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0
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1 /* Language independent return value optimizations
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2 Copyright (C) 2004, 2005, 2007, 2008 Free Software Foundation, Inc.
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3
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4 This file is part of GCC.
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5
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6 GCC is free software; you can redistribute it and/or modify
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7 it under the terms of the GNU General Public License as published by
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8 the Free Software Foundation; either version 3, or (at your option)
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9 any later version.
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10
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11 GCC is distributed in the hope that it will be useful,
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12 but WITHOUT ANY WARRANTY; without even the implied warranty of
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13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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14 GNU General Public License for more details.
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15
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16 You should have received a copy of the GNU General Public License
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17 along with GCC; see the file COPYING3. If not see
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18 <http://www.gnu.org/licenses/>. */
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19
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20 #include "config.h"
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21 #include "system.h"
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22 #include "coretypes.h"
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23 #include "tm.h"
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24 #include "tree.h"
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25 #include "rtl.h"
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26 #include "function.h"
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27 #include "basic-block.h"
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28 #include "expr.h"
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29 #include "diagnostic.h"
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30 #include "tree-flow.h"
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31 #include "timevar.h"
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32 #include "tree-dump.h"
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33 #include "tree-pass.h"
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34 #include "langhooks.h"
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35
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36 /* This file implements return value optimizations for functions which
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37 return aggregate types.
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38
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39 Basically this pass searches the function for return statements which
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40 return a local aggregate. When converted to RTL such statements will
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41 generate a copy from the local aggregate to final return value destination
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42 mandated by the target's ABI.
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43
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44 That copy can often be avoided by directly constructing the return value
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45 into the final destination mandated by the target's ABI.
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46
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47 This is basically a generic equivalent to the C++ front-end's
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48 Named Return Value optimization. */
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49
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50 struct nrv_data
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51 {
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52 /* This is the temporary (a VAR_DECL) which appears in all of
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53 this function's RETURN_EXPR statements. */
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54 tree var;
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55
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56 /* This is the function's RESULT_DECL. We will replace all occurrences
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57 of VAR with RESULT_DECL when we apply this optimization. */
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58 tree result;
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59 };
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60
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61 static tree finalize_nrv_r (tree *, int *, void *);
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62
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63 /* Callback for the tree walker.
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64
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65 If TP refers to a RETURN_EXPR, then set the expression being returned
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66 to nrv_data->result.
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67
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68 If TP refers to nrv_data->var, then replace nrv_data->var with
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69 nrv_data->result.
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70
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71 If we reach a node where we know all the subtrees are uninteresting,
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72 then set *WALK_SUBTREES to zero. */
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73
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74 static tree
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75 finalize_nrv_r (tree *tp, int *walk_subtrees, void *data)
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76 {
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77 struct walk_stmt_info *wi = (struct walk_stmt_info *) data;
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78 struct nrv_data *dp = (struct nrv_data *) wi->info;
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79
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80 /* No need to walk into types. */
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81 if (TYPE_P (*tp))
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82 *walk_subtrees = 0;
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83
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84 /* Otherwise replace all occurrences of VAR with RESULT. */
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85 else if (*tp == dp->var)
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86 *tp = dp->result;
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87
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88 /* Keep iterating. */
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89 return NULL_TREE;
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90 }
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91
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92 /* Main entry point for return value optimizations.
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93
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94 If this function always returns the same local variable, and that
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95 local variable is an aggregate type, then replace the variable with
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96 the function's DECL_RESULT.
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97
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98 This is the equivalent of the C++ named return value optimization
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99 applied to optimized trees in a language independent form. If we
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100 ever encounter languages which prevent this kind of optimization,
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101 then we could either have the languages register the optimization or
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102 we could change the gating function to check the current language. */
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103
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104 static unsigned int
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105 tree_nrv (void)
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106 {
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107 tree result = DECL_RESULT (current_function_decl);
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108 tree result_type = TREE_TYPE (result);
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109 tree found = NULL;
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110 basic_block bb;
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111 gimple_stmt_iterator gsi;
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112 struct nrv_data data;
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113
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114 /* If this function does not return an aggregate type in memory, then
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115 there is nothing to do. */
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116 if (!aggregate_value_p (result, current_function_decl))
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117 return 0;
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118
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119 /* If a GIMPLE type is returned in memory, finalize_nrv_r might create
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120 non-GIMPLE. */
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121 if (is_gimple_reg_type (result_type))
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122 return 0;
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123
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124 /* If the front end already did something like this, don't do it here. */
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125 if (DECL_NAME (result))
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126 return 0;
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127
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128 /* Look through each block for assignments to the RESULT_DECL. */
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129 FOR_EACH_BB (bb)
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130 {
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131 for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
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132 {
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133 gimple stmt = gsi_stmt (gsi);
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134 tree ret_val;
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135
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136 if (gimple_code (stmt) == GIMPLE_RETURN)
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137 {
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138 /* In a function with an aggregate return value, the
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139 gimplifier has changed all non-empty RETURN_EXPRs to
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140 return the RESULT_DECL. */
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141 ret_val = gimple_return_retval (stmt);
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142 if (ret_val)
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143 gcc_assert (ret_val == result);
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144 }
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145 else if (gimple_has_lhs (stmt)
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146 && gimple_get_lhs (stmt) == result)
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147 {
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148 tree rhs;
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149
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150 if (!gimple_assign_copy_p (stmt))
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151 return 0;
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152
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153 rhs = gimple_assign_rhs1 (stmt);
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154
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155 /* Now verify that this return statement uses the same value
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156 as any previously encountered return statement. */
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157 if (found != NULL)
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158 {
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159 /* If we found a return statement using a different variable
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160 than previous return statements, then we can not perform
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161 NRV optimizations. */
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162 if (found != rhs)
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163 return 0;
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164 }
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165 else
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166 found = rhs;
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167
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168 /* The returned value must be a local automatic variable of the
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169 same type and alignment as the function's result. */
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170 if (TREE_CODE (found) != VAR_DECL
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171 || TREE_THIS_VOLATILE (found)
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172 || DECL_CONTEXT (found) != current_function_decl
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173 || TREE_STATIC (found)
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174 || TREE_ADDRESSABLE (found)
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175 || DECL_ALIGN (found) > DECL_ALIGN (result)
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176 || !useless_type_conversion_p (result_type,
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177 TREE_TYPE (found)))
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178 return 0;
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179 }
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180 else if (gimple_has_lhs (stmt))
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181 {
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182 tree addr = get_base_address (gimple_get_lhs (stmt));
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183 /* If there's any MODIFY of component of RESULT,
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184 then bail out. */
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185 if (addr && addr == result)
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186 return 0;
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187 }
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188 }
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189 }
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190
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191 if (!found)
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192 return 0;
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193
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194 /* If dumping details, then note once and only the NRV replacement. */
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195 if (dump_file && (dump_flags & TDF_DETAILS))
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196 {
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197 fprintf (dump_file, "NRV Replaced: ");
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198 print_generic_expr (dump_file, found, dump_flags);
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199 fprintf (dump_file, " with: ");
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200 print_generic_expr (dump_file, result, dump_flags);
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201 fprintf (dump_file, "\n");
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202 }
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203
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204 /* At this point we know that all the return statements return the
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205 same local which has suitable attributes for NRV. Copy debugging
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206 information from FOUND to RESULT if it will be useful. But don't set
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207 DECL_ABSTRACT_ORIGIN to point at another function. */
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208 if (!DECL_IGNORED_P (found)
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209 && !(DECL_ABSTRACT_ORIGIN (found)
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210 && DECL_CONTEXT (DECL_ABSTRACT_ORIGIN (found)) != current_function_decl))
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211 {
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212 DECL_NAME (result) = DECL_NAME (found);
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213 DECL_SOURCE_LOCATION (result) = DECL_SOURCE_LOCATION (found);
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214 DECL_ABSTRACT_ORIGIN (result) = DECL_ABSTRACT_ORIGIN (found);
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215 }
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216
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217 TREE_ADDRESSABLE (result) = TREE_ADDRESSABLE (found);
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218
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219 /* Now walk through the function changing all references to VAR to be
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220 RESULT. */
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221 data.var = found;
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222 data.result = result;
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223 FOR_EACH_BB (bb)
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224 {
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225 for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); )
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226 {
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227 gimple stmt = gsi_stmt (gsi);
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228 /* If this is a copy from VAR to RESULT, remove it. */
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229 if (gimple_assign_copy_p (stmt)
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230 && gimple_assign_lhs (stmt) == result
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231 && gimple_assign_rhs1 (stmt) == found)
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232 gsi_remove (&gsi, true);
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233 else
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234 {
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235 struct walk_stmt_info wi;
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236 memset (&wi, 0, sizeof (wi));
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237 wi.info = &data;
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238 walk_gimple_op (stmt, finalize_nrv_r, &wi);
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239 gsi_next (&gsi);
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240 }
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241 }
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242 }
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243
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244 /* FOUND is no longer used. Ensure it gets removed. */
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245 var_ann (found)->used = 0;
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246 return 0;
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247 }
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248
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249 static bool
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250 gate_pass_return_slot (void)
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251 {
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252 return optimize > 0;
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253 }
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254
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255 struct gimple_opt_pass pass_nrv =
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256 {
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257 {
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258 GIMPLE_PASS,
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259 "nrv", /* name */
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260 gate_pass_return_slot, /* gate */
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261 tree_nrv, /* execute */
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262 NULL, /* sub */
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263 NULL, /* next */
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264 0, /* static_pass_number */
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265 TV_TREE_NRV, /* tv_id */
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266 PROP_cfg, /* properties_required */
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267 0, /* properties_provided */
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268 0, /* properties_destroyed */
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269 0, /* todo_flags_start */
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270 TODO_dump_func | TODO_ggc_collect /* todo_flags_finish */
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271 }
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272 };
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273
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274 /* Determine (pessimistically) whether DEST is available for NRV
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275 optimization, where DEST is expected to be the LHS of a modify
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276 expression where the RHS is a function returning an aggregate.
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277
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278 We search for a base VAR_DECL and look to see if it is call clobbered.
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279 Note that we could do better, for example, by
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280 attempting to doing points-to analysis on INDIRECT_REFs. */
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281
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282 static bool
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283 dest_safe_for_nrv_p (tree dest)
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284 {
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285 while (handled_component_p (dest))
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286 dest = TREE_OPERAND (dest, 0);
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287
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288 if (! SSA_VAR_P (dest))
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289 return false;
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290
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291 if (TREE_CODE (dest) == SSA_NAME)
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292 dest = SSA_NAME_VAR (dest);
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293
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294 if (is_call_used (dest))
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295 return false;
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296
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297 return true;
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298 }
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299
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300 /* Walk through the function looking for GIMPLE_ASSIGNs with calls that
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301 return in memory on the RHS. For each of these, determine whether it is
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302 safe to pass the address of the LHS as the return slot, and mark the
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303 call appropriately if so.
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304
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305 The NRV shares the return slot with a local variable in the callee; this
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306 optimization shares the return slot with the target of the call within
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307 the caller. If the NRV is performed (which we can't know in general),
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308 this optimization is safe if the address of the target has not
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309 escaped prior to the call. If it has, modifications to the local
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310 variable will produce visible changes elsewhere, as in PR c++/19317. */
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311
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312 static unsigned int
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313 execute_return_slot_opt (void)
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314 {
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315 basic_block bb;
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316
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317 FOR_EACH_BB (bb)
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318 {
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319 gimple_stmt_iterator gsi;
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320 for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
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321 {
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322 gimple stmt = gsi_stmt (gsi);
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323 bool slot_opt_p;
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324
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325 if (is_gimple_call (stmt)
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326 && gimple_call_lhs (stmt)
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327 && !gimple_call_return_slot_opt_p (stmt)
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328 && aggregate_value_p (TREE_TYPE (gimple_call_lhs (stmt)),
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329 gimple_call_fndecl (stmt))
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330 )
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331 {
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332 /* Check if the location being assigned to is
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333 call-clobbered. */
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334 slot_opt_p = dest_safe_for_nrv_p (gimple_call_lhs (stmt));
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335 gimple_call_set_return_slot_opt (stmt, slot_opt_p);
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336 }
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337 }
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338 }
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339 return 0;
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340 }
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341
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342 struct gimple_opt_pass pass_return_slot =
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343 {
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344 {
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345 GIMPLE_PASS,
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346 "retslot", /* name */
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347 NULL, /* gate */
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348 execute_return_slot_opt, /* execute */
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349 NULL, /* sub */
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350 NULL, /* next */
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351 0, /* static_pass_number */
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352 0, /* tv_id */
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353 PROP_ssa | PROP_alias, /* properties_required */
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354 0, /* properties_provided */
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355 0, /* properties_destroyed */
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356 0, /* todo_flags_start */
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357 0 /* todo_flags_finish */
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358 }
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359 };
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