--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/gcc/tree-nrv.c	Fri Jul 17 14:47:48 2009 +0900
@@ -0,0 +1,359 @@
+/* Language independent return value optimizations
+   Copyright (C) 2004, 2005, 2007, 2008 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 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;
+
+  /* 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;
+
+  /* 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)
+	    gsi_remove (&gsi, true);
+	  else
+	    {
+	      struct walk_stmt_info wi;
+	      memset (&wi, 0, sizeof (wi));
+	      wi.info = &data;
+	      walk_gimple_op (stmt, finalize_nrv_r, &wi);
+	      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_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 */
+  0,					/* tv_id */
+  PROP_ssa | PROP_alias,		/* properties_required */
+  0,					/* properties_provided */
+  0,					/* properties_destroyed */
+  0,					/* todo_flags_start */
+  0					/* todo_flags_finish */
+ }
+};