text: /* Language independent return value optimizations
     Copyright (C) 2004, 2005, 2007, 2008, 2009 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;
    int modified;
  };
  
  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 walk_stmt_info *wi = (struct walk_stmt_info *) data;
    struct nrv_data *dp = (struct nrv_data *) wi->info;
  
    /* 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;
        dp->modified = 1;
      }
  
    /* 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;
    gimple_stmt_iterator gsi;
    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;
  
    /* If a GIMPLE type is returned in memory, finalize_nrv_r might create
       non-GIMPLE.  */
    if (is_gimple_reg_type (result_type))
      return 0;
  
    /* If the front end already did something like this, don't do it here.  */
    if (DECL_NAME (result))
      return 0;
  
    /* If the result has its address taken then it might be modified
       by means not detected in the following loop.  Bail out in this
       case.  */
    if (TREE_ADDRESSABLE (result))
      return 0;
  
    /* Look through each block for assignments to the RESULT_DECL.  */
    FOR_EACH_BB (bb)
      {
        for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
  	{
  	  gimple stmt = gsi_stmt (gsi);
  	  tree ret_val;
  
  	  if (gimple_code (stmt) == GIMPLE_RETURN)
  	    {
  	      /* In a function with an aggregate return value, the
  		 gimplifier has changed all non-empty RETURN_EXPRs to
  		 return the RESULT_DECL.  */
  	      ret_val = gimple_return_retval (stmt);
  	      if (ret_val)
  		gcc_assert (ret_val == result);
  	    }
  	  else if (gimple_has_lhs (stmt)
  		   && gimple_get_lhs (stmt) == result)
  	    {
                tree rhs;
  
  	      if (!gimple_assign_copy_p (stmt))
  		return 0;
  
  	      rhs = gimple_assign_rhs1 (stmt);
  
  	      /* 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 != rhs)
  		    return 0;
  		}
  	      else
  		found = rhs;
  
  	      /* 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)
  		  || !useless_type_conversion_p (result_type,
  						 TREE_TYPE (found)))
  		return 0;
  	    }
  	  else if (gimple_has_lhs (stmt))
  	    {
  	      tree addr = get_base_address (gimple_get_lhs (stmt));
  	       /* 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 if it will be useful.  But don't set
       DECL_ABSTRACT_ORIGIN to point at another function.  */
    if (!DECL_IGNORED_P (found)
        && !(DECL_ABSTRACT_ORIGIN (found)
  	   && DECL_CONTEXT (DECL_ABSTRACT_ORIGIN (found)) != current_function_decl))
      {
        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 (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); )
  	{
  	  gimple stmt = gsi_stmt (gsi);
  	  /* If this is a copy from VAR to RESULT, remove it.  */
  	  if (gimple_assign_copy_p (stmt)
  	      && gimple_assign_lhs (stmt) == result
  	      && gimple_assign_rhs1 (stmt) == found)
  	    {
  	      unlink_stmt_vdef (stmt);
  	      gsi_remove (&gsi, true);
  	    }
  	  else
  	    {
  	      struct walk_stmt_info wi;
  	      memset (&wi, 0, sizeof (wi));
  	      wi.info = &data;
  	      data.modified = 0;
  	      walk_gimple_op (stmt, finalize_nrv_r, &wi);
  	      if (data.modified)
  		update_stmt (stmt);
  	      gsi_next (&gsi);
  	    }
  	}
      }
  
    /* FOUND is no longer used.  Ensure it gets removed.  */
    var_ann (found)->used = 0;
    return 0;
  }
  
  static bool
  gate_pass_return_slot (void)
  {
    return optimize > 0;
  }
  
  struct gimple_opt_pass pass_nrv =
  {
   {
    GIMPLE_PASS,
    "nrv",				/* name */
    gate_pass_return_slot,		/* gate */
    tree_nrv,				/* execute */
    NULL,					/* sub */
    NULL,					/* next */
    0,					/* static_pass_number */
    TV_TREE_NRV,				/* tv_id */
    PROP_ssa | 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 is call 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)
  {
    while (handled_component_p (dest))
      dest = TREE_OPERAND (dest, 0);
  
    if (! SSA_VAR_P (dest))
      return false;
  
    if (TREE_CODE (dest) == SSA_NAME)
      dest = SSA_NAME_VAR (dest);
  
    if (is_call_used (dest))
      return false;
  
    return true;
  }
  
  /* Walk through the function looking for GIMPLE_ASSIGNs 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)
      {
        gimple_stmt_iterator gsi;
        for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
  	{
  	  gimple stmt = gsi_stmt (gsi);
  	  bool slot_opt_p;
  
  	  if (is_gimple_call (stmt)
  	      && gimple_call_lhs (stmt)
  	      && !gimple_call_return_slot_opt_p (stmt)
  	      && aggregate_value_p (TREE_TYPE (gimple_call_lhs (stmt)),
  				    gimple_call_fndecl (stmt))
  	     )
  	    {
  	      /* Check if the location being assigned to is
  	         call-clobbered.  */
  	      slot_opt_p = dest_safe_for_nrv_p (gimple_call_lhs (stmt));
  	      gimple_call_set_return_slot_opt (stmt, slot_opt_p);
  	    }
  	}
      }
    return 0;
  }
  
  struct gimple_opt_pass pass_return_slot =
  {
   {
    GIMPLE_PASS,
    "retslot",				/* name */
    NULL,					/* gate */
    execute_return_slot_opt,		/* execute */
    NULL,					/* sub */
    NULL,					/* next */
    0,					/* static_pass_number */
    TV_NONE,				/* tv_id */
    PROP_ssa,				/* properties_required */
    0,					/* properties_provided */
    0,					/* properties_destroyed */
    0,					/* todo_flags_start */
    0					/* todo_flags_finish */
   }
  };
  