gating function to check the current language.  */
 106
 107 static unsigned int
 108 tree_nrv (void)
 109 {
 110   tree result = DECL_RESULT (current_function_decl);
 111   tree result_type = TREE_TYPE (result);
 112   tree found = NULL;
 113   basic_block bb;
 114   gimple_stmt_iterator gsi;
 115   struct nrv_data data;
 116
 117   /* If this function does not return an aggregate type in memory, then
 118      there is nothing to do.  */
 119   if (!aggregate_value_p (result, current_function_decl))
 120     return 0;
 121
 122   /* If a GIMPLE type is returned in memory, finalize_nrv_r might create
 123      non-GIMPLE.  */
 124   if (is_gimple_reg_type (result_type))
 125     return 0;
 126
 127   /* If the front end already did something like this, don't do it here.  */
 128   if (DECL_NAME (result))
 129     return 0;
 130
 131   /* If the result has its address taken then it might be modified
 132      by means not detected in the following loop.  Bail out in this
 133      case.  */
 134   if (TREE_ADDRESSABLE (result))
 135     return 0;
 136
 137   /* Look through each block for assignments to the RESULT_DECL.  */
 138   FOR_EACH_BB (bb)
 139     {
 140       for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 141         {
 142           gimple stmt = gsi_stmt (gsi);
 143           tree ret_val;
 144
 145           if (gimple_code (stmt) == GIMPLE_RETURN)
 146             {
 147               /* In a function with an aggregate return value, the
 148                  gimplifier has changed all non-empty RETURN_EXPRs to
 149                  return the RESULT_DECL.  */
 150               ret_val = gimple_return_retval (stmt);
 151               if (ret_val)
 152                 gcc_assert (ret_val == result);
 153             }
 154           else if (gimple_has_lhs (stmt)
 155                    && gimple_get_lhs (stmt) == result)
 156             {
 157               tree rhs;
 158
 159               if (!gimple_assign_copy_p (stmt))
 160                 return 0;
 161
 162               rhs = gimple_assign_rhs1 (stmt);
 163
 164               /* Now verify that this return statement uses the same value
 165                  as any previously encountered return statement.  */
 166               if (found != NULL)
 167                 {
 168                   /* If we found a return statement using a different variable
 169                      than previous return statements, then we can not perform
 170                      NRV optimizations.  */
 171                   if (found != rhs)
 172                     return 0;
 173                 }
 174               else
 175                 found = rhs;
 176
 177               /* The returned value must be a local automatic variable of the
 178                  same type and alignment as the function's result.  */
 179               if (TREE_CODE (found) != VAR_DECL
 180                   || TREE_THIS_VOLATILE (found)
 181                   || !auto_var_in_fn_p (found, current_function_decl)
 182                   || TREE_ADDRESSABLE (found)
 183                   || DECL_ALIGN (found) > DECL_ALIGN (result)
 184                   || !useless_type_conversion_p (result_type,
 185                                                  TREE_TYPE (found)))
 186                 return 0;
 187             }
 188           else if (gimple_has_lhs (stmt))
 189             {
 190               tree addr = get_base_address (gimple_get_lhs (stmt));
 191                /* If there's any MODIFY of component of RESULT,
 192                   then bail out.  */
 193               if (addr && addr == result)
 194                 return 0;
 195             }
 196         }
 197     }
 198
 199   if (!found)
 200     return 0;
 201
 202   /* If dumping details, then note once and only the NRV replacement.  */
 203   if (dump_file && (dump_flags & TDF_DETAILS))
 204     {
 205       fprintf (dump_file, "NRV Replaced: ");
 206       print_generic_expr (dump_file, found, dump_flags);
 207       fprintf (dump_file, "  with: ");
 208       print_generic_expr (dump_file, result, dump_flags);
 209       fprintf (dump_file, "\n");
 210     }
 211
 212   /* At this point we know that all the return statements return the
 213      same local which has suitable attributes for NRV.   Copy debugging
 214      information from FOUND to RESULT if it will be useful.  But don't set
 215      DECL_ABSTRACT_ORIGIN to point at another function.  */
 216   if (!DECL_IGNORED_P (found)
 217       && !(DECL_ABSTRACT_ORIGIN (found)
 218            && DECL_CONTEXT (DECL_ABSTRACT_ORIGIN (found)) != current_function_decl))
 219     {
 220       DECL_NAME (result) = DECL_NAME (found);
 221       DECL_SOURCE_LOCATION (result) = DECL_SOURCE_LOCATION (found);
 222       DECL_ABSTRACT_ORIGIN (result) = DECL_ABSTRACT_ORIGIN (found);
 223     }
 224
 225   TREE_ADDRESSABLE (result) |= TREE_ADDRESSABLE (found);
 226
 227   /* Now walk through the function changing all references to VAR to be
 228      RESULT.  */
 229   data.var = found;
 230   data.result = result;
 231   FOR_EACH_BB (bb)
 232     {
 233       for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); )
 234         {
 235           gimple stmt = gsi_stmt (gsi);
 236           /* If this is a copy from VAR to RESULT, remove it.  */
 237           if (gimple_assign_copy_p (stmt)
 238               && gimple_assign_lhs (stmt) == result
 239               && gimple_assign_rhs1 (stmt) == found)
 240             {
 241               unlink_stmt_vdef (stmt);
 242               gsi_remove (&gsi, true);
 243               release_defs (stmt);
 244             }
 245           else
 246             {
 247               struct walk_stmt_info wi;
 248               memset (&wi, 0, sizeof (wi));
 249               wi.info = &data;
 250               data.modified = 0;
 251               walk_gimple_op (stmt, finalize_nrv_r, &wi);
 252               if (data.modified)
 253                 update_stmt (stmt);
 254               gsi_next (&gsi);
 255             }
 256         }
 257     }
 258
 259   SET_DECL_VALUE_EXPR (found, result);
 260   DECL_HAS_VALUE_EXPR_P (found) = 1;
 261
 262   return 0;
 263 }
 264
 265 static bool
 266 gate_pass_return_slot (void)
 267 {
 268   return optimize > 0;
 269 }
 270
 271 struct gimple_opt_pass pass_nrv =
 272 {
 273  {
 274   GIMPLE_PASS,
 275   "nrv",                                /* name */
 276   OPTGROUP_NONE,                        /* optinfo_flags */
 277   gate_pass_return_slot,                /* gate */
 278   tree_nrv,                             /* execute */
 279   NULL,                                 /* sub */
 280   NULL,                                 /* next */
 281   0,                                    /* static_pass_number */
 282   TV_TREE_NRV,                          /* tv_id */
 283   PROP_ssa | PROP_cfg,                          /* properties_required */
 284   0,                                    /* properties_provided */
 285   0,                                    /* properties_destroyed */
 286   0,                                    /* todo_flags_start */
 287   TODO_ggc_collect                      /* todo_flags_finish */
 288  }
 289 };
 290
 291 /* Determine (pessimistically) whether DEST is available for NRV
 292    optimization, where DEST is expected to be the LHS of a modify
 293    expression where the RHS is a function returning an aggregate.
 294
 295    DEST is available if it is not clobbered or used by the call.  */
 296
 297 static bool
 298 dest_safe_for_nrv_p (gimple call)
 299 {
 300   tree dest = gimple_call_lhs (call);
 301
 302   dest = get_base_address (dest);
 303   if (! dest)
 304     return false;
 305
 306   if (TREE_CODE (dest) == SSA_NAME)
 307     return true;
 308
 309   if (call_may_clobber_ref_p (call, dest)
 310       || ref_maybe_used_by_stmt_p (call, dest))
 311     return false;
 312
 313   return true;
 314 }
 315
 316 /* Walk through the function looking for GIMPLE_ASSIGNs with calls that
 317    return in memory on the RHS.  For each of these, determine whether it is
 318    safe to pass the address of the LHS as the return slot, and mark the
 319    call appropriately if so.
 320
 321    The NRV shares the return slot with a local variable in the callee; this
 322    optimization shares the return slot with the target of the call within
 323    the caller.  If the NRV is performed (which we can't know in general),
 324    this optimization is safe if the address of the target has not
 325    escaped prior to the call.  If it has, modifications to the local
 326    variable will produce visible changes elsewhere, as in PR c++/19317.  */
 327
 328 static unsigned int
 329 execute_return_slot_opt (void)
 330 {
 331   basic_block bb;
 332
 333   FOR_EACH_BB (bb)
 334     {
 335       gimple_stmt_iterator gsi;
 336       for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 337         {
 338           gimple stmt = gsi_stmt (gsi);
 339           bool slot_opt_p;
 340
 341           if (is_gimple_call (stmt)
 342               && gimple_call_lhs (stmt)
 343               && !gimple_call_return_slot_opt_p (stmt)
 344               && aggregate_value_p (TREE_TYPE (gimple_call_lhs (stmt)),
 345                                     gimple_call_fndecl (stmt)))
 346             {
 347               /* Check if the location being assigned to is
 348                  clobbered by the call.  */
 349               slot_opt_p = dest_safe_for_nrv_p (stmt);
 350               gimple_call_set_return_slot_opt (stmt, slot_opt_p);
 351             }
 352         }
 353     }
 354   return 0;
 355 }
 356
 357 struct gimple_opt_pass pass_return_slot =
 358 {
 359  {
 360   GIMPLE_PASS,
 361   "retslot",                            /* name */
 362   OPTGROUP_NONE,                        /* optinfo_flags */
 363   NULL,                                 /* gate */
 364   execute_return_slot_opt,              /* execute */
 365   NULL,                                 /* sub */
 366   NULL,                                 /* next */
 367   0,                                    /* static_pass_number */
 368   TV_NONE,                              /* tv_id */
 369   PROP_ssa,                             /* properties_required */
 370   0,                                    /* properties_provided */
 371   0,                                    /* properties_destroyed */
 372   0,                                    /* todo_flags_start */
 373   0                                     /* todo_flags_finish */
 374  }
 375 };