330 lines
9.9 KiB
C
330 lines
9.9 KiB
C
/* Language independent return value optimizations
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Copyright (C) 2004, 2005 Free Software Foundation, Inc.
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This file is part of GCC.
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GCC is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 2, or (at your option)
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any later version.
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GCC is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with GCC; see the file COPYING. If not, write to
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the Free Software Foundation, 51 Franklin Street, Fifth Floor,
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Boston, MA 02110-1301, USA. */
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#include "config.h"
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#include "system.h"
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#include "coretypes.h"
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#include "tm.h"
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#include "tree.h"
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#include "rtl.h"
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#include "function.h"
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#include "basic-block.h"
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#include "expr.h"
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#include "diagnostic.h"
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#include "tree-flow.h"
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#include "timevar.h"
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#include "tree-dump.h"
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#include "tree-pass.h"
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#include "langhooks.h"
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/* This file implements return value optimizations for functions which
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return aggregate types.
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Basically this pass searches the function for return statements which
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return a local aggregate. When converted to RTL such statements will
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generate a copy from the local aggregate to final return value destination
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mandated by the target's ABI.
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That copy can often be avoided by directly constructing the return value
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into the final destination mandated by the target's ABI.
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This is basically a generic equivalent to the C++ front-end's
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Named Return Value optimization. */
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struct nrv_data
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{
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/* This is the temporary (a VAR_DECL) which appears in all of
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this function's RETURN_EXPR statements. */
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tree var;
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/* This is the function's RESULT_DECL. We will replace all occurrences
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of VAR with RESULT_DECL when we apply this optimization. */
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tree result;
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};
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static tree finalize_nrv_r (tree *, int *, void *);
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/* Callback for the tree walker.
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If TP refers to a RETURN_EXPR, then set the expression being returned
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to nrv_data->result.
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If TP refers to nrv_data->var, then replace nrv_data->var with
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nrv_data->result.
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If we reach a node where we know all the subtrees are uninteresting,
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then set *WALK_SUBTREES to zero. */
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static tree
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finalize_nrv_r (tree *tp, int *walk_subtrees, void *data)
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{
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struct nrv_data *dp = (struct nrv_data *)data;
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/* No need to walk into types. */
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if (TYPE_P (*tp))
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*walk_subtrees = 0;
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/* Otherwise replace all occurrences of VAR with RESULT. */
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else if (*tp == dp->var)
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*tp = dp->result;
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/* Keep iterating. */
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return NULL_TREE;
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}
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/* Main entry point for return value optimizations.
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If this function always returns the same local variable, and that
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local variable is an aggregate type, then replace the variable with
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the function's DECL_RESULT.
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This is the equivalent of the C++ named return value optimization
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applied to optimized trees in a language independent form. If we
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ever encounter languages which prevent this kind of optimization,
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then we could either have the languages register the optimization or
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we could change the gating function to check the current language. */
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static unsigned int
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tree_nrv (void)
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{
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tree result = DECL_RESULT (current_function_decl);
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tree result_type = TREE_TYPE (result);
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tree found = NULL;
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basic_block bb;
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block_stmt_iterator bsi;
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struct nrv_data data;
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/* If this function does not return an aggregate type in memory, then
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there is nothing to do. */
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if (!aggregate_value_p (result, current_function_decl))
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return 0;
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/* Look through each block for assignments to the RESULT_DECL. */
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FOR_EACH_BB (bb)
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{
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for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi))
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{
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tree stmt = bsi_stmt (bsi);
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tree ret_expr;
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if (TREE_CODE (stmt) == RETURN_EXPR)
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{
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/* In a function with an aggregate return value, the
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gimplifier has changed all non-empty RETURN_EXPRs to
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return the RESULT_DECL. */
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ret_expr = TREE_OPERAND (stmt, 0);
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if (ret_expr)
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gcc_assert (ret_expr == result);
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}
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else if (TREE_CODE (stmt) == MODIFY_EXPR
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&& TREE_OPERAND (stmt, 0) == result)
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{
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ret_expr = TREE_OPERAND (stmt, 1);
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/* Now verify that this return statement uses the same value
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as any previously encountered return statement. */
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if (found != NULL)
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{
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/* If we found a return statement using a different variable
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than previous return statements, then we can not perform
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NRV optimizations. */
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if (found != ret_expr)
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return 0;
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}
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else
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found = ret_expr;
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/* The returned value must be a local automatic variable of the
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same type and alignment as the function's result. */
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if (TREE_CODE (found) != VAR_DECL
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|| TREE_THIS_VOLATILE (found)
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|| DECL_CONTEXT (found) != current_function_decl
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|| TREE_STATIC (found)
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|| TREE_ADDRESSABLE (found)
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|| DECL_ALIGN (found) > DECL_ALIGN (result)
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|| !lang_hooks.types_compatible_p (TREE_TYPE (found),
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result_type))
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return 0;
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}
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else if (TREE_CODE (stmt) == MODIFY_EXPR)
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{
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tree addr = get_base_address (TREE_OPERAND (stmt, 0));
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/* If there's any MODIFY of component of RESULT,
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then bail out. */
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if (addr && addr == result)
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return 0;
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}
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}
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}
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if (!found)
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return 0;
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/* If dumping details, then note once and only the NRV replacement. */
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if (dump_file && (dump_flags & TDF_DETAILS))
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{
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fprintf (dump_file, "NRV Replaced: ");
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print_generic_expr (dump_file, found, dump_flags);
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fprintf (dump_file, " with: ");
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print_generic_expr (dump_file, result, dump_flags);
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fprintf (dump_file, "\n");
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}
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/* At this point we know that all the return statements return the
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same local which has suitable attributes for NRV. Copy debugging
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information from FOUND to RESULT. */
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DECL_NAME (result) = DECL_NAME (found);
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DECL_SOURCE_LOCATION (result) = DECL_SOURCE_LOCATION (found);
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DECL_ABSTRACT_ORIGIN (result) = DECL_ABSTRACT_ORIGIN (found);
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TREE_ADDRESSABLE (result) = TREE_ADDRESSABLE (found);
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/* Now walk through the function changing all references to VAR to be
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RESULT. */
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data.var = found;
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data.result = result;
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FOR_EACH_BB (bb)
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{
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for (bsi = bsi_start (bb); !bsi_end_p (bsi); )
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{
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tree *tp = bsi_stmt_ptr (bsi);
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/* If this is a copy from VAR to RESULT, remove it. */
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if (TREE_CODE (*tp) == MODIFY_EXPR
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&& TREE_OPERAND (*tp, 0) == result
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&& TREE_OPERAND (*tp, 1) == found)
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bsi_remove (&bsi, true);
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else
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{
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walk_tree (tp, finalize_nrv_r, &data, 0);
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bsi_next (&bsi);
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}
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}
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}
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/* FOUND is no longer used. Ensure it gets removed. */
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var_ann (found)->used = 0;
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return 0;
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}
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struct tree_opt_pass pass_nrv =
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{
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"nrv", /* name */
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NULL, /* gate */
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tree_nrv, /* execute */
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NULL, /* sub */
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NULL, /* next */
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0, /* static_pass_number */
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TV_TREE_NRV, /* tv_id */
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PROP_cfg, /* properties_required */
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0, /* properties_provided */
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0, /* properties_destroyed */
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0, /* todo_flags_start */
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TODO_dump_func | TODO_ggc_collect, /* todo_flags_finish */
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0 /* letter */
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};
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/* Determine (pessimistically) whether DEST is available for NRV
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optimization, where DEST is expected to be the LHS of a modify
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expression where the RHS is a function returning an aggregate.
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We search for a base VAR_DECL and look to see if it, or any of its
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subvars are clobbered. Note that we could do better, for example, by
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attempting to doing points-to analysis on INDIRECT_REFs. */
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static bool
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dest_safe_for_nrv_p (tree dest)
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{
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switch (TREE_CODE (dest))
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{
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case VAR_DECL:
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{
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subvar_t subvar;
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if (is_call_clobbered (dest))
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return false;
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for (subvar = get_subvars_for_var (dest);
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subvar;
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subvar = subvar->next)
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if (is_call_clobbered (subvar->var))
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return false;
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return true;
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}
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case ARRAY_REF:
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case COMPONENT_REF:
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return dest_safe_for_nrv_p (TREE_OPERAND (dest, 0));
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default:
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return false;
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}
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}
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/* Walk through the function looking for MODIFY_EXPRs with calls that
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return in memory on the RHS. For each of these, determine whether it is
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safe to pass the address of the LHS as the return slot, and mark the
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call appropriately if so.
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The NRV shares the return slot with a local variable in the callee; this
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optimization shares the return slot with the target of the call within
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the caller. If the NRV is performed (which we can't know in general),
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this optimization is safe if the address of the target has not
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escaped prior to the call. If it has, modifications to the local
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variable will produce visible changes elsewhere, as in PR c++/19317. */
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static unsigned int
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execute_return_slot_opt (void)
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{
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basic_block bb;
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FOR_EACH_BB (bb)
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{
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block_stmt_iterator i;
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for (i = bsi_start (bb); !bsi_end_p (i); bsi_next (&i))
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{
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tree stmt = bsi_stmt (i);
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tree call;
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if (TREE_CODE (stmt) == MODIFY_EXPR
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&& (call = TREE_OPERAND (stmt, 1),
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TREE_CODE (call) == CALL_EXPR)
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&& !CALL_EXPR_RETURN_SLOT_OPT (call)
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&& aggregate_value_p (call, call))
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/* Check if the location being assigned to is
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call-clobbered. */
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CALL_EXPR_RETURN_SLOT_OPT (call) =
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dest_safe_for_nrv_p (TREE_OPERAND (stmt, 0)) ? 1 : 0;
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}
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}
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return 0;
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}
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struct tree_opt_pass pass_return_slot =
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{
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"retslot", /* name */
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NULL, /* gate */
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execute_return_slot_opt, /* execute */
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NULL, /* sub */
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NULL, /* next */
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0, /* static_pass_number */
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0, /* tv_id */
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PROP_ssa | PROP_alias, /* properties_required */
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0, /* properties_provided */
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0, /* properties_destroyed */
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0, /* todo_flags_start */
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0, /* todo_flags_finish */
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0 /* letter */
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};
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