gcc/tree-nrv.c

   1 /* Language independent return value optimizations
   2    Copyright (C) 2004-2017 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 "backend.h"
  24 #include "tree.h"
  25 #include "gimple.h"
  26 #include "tree-pass.h"
  27 #include "ssa.h"
  28 #include "tree-pretty-print.h"
  29 #include "gimple-iterator.h"
  30 #include "gimple-walk.h"
  31
  32 /* This file implements return value optimizations for functions which
  33    return aggregate types.
  34
  35    Basically this pass searches the function for return statements which
  36    return a local aggregate.  When converted to RTL such statements will
  37    generate a copy from the local aggregate to final return value destination
  38    mandated by the target's ABI.
  39
  40    That copy can often be avoided by directly constructing the return value
  41    into the final destination mandated by the target's ABI.
  42
  43    This is basically a generic equivalent to the C++ front-end's
  44    Named Return Value optimization.  */
  45
  46 struct nrv_data_t
  47 {
  48   /* This is the temporary (a VAR_DECL) which appears in all of
  49      this function's RETURN_EXPR statements.  */
  50   tree var;
  51
  52   /* This is the function's RESULT_DECL.  We will replace all occurrences
  53      of VAR with RESULT_DECL when we apply this optimization.  */
  54   tree result;
  55   int modified;
  56 };
  57
  58 static tree finalize_nrv_r (tree *, int *, void *);
  59
  60 /* Callback for the tree walker.
  61
  62    If TP refers to a RETURN_EXPR, then set the expression being returned
  63    to nrv_data->result.
  64
  65    If TP refers to nrv_data->var, then replace nrv_data->var with
  66    nrv_data->result.
  67
  68    If we reach a node where we know all the subtrees are uninteresting,
  69    then set *WALK_SUBTREES to zero.  */
  70
  71 static tree
  72 finalize_nrv_r (tree *tp, int *walk_subtrees, void *data)
  73 {
  74   struct walk_stmt_info *wi = (struct walk_stmt_info *) data;
  75   struct nrv_data_t *dp = (struct nrv_data_t *) wi->info;
  76
  77   /* No need to walk into types.  */
  78   if (TYPE_P (*tp))
  79     *walk_subtrees = 0;
  80
  81   /* Otherwise replace all occurrences of VAR with RESULT.  */
  82   else if (*tp == dp->var)
  83     {
  84       *tp = dp->result;
  85       dp->modified = 1;
  86     }
  87
  88   /* Keep iterating.  */
  89   return NULL_TREE;
  90 }
  91
  92 /* Main entry point for return value optimizations.
  93
  94    If this function always returns the same local variable, and that
  95    local variable is an aggregate type, then replace the variable with
  96    the function's DECL_RESULT.
  97
  98    This is the equivalent of the C++ named return value optimization
  99    applied to optimized trees in a language independent form.  If we
 100    ever encounter languages which prevent this kind of optimization,
 101    then we could either have the languages register the optimization or
 102    we could change the gating function to check the current language.  */
 103
 104 namespace {
 105
 106 const pass_data pass_data_nrv =
 107 {
 108   GIMPLE_PASS, /* type */
 109   "nrv", /* name */
 110   OPTGROUP_NONE, /* optinfo_flags */
 111   TV_TREE_NRV, /* tv_id */
 112   ( PROP_ssa | PROP_cfg ), /* properties_required */
 113   0, /* properties_provided */
 114   0, /* properties_destroyed */
 115   0, /* todo_flags_start */
 116   0, /* todo_flags_finish */
 117 };
 118
 119 class pass_nrv : public gimple_opt_pass
 120 {
 121 public:
 122   pass_nrv (gcc::context *ctxt)
 123     : gimple_opt_pass (pass_data_nrv, ctxt)
 124   {}
 125
 126   /* opt_pass methods: */
 127   virtual bool gate (function *) { return optimize > 0; }
 128
 129   virtual unsigned int execute (function *);
 130
 131 }; // class pass_nrv
 132
 133 unsigned int
 134 pass_nrv::execute (function *fun)
 135 {
 136   tree result = DECL_RESULT (current_function_decl);
 137   tree result_type = TREE_TYPE (result);
 138   tree found = NULL;
 139   basic_block bb;
 140   gimple_stmt_iterator gsi;
 141   struct nrv_data_t data;
 142
 143   /* If this function does not return an aggregate type in memory, then
 144      there is nothing to do.  */
 145   if (!aggregate_value_p (result, current_function_decl))
 146     return 0;
 147
 148   /* If a GIMPLE type is returned in memory, finalize_nrv_r might create
 149      non-GIMPLE.  */
 150   if (is_gimple_reg_type (result_type))
 151     return 0;
 152
 153   /* If the front end already did something like this, don't do it here.  */
 154   if (DECL_NAME (result))
 155     return 0;
 156
 157   /* If the result has its address taken then it might be modified
 158      by means not detected in the following loop.  Bail out in this
 159      case.  */
 160   if (TREE_ADDRESSABLE (result))
 161     return 0;
 162
 163   /* Look through each block for assignments to the RESULT_DECL.  */
 164   FOR_EACH_BB_FN (bb, fun)
 165     {
 166       for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 167         {
 168           gimple *stmt = gsi_stmt (gsi);
 169           tree ret_val;
 170
 171           if (greturn *return_stmt = dyn_cast <greturn *> (stmt))
 172             {
 173               /* In a function with an aggregate return value, the
 174                  gimplifier has changed all non-empty RETURN_EXPRs to
 175                  return the RESULT_DECL.  */
 176               ret_val = gimple_return_retval (return_stmt);
 177               if (ret_val)
 178                 gcc_assert (ret_val == result);
 179             }
 180           else if (gimple_has_lhs (stmt)
 181                    && gimple_get_lhs (stmt) == result)
 182             {
 183               tree rhs;
 184
 185               if (!gimple_assign_copy_p (stmt))
 186                 return 0;
 187
 188               rhs = gimple_assign_rhs1 (stmt);
 189
 190               /* Now verify that this return statement uses the same value
 191                  as any previously encountered return statement.  */
 192               if (found != NULL)
 193                 {
 194                   /* If we found a return statement using a different variable
 195                      than previous return statements, then we can not perform
 196                      NRV optimizations.  */
 197                   if (found != rhs)
 198                     return 0;
 199                 }
 200               else
 201                 found = rhs;
 202
 203               /* The returned value must be a local automatic variable of the
 204                  same type and alignment as the function's result.  */
 205               if (!VAR_P (found)
 206                   || TREE_THIS_VOLATILE (found)
 207                   || !auto_var_in_fn_p (found, current_function_decl)
 208                   || TREE_ADDRESSABLE (found)
 209                   || DECL_ALIGN (found) > DECL_ALIGN (result)
 210                   || !useless_type_conversion_p (result_type,
 211                                                  TREE_TYPE (found)))
 212                 return 0;
 213             }
 214           else if (gimple_has_lhs (stmt))
 215             {
 216               tree addr = get_base_address (gimple_get_lhs (stmt));
 217                /* If there's any MODIFY of component of RESULT,
 218                   then bail out.  */
 219               if (addr && addr == result)
 220                 return 0;
 221             }
 222         }
 223     }
 224
 225   if (!found)
 226     return 0;
 227
 228   /* If dumping details, then note once and only the NRV replacement.  */
 229   if (dump_file && (dump_flags & TDF_DETAILS))
 230     {
 231       fprintf (dump_file, "NRV Replaced: ");
 232       print_generic_expr (dump_file, found, dump_flags);
 233       fprintf (dump_file, "  with: ");
 234       print_generic_expr (dump_file, result, dump_flags);
 235       fprintf (dump_file, "\n");
 236     }
 237
 238   /* At this point we know that all the return statements return the
 239      same local which has suitable attributes for NRV.   Copy debugging
 240      information from FOUND to RESULT if it will be useful.  But don't set
 241      DECL_ABSTRACT_ORIGIN to point at another function.  */
 242   if (!DECL_IGNORED_P (found)
 243       && !(DECL_ABSTRACT_ORIGIN (found)
 244            && DECL_CONTEXT (DECL_ABSTRACT_ORIGIN (found)) != current_function_decl))
 245     {
 246       DECL_NAME (result) = DECL_NAME (found);
 247       DECL_SOURCE_LOCATION (result) = DECL_SOURCE_LOCATION (found);
 248       DECL_ABSTRACT_ORIGIN (result) = DECL_ABSTRACT_ORIGIN (found);
 249     }
 250
 251   TREE_ADDRESSABLE (result) |= TREE_ADDRESSABLE (found);
 252
 253   /* Now walk through the function changing all references to VAR to be
 254      RESULT.  */
 255   data.var = found;
 256   data.result = result;
 257   FOR_EACH_BB_FN (bb, fun)
 258     {
 259       for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); )
 260         {
 261           gimple *stmt = gsi_stmt (gsi);
 262           /* If this is a copy from VAR to RESULT, remove it.  */
 263           if (gimple_assign_copy_p (stmt)
 264               && gimple_assign_lhs (stmt) == result
 265               && gimple_assign_rhs1 (stmt) == found)
 266             {
 267               unlink_stmt_vdef (stmt);
 268               gsi_remove (&gsi, true);
 269               release_defs (stmt);
 270             }
 271           else
 272             {
 273               struct walk_stmt_info wi;
 274               memset (&wi, 0, sizeof (wi));
 275               wi.info = &data;
 276               data.modified = 0;
 277               walk_gimple_op (stmt, finalize_nrv_r, &wi);
 278               if (data.modified)
 279                 update_stmt (stmt);
 280               gsi_next (&gsi);
 281             }
 282         }
 283     }
 284
 285   SET_DECL_VALUE_EXPR (found, result);
 286   DECL_HAS_VALUE_EXPR_P (found) = 1;
 287
 288   return 0;
 289 }
 290
 291 } // anon namespace
 292
 293 gimple_opt_pass *
 294 make_pass_nrv (gcc::context *ctxt)
 295 {
 296   return new pass_nrv (ctxt);
 297 }
 298
 299 /* Determine (pessimistically) whether DEST is available for NRV
 300    optimization, where DEST is expected to be the LHS of a modify
 301    expression where the RHS is a function returning an aggregate.
 302
 303    DEST is available if it is not clobbered or used by the call.  */
 304
 305 static bool
 306 dest_safe_for_nrv_p (gcall *call)
 307 {
 308   tree dest = gimple_call_lhs (call);
 309
 310   dest = get_base_address (dest);
 311   if (! dest)
 312     return false;
 313
 314   if (TREE_CODE (dest) == SSA_NAME)
 315     return true;
 316
 317   if (call_may_clobber_ref_p (call, dest)
 318       || ref_maybe_used_by_stmt_p (call, dest))
 319     return false;
 320
 321   return true;
 322 }
 323
 324 /* Walk through the function looking for GIMPLE_ASSIGNs with calls that
 325    return in memory on the RHS.  For each of these, determine whether it is
 326    safe to pass the address of the LHS as the return slot, and mark the
 327    call appropriately if so.
 328
 329    The NRV shares the return slot with a local variable in the callee; this
 330    optimization shares the return slot with the target of the call within
 331    the caller.  If the NRV is performed (which we can't know in general),
 332    this optimization is safe if the address of the target has not
 333    escaped prior to the call.  If it has, modifications to the local
 334    variable will produce visible changes elsewhere, as in PR c++/19317.  */
 335
 336 namespace {
 337
 338 const pass_data pass_data_return_slot =
 339 {
 340   GIMPLE_PASS, /* type */
 341   "retslot", /* name */
 342   OPTGROUP_NONE, /* optinfo_flags */
 343   TV_NONE, /* tv_id */
 344   PROP_ssa, /* properties_required */
 345   0, /* properties_provided */
 346   0, /* properties_destroyed */
 347   0, /* todo_flags_start */
 348   0, /* todo_flags_finish */
 349 };
 350
 351 class pass_return_slot : public gimple_opt_pass
 352 {
 353 public:
 354   pass_return_slot (gcc::context *ctxt)
 355     : gimple_opt_pass (pass_data_return_slot, ctxt)
 356   {}
 357
 358   /* opt_pass methods: */
 359   virtual unsigned int execute (function *);
 360
 361 }; // class pass_return_slot
 362
 363 unsigned int
 364 pass_return_slot::execute (function *fun)
 365 {
 366   basic_block bb;
 367
 368   FOR_EACH_BB_FN (bb, fun)
 369     {
 370       gimple_stmt_iterator gsi;
 371       for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 372         {
 373           gcall *stmt;
 374           bool slot_opt_p;
 375
 376           stmt = dyn_cast <gcall *> (gsi_stmt (gsi));
 377           if (stmt
 378               && gimple_call_lhs (stmt)
 379               && !gimple_call_return_slot_opt_p (stmt)
 380               && aggregate_value_p (TREE_TYPE (gimple_call_lhs (stmt)),
 381                                     gimple_call_fndecl (stmt)))
 382             {
 383               /* Check if the location being assigned to is
 384                  clobbered by the call.  */
 385               slot_opt_p = dest_safe_for_nrv_p (stmt);
 386               gimple_call_set_return_slot_opt (stmt, slot_opt_p);
 387             }
 388         }
 389     }
 390   return 0;
 391 }
 392
 393 } // anon namespace
 394
 395 gimple_opt_pass *
 396 make_pass_return_slot (gcc::context *ctxt)
 397 {
 398   return new pass_return_slot (ctxt);
 399 }