gcc/tree-nrv.cc

   1 /* Language independent return value optimizations
   2    Copyright (C) 2004-2023 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 #include "internal-fn.h"
  32
  33 /* This file implements return value optimizations for functions which
  34    return aggregate types.
  35
  36    Basically this pass searches the function for return statements which
  37    return a local aggregate.  When converted to RTL such statements will
  38    generate a copy from the local aggregate to final return value destination
  39    mandated by the target's ABI.
  40
  41    That copy can often be avoided by directly constructing the return value
  42    into the final destination mandated by the target's ABI.
  43
  44    This is basically a generic equivalent to the C++ front-end's
  45    Named Return Value optimization.  */
  46
  47 struct nrv_data_t
  48 {
  49   /* This is the temporary (a VAR_DECL) which appears in all of
  50      this function's RETURN_EXPR statements.  */
  51   tree var;
  52
  53   /* This is the function's RESULT_DECL.  We will replace all occurrences
  54      of VAR with RESULT_DECL when we apply this optimization.  */
  55   tree result;
  56   int modified;
  57 };
  58
  59 static tree finalize_nrv_r (tree *, int *, void *);
  60
  61 /* Callback for the tree walker.
  62
  63    If TP refers to a RETURN_EXPR, then set the expression being returned
  64    to nrv_data->result.
  65
  66    If TP refers to nrv_data->var, then replace nrv_data->var with
  67    nrv_data->result.
  68
  69    If we reach a node where we know all the subtrees are uninteresting,
  70    then set *WALK_SUBTREES to zero.  */
  71
  72 static tree
  73 finalize_nrv_r (tree *tp, int *walk_subtrees, void *data)
  74 {
  75   struct walk_stmt_info *wi = (struct walk_stmt_info *) data;
  76   struct nrv_data_t *dp = (struct nrv_data_t *) wi->info;
  77
  78   /* No need to walk into types.  */
  79   if (TYPE_P (*tp))
  80     *walk_subtrees = 0;
  81
  82   /* Otherwise replace all occurrences of VAR with RESULT.  */
  83   else if (*tp == dp->var)
  84     {
  85       *tp = dp->result;
  86       dp->modified = 1;
  87     }
  88
  89   /* Keep iterating.  */
  90   return NULL_TREE;
  91 }
  92
  93 /* Main entry point for return value optimizations.
  94
  95    If this function always returns the same local variable, and that
  96    local variable is an aggregate type, then replace the variable with
  97    the function's DECL_RESULT.
  98
  99    This is the equivalent of the C++ named return value optimization
 100    applied to optimized trees in a language independent form.  If we
 101    ever encounter languages which prevent this kind of optimization,
 102    then we could either have the languages register the optimization or
 103    we could change the gating function to check the current language.  */
 104
 105 namespace {
 106
 107 const pass_data pass_data_nrv =
 108 {
 109   GIMPLE_PASS, /* type */
 110   "nrv", /* name */
 111   OPTGROUP_NONE, /* optinfo_flags */
 112   TV_TREE_NRV, /* tv_id */
 113   ( PROP_ssa | PROP_cfg ), /* properties_required */
 114   0, /* properties_provided */
 115   0, /* properties_destroyed */
 116   0, /* todo_flags_start */
 117   0, /* todo_flags_finish */
 118 };
 119
 120 class pass_nrv : public gimple_opt_pass
 121 {
 122 public:
 123   pass_nrv (gcc::context *ctxt)
 124     : gimple_opt_pass (pass_data_nrv, ctxt)
 125   {}
 126
 127   /* opt_pass methods: */
 128   bool gate (function *) final override { return optimize > 0; }
 129
 130   unsigned int execute (function *) final override;
 131
 132 }; // class pass_nrv
 133
 134 unsigned int
 135 pass_nrv::execute (function *fun)
 136 {
 137   tree result = DECL_RESULT (current_function_decl);
 138   tree result_type = TREE_TYPE (result);
 139   tree found = NULL;
 140   basic_block bb;
 141   gimple_stmt_iterator gsi;
 142   struct nrv_data_t data;
 143
 144   /* If this function does not return an aggregate type in memory, then
 145      there is nothing to do.  */
 146   if (!aggregate_value_p (result, current_function_decl))
 147     return 0;
 148
 149   /* If a GIMPLE type is returned in memory, finalize_nrv_r might create
 150      non-GIMPLE.  */
 151   if (is_gimple_reg_type (result_type))
 152     return 0;
 153
 154   /* If the front end already did something like this, don't do it here.  */
 155   if (DECL_NAME (result))
 156     return 0;
 157
 158   /* If the result has its address taken then it might be modified
 159      by means not detected in the following loop.  Bail out in this
 160      case.  */
 161   if (TREE_ADDRESSABLE (result))
 162     return 0;
 163
 164   /* Look through each block for assignments to the RESULT_DECL.  */
 165   FOR_EACH_BB_FN (bb, fun)
 166     {
 167       for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 168         {
 169           gimple *stmt = gsi_stmt (gsi);
 170           tree ret_val;
 171
 172           if (greturn *return_stmt = dyn_cast <greturn *> (stmt))
 173             {
 174               /* In a function with an aggregate return value, the
 175                  gimplifier has changed all non-empty RETURN_EXPRs to
 176                  return the RESULT_DECL.  */
 177               ret_val = gimple_return_retval (return_stmt);
 178               if (ret_val)
 179                 gcc_assert (ret_val == result);
 180             }
 181           else if (gimple_has_lhs (stmt)
 182                    && gimple_get_lhs (stmt) == result)
 183             {
 184               tree rhs;
 185
 186               if (!gimple_assign_copy_p (stmt))
 187                 return 0;
 188
 189               rhs = gimple_assign_rhs1 (stmt);
 190
 191               /* Now verify that this return statement uses the same value
 192                  as any previously encountered return statement.  */
 193               if (found != NULL)
 194                 {
 195                   /* If we found a return statement using a different variable
 196                      than previous return statements, then we cannot perform
 197                      NRV optimizations.  */
 198                   if (found != rhs)
 199                     return 0;
 200                 }
 201               else
 202                 found = rhs;
 203
 204               /* The returned value must be a local automatic variable of the
 205                  same type and alignment as the function's result.  */
 206               if (!VAR_P (found)
 207                   || TREE_THIS_VOLATILE (found)
 208                   || !auto_var_in_fn_p (found, current_function_decl)
 209                   || TREE_ADDRESSABLE (found)
 210                   || DECL_ALIGN (found) > DECL_ALIGN (result)
 211                   || !useless_type_conversion_p (result_type,
 212                                                  TREE_TYPE (found)))
 213                 return 0;
 214             }
 215           else if (gimple_has_lhs (stmt))
 216             {
 217               tree addr = get_base_address (gimple_get_lhs (stmt));
 218                /* If there's any MODIFY of component of RESULT,
 219                   then bail out.  */
 220               if (addr && addr == result)
 221                 return 0;
 222             }
 223         }
 224     }
 225
 226   if (!found)
 227     return 0;
 228
 229   /* If dumping details, then note once and only the NRV replacement.  */
 230   if (dump_file && (dump_flags & TDF_DETAILS))
 231     {
 232       fprintf (dump_file, "NRV Replaced: ");
 233       print_generic_expr (dump_file, found, dump_flags);
 234       fprintf (dump_file, "  with: ");
 235       print_generic_expr (dump_file, result, dump_flags);
 236       fprintf (dump_file, "\n");
 237     }
 238
 239   TREE_ADDRESSABLE (result) |= TREE_ADDRESSABLE (found);
 240
 241   /* Now walk through the function changing all references to VAR to be
 242      RESULT.  */
 243   data.var = found;
 244   data.result = result;
 245   FOR_EACH_BB_FN (bb, fun)
 246     {
 247       for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); )
 248         {
 249           gimple *stmt = gsi_stmt (gsi);
 250           /* If this is a copy from VAR to RESULT, remove it.  */
 251           if (gimple_assign_copy_p (stmt)
 252               && gimple_assign_lhs (stmt) == result
 253               && gimple_assign_rhs1 (stmt) == found)
 254             {
 255               unlink_stmt_vdef (stmt);
 256               gsi_remove (&gsi, true);
 257               release_defs (stmt);
 258             }
 259           else
 260             {
 261               struct walk_stmt_info wi;
 262               memset (&wi, 0, sizeof (wi));
 263               wi.info = &data;
 264               data.modified = 0;
 265               walk_gimple_op (stmt, finalize_nrv_r, &wi);
 266               if (data.modified)
 267                 update_stmt (stmt);
 268               gsi_next (&gsi);
 269             }
 270         }
 271     }
 272
 273   SET_DECL_VALUE_EXPR (found, result);
 274   DECL_HAS_VALUE_EXPR_P (found) = 1;
 275
 276   return 0;
 277 }
 278
 279 } // anon namespace
 280
 281 gimple_opt_pass *
 282 make_pass_nrv (gcc::context *ctxt)
 283 {
 284   return new pass_nrv (ctxt);
 285 }
 286
 287 /* Determine (pessimistically) whether DEST is available for NRV
 288    optimization, where DEST is expected to be the LHS of a modify
 289    expression where the RHS is a function returning an aggregate.
 290
 291    DEST is available if it is not clobbered or used by the call.  */
 292
 293 static bool
 294 dest_safe_for_nrv_p (gcall *call)
 295 {
 296   tree dest = gimple_call_lhs (call);
 297
 298   dest = get_base_address (dest);
 299   if (! dest)
 300     return false;
 301
 302   if (TREE_CODE (dest) == SSA_NAME)
 303     return true;
 304
 305   if (call_may_clobber_ref_p (call, dest, false)
 306       || ref_maybe_used_by_stmt_p (call, dest, false))
 307     return false;
 308
 309   return true;
 310 }
 311
 312 /* Walk through the function looking for GIMPLE_ASSIGNs with calls that
 313    return in memory on the RHS.  For each of these, determine whether it is
 314    safe to pass the address of the LHS as the return slot, and mark the
 315    call appropriately if so.
 316
 317    The NRV shares the return slot with a local variable in the callee; this
 318    optimization shares the return slot with the target of the call within
 319    the caller.  If the NRV is performed (which we can't know in general),
 320    this optimization is safe if the address of the target has not
 321    escaped prior to the call.  If it has, modifications to the local
 322    variable will produce visible changes elsewhere, as in PR c++/19317.  */
 323
 324 namespace {
 325
 326 const pass_data pass_data_return_slot =
 327 {
 328   GIMPLE_PASS, /* type */
 329   "retslot", /* name */
 330   OPTGROUP_NONE, /* optinfo_flags */
 331   TV_NONE, /* tv_id */
 332   PROP_ssa, /* properties_required */
 333   0, /* properties_provided */
 334   0, /* properties_destroyed */
 335   0, /* todo_flags_start */
 336   0, /* todo_flags_finish */
 337 };
 338
 339 class pass_return_slot : public gimple_opt_pass
 340 {
 341 public:
 342   pass_return_slot (gcc::context *ctxt)
 343     : gimple_opt_pass (pass_data_return_slot, ctxt)
 344   {}
 345
 346   /* opt_pass methods: */
 347   unsigned int execute (function *) final override;
 348
 349 }; // class pass_return_slot
 350
 351 unsigned int
 352 pass_return_slot::execute (function *fun)
 353 {
 354   basic_block bb;
 355
 356   FOR_EACH_BB_FN (bb, fun)
 357     {
 358       gimple_stmt_iterator gsi;
 359       for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 360         {
 361           gcall *stmt;
 362           bool slot_opt_p;
 363
 364           stmt = dyn_cast <gcall *> (gsi_stmt (gsi));
 365           if (stmt
 366               && gimple_call_lhs (stmt)
 367               && !gimple_call_return_slot_opt_p (stmt)
 368               /* Ignore internal functions, those are expanded specially
 369                  and aggregate_value_p on their result might result in
 370                  undesirable warnings with some backends.  */
 371               && !gimple_call_internal_p (stmt)
 372               && aggregate_value_p (TREE_TYPE (gimple_call_lhs (stmt)),
 373                                     gimple_call_fndecl (stmt)))
 374             {
 375               /* Check if the location being assigned to is
 376                  clobbered by the call.  */
 377               slot_opt_p = dest_safe_for_nrv_p (stmt);
 378               gimple_call_set_return_slot_opt (stmt, slot_opt_p);
 379             }
 380         }
 381     }
 382   return 0;
 383 }
 384
 385 } // anon namespace
 386
 387 gimple_opt_pass *
 388 make_pass_return_slot (gcc::context *ctxt)
 389 {
 390   return new pass_return_slot (ctxt);
 391 }