--- /dev/null
+/* 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 */
+ }
+};
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