gcc/tree-nrv.c

   1 /* Language independent return value optimizations
   2    Copyright (C) 2004-2021 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   virtual bool gate (function *) { return optimize > 0; }
 129
 130   virtual unsigned int execute (function *);
 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   /* At this point we know that all the return statements return the
 240      same local which has suitable attributes for NRV.   Copy debugging
 241      information from FOUND to RESULT if it will be useful.  But don't set
 242      DECL_ABSTRACT_ORIGIN to point at another function.  */
 243   if (!DECL_IGNORED_P (found)
 244       && !(DECL_ABSTRACT_ORIGIN (found)
 245            && DECL_CONTEXT (DECL_ABSTRACT_ORIGIN (found)) != current_function_decl))
 246     {
 247       DECL_NAME (result) = DECL_NAME (found);
 248       DECL_SOURCE_LOCATION (result) = DECL_SOURCE_LOCATION (found);
 249       DECL_ABSTRACT_ORIGIN (result) = DECL_ABSTRACT_ORIGIN (found);
 250     }
 251
 252   TREE_ADDRESSABLE (result) |= TREE_ADDRESSABLE (found);
 253
 254   /* Now walk through the function changing all references to VAR to be
 255      RESULT.  */
 256   data.var = found;
 257   data.result = result;
 258   FOR_EACH_BB_FN (bb, fun)
 259     {
 260       for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); )
 261         {
 262           gimple *stmt = gsi_stmt (gsi);
 263           /* If this is a copy from VAR to RESULT, remove it.  */
 264           if (gimple_assign_copy_p (stmt)
 265               && gimple_assign_lhs (stmt) == result
 266               && gimple_assign_rhs1 (stmt) == found)
 267             {
 268               unlink_stmt_vdef (stmt);
 269               gsi_remove (&gsi, true);
 270               release_defs (stmt);
 271             }
 272           else
 273             {
 274               struct walk_stmt_info wi;
 275               memset (&wi, 0, sizeof (wi));
 276               wi.info = &data;
 277               data.modified = 0;
 278               walk_gimple_op (stmt, finalize_nrv_r, &wi);
 279               if (data.modified)
 280                 update_stmt (stmt);
 281               gsi_next (&gsi);
 282             }
 283         }
 284     }
 285
 286   SET_DECL_VALUE_EXPR (found, result);
 287   DECL_HAS_VALUE_EXPR_P (found) = 1;
 288
 289   return 0;
 290 }
 291
 292 } // anon namespace
 293
 294 gimple_opt_pass *
 295 make_pass_nrv (gcc::context *ctxt)
 296 {
 297   return new pass_nrv (ctxt);
 298 }
 299
 300 /* Determine (pessimistically) whether DEST is available for NRV
 301    optimization, where DEST is expected to be the LHS of a modify
 302    expression where the RHS is a function returning an aggregate.
 303
 304    DEST is available if it is not clobbered or used by the call.  */
 305
 306 static bool
 307 dest_safe_for_nrv_p (gcall *call)
 308 {
 309   tree dest = gimple_call_lhs (call);
 310
 311   dest = get_base_address (dest);
 312   if (! dest)
 313     return false;
 314
 315   if (TREE_CODE (dest) == SSA_NAME)
 316     return true;
 317
 318   if (call_may_clobber_ref_p (call, dest, false)
 319       || ref_maybe_used_by_stmt_p (call, dest, false))
 320     return false;
 321
 322   return true;
 323 }
 324
 325 /* Walk through the function looking for GIMPLE_ASSIGNs with calls that
 326    return in memory on the RHS.  For each of these, determine whether it is
 327    safe to pass the address of the LHS as the return slot, and mark the
 328    call appropriately if so.
 329
 330    The NRV shares the return slot with a local variable in the callee; this
 331    optimization shares the return slot with the target of the call within
 332    the caller.  If the NRV is performed (which we can't know in general),
 333    this optimization is safe if the address of the target has not
 334    escaped prior to the call.  If it has, modifications to the local
 335    variable will produce visible changes elsewhere, as in PR c++/19317.  */
 336
 337 namespace {
 338
 339 const pass_data pass_data_return_slot =
 340 {
 341   GIMPLE_PASS, /* type */
 342   "retslot", /* name */
 343   OPTGROUP_NONE, /* optinfo_flags */
 344   TV_NONE, /* tv_id */
 345   PROP_ssa, /* properties_required */
 346   0, /* properties_provided */
 347   0, /* properties_destroyed */
 348   0, /* todo_flags_start */
 349   0, /* todo_flags_finish */
 350 };
 351
 352 class pass_return_slot : public gimple_opt_pass
 353 {
 354 public:
 355   pass_return_slot (gcc::context *ctxt)
 356     : gimple_opt_pass (pass_data_return_slot, ctxt)
 357   {}
 358
 359   /* opt_pass methods: */
 360   virtual unsigned int execute (function *);
 361
 362 }; // class pass_return_slot
 363
 364 unsigned int
 365 pass_return_slot::execute (function *fun)
 366 {
 367   basic_block bb;
 368
 369   FOR_EACH_BB_FN (bb, fun)
 370     {
 371       gimple_stmt_iterator gsi;
 372       for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 373         {
 374           gcall *stmt;
 375           bool slot_opt_p;
 376
 377           stmt = dyn_cast <gcall *> (gsi_stmt (gsi));
 378           if (stmt
 379               && gimple_call_lhs (stmt)
 380               && !gimple_call_return_slot_opt_p (stmt)
 381               /* Ignore internal functions, those are expanded specially
 382                  and aggregate_value_p on their result might result in
 383                  undesirable warnings with some backends.  */
 384               && !gimple_call_internal_p (stmt)
 385               && aggregate_value_p (TREE_TYPE (gimple_call_lhs (stmt)),
 386                                     gimple_call_fndecl (stmt)))
 387             {
 388               /* Check if the location being assigned to is
 389                  clobbered by the call.  */
 390               slot_opt_p = dest_safe_for_nrv_p (stmt);
 391               gimple_call_set_return_slot_opt (stmt, slot_opt_p);
 392             }
 393         }
 394     }
 395   return 0;
 396 }
 397
 398 } // anon namespace
 399
 400 gimple_opt_pass *
 401 make_pass_return_slot (gcc::context *ctxt)
 402 {
 403   return new pass_return_slot (ctxt);
 404 }