gcc/tree-nrv.c

   1 /* Language independent return value optimizations
   2    Copyright (C) 2004, 2005, 2007, 2008, 2009, 2010
   3    Free Software Foundation, Inc.
   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 "tree.h"
  26 #include "function.h"
  27 #include "basic-block.h"
  28 #include "tree-pretty-print.h"
  29 #include "tree-flow.h"
  30 #include "tree-pass.h"
  31 #include "langhooks.h"
  32 #include "flags.h"      /* For "optimize" in gate_pass_return_slot.
  33                            FIXME: That should be up to the pass manager,
  34                            but pass_nrv is not in pass_all_optimizations.  */
  35
  36 /* This file implements return value optimizations for functions which
  37    return aggregate types.
  38
  39    Basically this pass searches the function for return statements which
  40    return a local aggregate.  When converted to RTL such statements will
  41    generate a copy from the local aggregate to final return value destination
  42    mandated by the target's ABI.
  43
  44    That copy can often be avoided by directly constructing the return value
  45    into the final destination mandated by the target's ABI.
  46
  47    This is basically a generic equivalent to the C++ front-end's
  48    Named Return Value optimization.  */
  49
  50 struct nrv_data
  51 {
  52   /* This is the temporary (a VAR_DECL) which appears in all of
  53      this function's RETURN_EXPR statements.  */
  54   tree var;
  55
  56   /* This is the function's RESULT_DECL.  We will replace all occurrences
  57      of VAR with RESULT_DECL when we apply this optimization.  */
  58   tree result;
  59   int modified;
  60 };
  61
  62 static tree finalize_nrv_r (tree *, int *, void *);
  63
  64 /* Callback for the tree walker.
  65
  66    If TP refers to a RETURN_EXPR, then set the expression being returned
  67    to nrv_data->result.
  68
  69    If TP refers to nrv_data->var, then replace nrv_data->var with
  70    nrv_data->result.
  71
  72    If we reach a node where we know all the subtrees are uninteresting,
  73    then set *WALK_SUBTREES to zero.  */
  74
  75 static tree
  76 finalize_nrv_r (tree *tp, int *walk_subtrees, void *data)
  77 {
  78   struct walk_stmt_info *wi = (struct walk_stmt_info *) data;
  79   struct nrv_data *dp = (struct nrv_data *) wi->info;
  80
  81   /* No need to walk into types.  */
  82   if (TYPE_P (*tp))
  83     *walk_subtrees = 0;
  84
  85   /* Otherwise replace all occurrences of VAR with RESULT.  */
  86   else if (*tp == dp->var)
  87     {
  88       *tp = dp->result;
  89       dp->modified = 1;
  90     }
  91
  92   /* Keep iterating.  */
  93   return NULL_TREE;
  94 }
  95
  96 /* Main entry point for return value optimizations.
  97
  98    If this function always returns the same local variable, and that
  99    local variable is an aggregate type, then replace the variable with
 100    the function's DECL_RESULT.
 101
 102    This is the equivalent of the C++ named return value optimization
 103    applied to optimized trees in a language independent form.  If we
 104    ever encounter languages which prevent this kind of optimization,
 105    then we could either have the languages register the optimization or
 106    we could change the gating function to check the current language.  */
 107
 108 static unsigned int
 109 tree_nrv (void)
 110 {
 111   tree result = DECL_RESULT (current_function_decl);
 112   tree result_type = TREE_TYPE (result);
 113   tree found = NULL;
 114   basic_block bb;
 115   gimple_stmt_iterator gsi;
 116   struct nrv_data data;
 117
 118   /* If this function does not return an aggregate type in memory, then
 119      there is nothing to do.  */
 120   if (!aggregate_value_p (result, current_function_decl))
 121     return 0;
 122
 123   /* If a GIMPLE type is returned in memory, finalize_nrv_r might create
 124      non-GIMPLE.  */
 125   if (is_gimple_reg_type (result_type))
 126     return 0;
 127
 128   /* If the front end already did something like this, don't do it here.  */
 129   if (DECL_NAME (result))
 130     return 0;
 131
 132   /* If the result has its address taken then it might be modified
 133      by means not detected in the following loop.  Bail out in this
 134      case.  */
 135   if (TREE_ADDRESSABLE (result))
 136     return 0;
 137
 138   /* Look through each block for assignments to the RESULT_DECL.  */
 139   FOR_EACH_BB (bb)
 140     {
 141       for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 142         {
 143           gimple stmt = gsi_stmt (gsi);
 144           tree ret_val;
 145
 146           if (gimple_code (stmt) == GIMPLE_RETURN)
 147             {
 148               /* In a function with an aggregate return value, the
 149                  gimplifier has changed all non-empty RETURN_EXPRs to
 150                  return the RESULT_DECL.  */
 151               ret_val = gimple_return_retval (stmt);
 152               if (ret_val)
 153                 gcc_assert (ret_val == result);
 154             }
 155           else if (gimple_has_lhs (stmt)
 156                    && gimple_get_lhs (stmt) == result)
 157             {
 158               tree rhs;
 159
 160               if (!gimple_assign_copy_p (stmt))
 161                 return 0;
 162
 163               rhs = gimple_assign_rhs1 (stmt);
 164
 165               /* Now verify that this return statement uses the same value
 166                  as any previously encountered return statement.  */
 167               if (found != NULL)
 168                 {
 169                   /* If we found a return statement using a different variable
 170                      than previous return statements, then we can not perform
 171                      NRV optimizations.  */
 172                   if (found != rhs)
 173                     return 0;
 174                 }
 175               else
 176                 found = rhs;
 177
 178               /* The returned value must be a local automatic variable of the
 179                  same type and alignment as the function's result.  */
 180               if (TREE_CODE (found) != VAR_DECL
 181                   || TREE_THIS_VOLATILE (found)
 182                   || DECL_CONTEXT (found) != current_function_decl
 183                   || TREE_STATIC (found)
 184                   || TREE_ADDRESSABLE (found)
 185                   || DECL_ALIGN (found) > DECL_ALIGN (result)
 186                   || !useless_type_conversion_p (result_type,
 187                                                  TREE_TYPE (found)))
 188                 return 0;
 189             }
 190           else if (gimple_has_lhs (stmt))
 191             {
 192               tree addr = get_base_address (gimple_get_lhs (stmt));
 193                /* If there's any MODIFY of component of RESULT,
 194                   then bail out.  */
 195               if (addr && addr == result)
 196                 return 0;
 197             }
 198         }
 199     }
 200
 201   if (!found)
 202     return 0;
 203
 204   /* If dumping details, then note once and only the NRV replacement.  */
 205   if (dump_file && (dump_flags & TDF_DETAILS))
 206     {
 207       fprintf (dump_file, "NRV Replaced: ");
 208       print_generic_expr (dump_file, found, dump_flags);
 209       fprintf (dump_file, "  with: ");
 210       print_generic_expr (dump_file, result, dump_flags);
 211       fprintf (dump_file, "\n");
 212     }
 213
 214   /* At this point we know that all the return statements return the
 215      same local which has suitable attributes for NRV.   Copy debugging
 216      information from FOUND to RESULT if it will be useful.  But don't set
 217      DECL_ABSTRACT_ORIGIN to point at another function.  */
 218   if (!DECL_IGNORED_P (found)
 219       && !(DECL_ABSTRACT_ORIGIN (found)
 220            && DECL_CONTEXT (DECL_ABSTRACT_ORIGIN (found)) != current_function_decl))
 221     {
 222       DECL_NAME (result) = DECL_NAME (found);
 223       DECL_SOURCE_LOCATION (result) = DECL_SOURCE_LOCATION (found);
 224       DECL_ABSTRACT_ORIGIN (result) = DECL_ABSTRACT_ORIGIN (found);
 225     }
 226
 227   TREE_ADDRESSABLE (result) |= TREE_ADDRESSABLE (found);
 228
 229   /* Now walk through the function changing all references to VAR to be
 230      RESULT.  */
 231   data.var = found;
 232   data.result = result;
 233   FOR_EACH_BB (bb)
 234     {
 235       for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); )
 236         {
 237           gimple stmt = gsi_stmt (gsi);
 238           /* If this is a copy from VAR to RESULT, remove it.  */
 239           if (gimple_assign_copy_p (stmt)
 240               && gimple_assign_lhs (stmt) == result
 241               && gimple_assign_rhs1 (stmt) == found)
 242             {
 243               unlink_stmt_vdef (stmt);
 244               gsi_remove (&gsi, true);
 245               release_defs (stmt);
 246             }
 247           else
 248             {
 249               struct walk_stmt_info wi;
 250               memset (&wi, 0, sizeof (wi));
 251               wi.info = &data;
 252               data.modified = 0;
 253               walk_gimple_op (stmt, finalize_nrv_r, &wi);
 254               if (data.modified)
 255                 update_stmt (stmt);
 256               gsi_next (&gsi);
 257             }
 258         }
 259     }
 260
 261   SET_DECL_VALUE_EXPR (found, result);
 262   DECL_HAS_VALUE_EXPR_P (found) = 1;
 263
 264   return 0;
 265 }
 266
 267 static bool
 268 gate_pass_return_slot (void)
 269 {
 270   return optimize > 0;
 271 }
 272
 273 struct gimple_opt_pass pass_nrv =
 274 {
 275  {
 276   GIMPLE_PASS,
 277   "nrv",                                /* name */
 278   gate_pass_return_slot,                /* gate */
 279   tree_nrv,                             /* execute */
 280   NULL,                                 /* sub */
 281   NULL,                                 /* next */
 282   0,                                    /* static_pass_number */
 283   TV_TREE_NRV,                          /* tv_id */
 284   PROP_ssa | PROP_cfg,                          /* properties_required */
 285   0,                                    /* properties_provided */
 286   0,                                    /* properties_destroyed */
 287   0,                                    /* todo_flags_start */
 288   TODO_ggc_collect                      /* todo_flags_finish */
 289  }
 290 };
 291
 292 /* Determine (pessimistically) whether DEST is available for NRV
 293    optimization, where DEST is expected to be the LHS of a modify
 294    expression where the RHS is a function returning an aggregate.
 295
 296    DEST is available if it is not clobbered or used by the call.  */
 297
 298 static bool
 299 dest_safe_for_nrv_p (gimple call)
 300 {
 301   tree dest = gimple_call_lhs (call);
 302
 303   dest = get_base_address (dest);
 304   if (! dest)
 305     return false;
 306
 307   if (TREE_CODE (dest) == SSA_NAME)
 308     return true;
 309
 310   if (call_may_clobber_ref_p (call, dest)
 311       || ref_maybe_used_by_stmt_p (call, dest))
 312     return false;
 313
 314   return true;
 315 }
 316
 317 /* Walk through the function looking for GIMPLE_ASSIGNs with calls that
 318    return in memory on the RHS.  For each of these, determine whether it is
 319    safe to pass the address of the LHS as the return slot, and mark the
 320    call appropriately if so.
 321
 322    The NRV shares the return slot with a local variable in the callee; this
 323    optimization shares the return slot with the target of the call within
 324    the caller.  If the NRV is performed (which we can't know in general),
 325    this optimization is safe if the address of the target has not
 326    escaped prior to the call.  If it has, modifications to the local
 327    variable will produce visible changes elsewhere, as in PR c++/19317.  */
 328
 329 static unsigned int
 330 execute_return_slot_opt (void)
 331 {
 332   basic_block bb;
 333
 334   FOR_EACH_BB (bb)
 335     {
 336       gimple_stmt_iterator gsi;
 337       for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 338         {
 339           gimple stmt = gsi_stmt (gsi);
 340           bool slot_opt_p;
 341
 342           if (is_gimple_call (stmt)
 343               && gimple_call_lhs (stmt)
 344               && !gimple_call_return_slot_opt_p (stmt)
 345               && aggregate_value_p (TREE_TYPE (gimple_call_lhs (stmt)),
 346                                     gimple_call_fndecl (stmt)))
 347             {
 348               /* Check if the location being assigned to is
 349                  clobbered by the call.  */
 350               slot_opt_p = dest_safe_for_nrv_p (stmt);
 351               gimple_call_set_return_slot_opt (stmt, slot_opt_p);
 352             }
 353         }
 354     }
 355   return 0;
 356 }
 357
 358 struct gimple_opt_pass pass_return_slot =
 359 {
 360  {
 361   GIMPLE_PASS,
 362   "retslot",                            /* name */
 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 };