/* Language independent return value optimizations
- Copyright (C) 2004 Free Software Foundation, Inc.
+ Copyright (C) 2004, 2005 Free Software Foundation, Inc.
This file is part of GCC.
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. */
+the Free Software Foundation, 51 Franklin Street, Fifth Floor,
+Boston, MA 02110-1301, USA. */
#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) == MODIFY_EXPR
+ && TREE_OPERAND (stmt, 0) == result)
+ {
+ ret_expr = TREE_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)
+ || !lang_hooks.types_compatible_p (TREE_TYPE (found),
+ result_type))
+ 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) == MODIFY_EXPR
+ && TREE_OPERAND (*tp, 0) == result
+ && TREE_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 =
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)
+{
+ switch (TREE_CODE (dest))
+ {
+ case VAR_DECL:
+ {
+ subvar_t subvar;
+ 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;
+ }
+ case ARRAY_REF:
+ case COMPONENT_REF:
+ return dest_safe_for_nrv_p (TREE_OPERAND (dest, 0));
+ default:
+ return false;
+ }
+}
+
+/* Walk through the function looking for MODIFY_EXPRs 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) == MODIFY_EXPR
+ && (call = TREE_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 (TREE_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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