Merge tree-ssa-20020619-branch into mainline.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@81764 138bc75d-0d04-0410-961f-82ee72b054a4

1 files changed, 217 insertions, 0 deletions

diff --git a/gcc/tree-nrv.c b/gcc/tree-nrv.c
new file mode 100644
index 00000000000..2c344967d66
--- /dev/null
+++ b/gcc/tree-nrv.c
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+/* Language independent return value optimizations
+   Copyright (C) 2004 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 2, 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 COPYING.  If not, write to
+the Free Software Foundation, 59 Temple Place - Suite 330,
+Boston, MA 02111-1307, USA.  */
+
+#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 occurences
+     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;
+  /* If this is a RETURN_EXPR, then set the expression being returned
+     to RESULT.  */
+  else if (TREE_CODE (*tp) == RETURN_EXPR)
+    TREE_OPERAND (*tp, 0) = dp->result;
+  /* Replace all occurences 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 void
+tree_nrv (void)
+{
+  tree result = DECL_RESULT (current_function_decl);
+  tree result_type = TREE_TYPE (result);
+  tree found = NULL;
+  basic_block bb;
+  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;
+
+  /* Look through each block for suitable return expressions.   RETURN_EXPRs
+     end basic blocks, so we only have to look at the last statement in
+     each block.  That makes this very fast.  */
+  FOR_EACH_BB (bb)
+    {
+      tree stmt = last_stmt (bb);
+
+      if (stmt && TREE_CODE (stmt) == RETURN_EXPR)
+	{
+	  tree ret_expr = TREE_OPERAND (stmt, 0);
+
+	  /* This probably should not happen, but just to be safe do
+	     not perform NRV optimizations if only some of the return
+	     statement return a value.  */
+	  if (!ret_expr
+	      || TREE_CODE (ret_expr) != MODIFY_EXPR
+	      || TREE_CODE (TREE_OPERAND (ret_expr, 0)) != RESULT_DECL)
+	    return;
+
+	  /* 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 != TREE_OPERAND (ret_expr, 1))
+		return;
+	    }
+	  else
+	    found = TREE_OPERAND (ret_expr, 1);
+
+	  /* 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
+	      || 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;
+	}
+    }
+
+  if (!found)
+    return;
+
+  /* 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)
+    {
+      block_stmt_iterator bsi;
+
+      for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi))
+	walk_tree (bsi_stmt_ptr (bsi), finalize_nrv_r, &data, 0);
+    }
+
+  /* FOUND is no longer used.  Ensure it gets removed.  */
+  var_ann (found)->used = 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 */
+};