/* Language independent return value optimizations
- Copyright (C) 2004 Free Software Foundation, Inc.
+ 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 2, or (at your option)
+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,
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. */
+along with GCC; see the file COPYING3. If not see
+<http://www.gnu.org/licenses/>. */
#include "config.h"
#include "system.h"
if (TYPE_P (*tp))
*walk_subtrees = 0;
- /* If this is a RETURN_EXPR, set the expression being returned to RESULT. */
- else if (TREE_CODE (*tp) == RETURN_EXPR)
- TREE_OPERAND (*tp, 0) = dp->result;
-
/* Otherwise replace all occurrences of VAR with RESULT. */
else if (*tp == dp->var)
*tp = dp->result;
then we could either have the languages register the optimization or
we could change the gating function to check the current language. */
-static void
+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;
+ return 0;
- /* 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. */
+ /* Look through each block for assignments to the RESULT_DECL. */
FOR_EACH_BB (bb)
{
- tree stmt = last_stmt (bb);
-
- if (stmt && TREE_CODE (stmt) == RETURN_EXPR)
+ for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi))
{
- 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)
+ tree stmt = bsi_stmt (bsi);
+ tree ret_expr;
+
+ if (TREE_CODE (stmt) == RETURN_EXPR)
{
- /* 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;
+ /* 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) == GIMPLE_MODIFY_STMT
+ && GIMPLE_STMT_OPERAND (stmt, 0) == result)
+ {
+ ret_expr = GIMPLE_STMT_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)
+ || !useless_type_conversion_p (result_type,
+ TREE_TYPE (found)))
+ return 0;
+ }
+ else if (TREE_CODE (stmt) == GIMPLE_MODIFY_STMT)
+ {
+ tree addr = get_base_address (GIMPLE_STMT_OPERAND (stmt, 0));
+ /* If there's any MODIFY of component of RESULT,
+ then bail out. */
+ if (addr && addr == result)
+ return 0;
}
- 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;
+ return 0;
/* If dumping details, then note once and only the NRV replacement. */
if (dump_file && (dump_flags & TDF_DETAILS))
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);
+ 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) == GIMPLE_MODIFY_STMT
+ && GIMPLE_STMT_OPERAND (*tp, 0) == result
+ && GIMPLE_STMT_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 =
0, /* properties_provided */
0, /* properties_destroyed */
0, /* todo_flags_start */
- TODO_dump_func | TODO_ggc_collect /* todo_flags_finish */
+ TODO_dump_func | TODO_ggc_collect, /* todo_flags_finish */
+ 0 /* letter */
+};
+
+/* 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)
+{
+ subvar_t subvar;
+
+ 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_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;
+}
+
+/* Walk through the function looking for GIMPLE_MODIFY_STMTs 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) == GIMPLE_MODIFY_STMT
+ && (call = GIMPLE_STMT_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 (GIMPLE_STMT_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 */
+ 0 /* letter */
};
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