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/* Language independent return value optimizations

   Copyright (C) 2004, 2005, 2007 Free Software Foundation, Inc.

This file is part of GCC.

GCC is free software; you can redistribute it and/or modify

it under the terms of the GNU General Public License as published by

the Free Software Foundation; either version 3, or (at your option)

any later version.

GCC is distributed in the hope that it will be useful,

but WITHOUT ANY WARRANTY; without even the implied warranty of

MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

GNU General Public License for more details.

You should have received a copy of the GNU General Public License

along with GCC; see the file COPYING3.  If not see

<http://www.gnu.org/licenses/>.  */

#include "config.h"

#include "system.h"

#include "coretypes.h"

#include "tm.h"

#include "tree.h"

#include "rtl.h"

#include "function.h"

#include "basic-block.h"

#include "expr.h"

#include "diagnostic.h"

#include "tree-flow.h"

#include "timevar.h"

#include "tree-dump.h"

#include "tree-pass.h"

#include "langhooks.h"

/* This file implements return value optimizations for functions which

   return aggregate types.

   Basically this pass searches the function for return statements which

   return a local aggregate.  When converted to RTL such statements will

   generate a copy from the local aggregate to final return value destination

   mandated by the target's ABI.

   That copy can often be avoided by directly constructing the return value

   into the final destination mandated by the target's ABI.

   This is basically a generic equivalent to the C++ front-end's

   Named Return Value optimization.  */

struct nrv_data

{

  /* This is the temporary (a VAR_DECL) which appears in all of

     this function's RETURN_EXPR statements.  */

  tree var;

  /* This is the function's RESULT_DECL.  We will replace all occurrences

     of VAR with RESULT_DECL when we apply this optimization.  */

  tree result;

};

static tree finalize_nrv_r (tree *, int *, void *);

/* Callback for the tree walker.

   If TP refers to a RETURN_EXPR, then set the expression being returned

   to nrv_data->result.

   If TP refers to nrv_data->var, then replace nrv_data->var with

   nrv_data->result.

   If we reach a node where we know all the subtrees are uninteresting,

   then set *WALK_SUBTREES to zero.  */

static tree

finalize_nrv_r (tree *tp, int *walk_subtrees, void *data)

{

  struct nrv_data *dp = (struct nrv_data *)data;

  /* No need to walk into types.  */

  if (TYPE_P (*tp))

    *walk_subtrees = 0;

  /* Otherwise replace all occurrences of VAR with RESULT.  */

  else if (*tp == dp->var)

    *tp = dp->result;

  /* Keep iterating.  */

  return NULL_TREE;

}

/* Main entry point for return value optimizations.

   If this function always returns the same local variable, and that

   local variable is an aggregate type, then replace the variable with

   the function's DECL_RESULT.

   This is the equivalent of the C++ named return value optimization

   applied to optimized trees in a language independent form.  If we

   ever encounter languages which prevent this kind of optimization,

   then we could either have the languages register the optimization or

   we could change the gating function to check the current language.  */

static unsigned int

tree_nrv (void)

{

  tree result = DECL_RESULT (current_function_decl);

  tree result_type = TREE_TYPE (result);

  tree found = NULL;

  basic_block bb;

  block_stmt_iterator bsi;

  struct nrv_data data;

  /* If this function does not return an aggregate type in memory, then

     there is nothing to do.  */

  if (!aggregate_value_p (result, current_function_decl))

    return 0;

  /* Look through each block for assignments to the RESULT_DECL.  */

  FOR_EACH_BB (bb)

    {

      for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi))

        {

          tree stmt = bsi_stmt (bsi);

          tree ret_expr;

          if (TREE_CODE (stmt) == RETURN_EXPR)

            {

              /* In a function with an aggregate return value, the

                 gimplifier has changed all non-empty RETURN_EXPRs to

                 return the RESULT_DECL.  */

              ret_expr = TREE_OPERAND (stmt, 0);

              if (ret_expr)

                gcc_assert (ret_expr == result);

            }

          else if (TREE_CODE (stmt) == MODIFY_EXPR

                   && TREE_OPERAND (stmt, 0) == result)

            {

              ret_expr = TREE_OPERAND (stmt, 1);

              /* Now verify that this return statement uses the same value

                 as any previously encountered return statement.  */

              if (found != NULL)

                {

                  /* If we found a return statement using a different variable

                     than previous return statements, then we can not perform

                     NRV optimizations.  */

                  if (found != ret_expr)

                    return 0;

                }

              else

                found = ret_expr;

              /* The returned value must be a local automatic variable of the

                 same type and alignment as the function's result.  */

              if (TREE_CODE (found) != VAR_DECL

                  || TREE_THIS_VOLATILE (found)

                  || DECL_CONTEXT (found) != current_function_decl

                  || TREE_STATIC (found)

                  || TREE_ADDRESSABLE (found)

                  || DECL_ALIGN (found) > DECL_ALIGN (result)

                  || !lang_hooks.types_compatible_p (TREE_TYPE (found),

                                                     result_type))

                return 0;

            }

          else if (TREE_CODE (stmt) == MODIFY_EXPR)

            {

              tree addr = get_base_address (TREE_OPERAND (stmt, 0));

               /* If there's any MODIFY of component of RESULT,

                  then bail out.  */

              if (addr && addr == result)

                return 0;

            }

        }

    }

  if (!found)

    return 0;

  /* If dumping details, then note once and only the NRV replacement.  */

  if (dump_file && (dump_flags & TDF_DETAILS))

    {

      fprintf (dump_file, "NRV Replaced: ");

      print_generic_expr (dump_file, found, dump_flags);

      fprintf (dump_file, "  with: ");

      print_generic_expr (dump_file, result, dump_flags);

      fprintf (dump_file, "\n");

    }

  /* At this point we know that all the return statements return the

     same local which has suitable attributes for NRV.   Copy debugging

     information from FOUND to RESULT.  */

  DECL_NAME (result) = DECL_NAME (found);

  DECL_SOURCE_LOCATION (result) = DECL_SOURCE_LOCATION (found);

  DECL_ABSTRACT_ORIGIN (result) = DECL_ABSTRACT_ORIGIN (found);

  TREE_ADDRESSABLE (result) = TREE_ADDRESSABLE (found);

  /* Now walk through the function changing all references to VAR to be

     RESULT.  */

  data.var = found;

  data.result = result;

  FOR_EACH_BB (bb)

    {

      for (bsi = bsi_start (bb); !bsi_end_p (bsi); )

        {

          tree *tp = bsi_stmt_ptr (bsi);

          /* If this is a copy from VAR to RESULT, remove it.  */

          if (TREE_CODE (*tp) == MODIFY_EXPR

              && TREE_OPERAND (*tp, 0) == result

              && TREE_OPERAND (*tp, 1) == found)

            bsi_remove (&bsi, true);

          else

            {

              walk_tree (tp, finalize_nrv_r, &data, 0);

              bsi_next (&bsi);

            }

        }

    }

  /* FOUND is no longer used.  Ensure it gets removed.  */

  var_ann (found)->used = 0;

  return 0;

}

struct tree_opt_pass pass_nrv =

{

  "nrv",                                /* name */

  NULL,                                 /* gate */

  tree_nrv,                             /* execute */

  NULL,                                 /* sub */

  NULL,                                 /* next */

  0,                                     /* static_pass_number */

  TV_TREE_NRV,                          /* tv_id */

  PROP_cfg,                             /* properties_required */

  0,                                     /* properties_provided */

  0,                                     /* properties_destroyed */

  0,                                     /* todo_flags_start */

  TODO_dump_func | TODO_ggc_collect,                    /* todo_flags_finish */

};

/* Determine (pessimistically) whether DEST is available for NRV

   optimization, where DEST is expected to be the LHS of a modify

   expression where the RHS is a function returning an aggregate.

   We search for a base VAR_DECL and look to see if it, or any of its

   subvars are clobbered.  Note that we could do better, for example, by

   attempting to doing points-to analysis on INDIRECT_REFs.  */

static bool

dest_safe_for_nrv_p (tree dest)

{

  switch (TREE_CODE (dest))

    {

      case VAR_DECL:

        {

          subvar_t subvar;

          if (is_call_clobbered (dest))

            return false;

          for (subvar = get_subvars_for_var (dest);

               subvar;

               subvar = subvar->next)

            if (is_call_clobbered (subvar->var))

              return false;

          return true;

        }

      case ARRAY_REF:

      case COMPONENT_REF:

        return dest_safe_for_nrv_p (TREE_OPERAND (dest, 0));

      default:

        return false;

    }

}

/* Walk through the function looking for MODIFY_EXPRs with calls that

   return in memory on the RHS.  For each of these, determine whether it is

   safe to pass the address of the LHS as the return slot, and mark the

   call appropriately if so.

   The NRV shares the return slot with a local variable in the callee; this

   optimization shares the return slot with the target of the call within

   the caller.  If the NRV is performed (which we can't know in general),

   this optimization is safe if the address of the target has not

   escaped prior to the call.  If it has, modifications to the local

   variable will produce visible changes elsewhere, as in PR c++/19317.  */

static unsigned int

execute_return_slot_opt (void)

{

  basic_block bb;

  FOR_EACH_BB (bb)

    {

      block_stmt_iterator i;

      for (i = bsi_start (bb); !bsi_end_p (i); bsi_next (&i))

        {

          tree stmt = bsi_stmt (i);

          tree call;

          if (TREE_CODE (stmt) == MODIFY_EXPR

              && (call = TREE_OPERAND (stmt, 1),

                  TREE_CODE (call) == CALL_EXPR)

              && !CALL_EXPR_RETURN_SLOT_OPT (call)

              && aggregate_value_p (call, call))

            /* Check if the location being assigned to is

               call-clobbered.  */

            CALL_EXPR_RETURN_SLOT_OPT (call) =

              dest_safe_for_nrv_p (TREE_OPERAND (stmt, 0)) ? 1 : 0;

        }

    }

  return 0;

}

struct tree_opt_pass pass_return_slot =

{

  "retslot",                            /* name */

  NULL,                                 /* gate */

  execute_return_slot_opt,              /* execute */

  NULL,                                 /* sub */

  NULL,                                 /* next */

  0,                                     /* static_pass_number */

  0,                                     /* tv_id */

  PROP_ssa | PROP_alias,                /* properties_required */

  0,                                     /* properties_provided */

  0,                                     /* properties_destroyed */

  0,                                     /* todo_flags_start */

  0,                                     /* todo_flags_finish */

};