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111
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1 /* Localize comdats.
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145
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2 Copyright (C) 2014-2020 Free Software Foundation, Inc.
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111
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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 it under
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7 the terms of the GNU General Public License as published by the Free
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8 Software Foundation; either version 3, or (at your option) any later
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9 version.
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10
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11 GCC is distributed in the hope that it will be useful, but WITHOUT ANY
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12 WARRANTY; without even the implied warranty of MERCHANTABILITY or
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13 FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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14 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 /* This is very simple pass that looks for static symbols that are used
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21 exclusively by symbol within one comdat group. In this case it makes
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22 sense to bring the symbol itself into the group to avoid dead code
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23 that would arrise when the comdat group from current unit is replaced
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24 by a different copy. Consider for example:
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25
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26 static int q(void)
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27 {
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28 ....
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29 }
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30 inline int t(void)
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31 {
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32 return q();
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33 }
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34
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35 if Q is used only by T, it makes sense to put Q into T's comdat group.
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36
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37 The pass solve simple dataflow across the callgraph trying to prove what
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38 symbols are used exclusively from a given comdat group.
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39
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40 The implementation maintains a queue linked by AUX pointer terminated by
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41 pointer value 1. Lattice values are NULL for TOP, actual comdat group, or
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42 ERROR_MARK_NODE for bottom.
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43
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44 TODO: When symbol is used only by comdat symbols, but from different groups,
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45 it would make sense to produce a new comdat group for it with anonymous name.
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46
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47 TODO2: We can't mix variables and functions within one group. Currently
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48 we just give up on references of symbols of different types. We also should
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49 handle this by anonymous comdat group section. */
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50
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51 #include "config.h"
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52 #include "system.h"
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53 #include "coretypes.h"
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54 #include "tm.h"
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55 #include "tree.h"
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56 #include "tree-pass.h"
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57 #include "cgraph.h"
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58
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59 /* Main dataflow loop propagating comdat groups across
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60 the symbol table. All references to SYMBOL are examined
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61 and NEWGROUP is updated accordingly. MAP holds current lattice
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62 values for individual symbols. */
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63
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64 tree
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65 propagate_comdat_group (struct symtab_node *symbol,
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66 tree newgroup, hash_map<symtab_node *, tree> &map)
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67 {
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68 int i;
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69 struct ipa_ref *ref;
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70
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71 /* Walk all references to SYMBOL, recursively dive into aliases. */
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72
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73 for (i = 0;
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74 symbol->iterate_referring (i, ref)
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75 && newgroup != error_mark_node; i++)
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76 {
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77 struct symtab_node *symbol2 = ref->referring;
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78
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79 if (ref->use == IPA_REF_ALIAS)
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80 {
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81 newgroup = propagate_comdat_group (symbol2, newgroup, map);
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82 continue;
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83 }
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84
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85 /* One COMDAT group cannot hold both variables and functions at
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86 a same time. For now we just go to BOTTOM, in future we may
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87 invent special comdat groups for this case. */
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88
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89 if (symbol->type != symbol2->type)
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90 {
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91 newgroup = error_mark_node;
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92 break;
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93 }
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94
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95 /* If we see inline clone, its comdat group actually
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96 corresponds to the comdat group of the function it is inlined
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97 to. */
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98
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99 if (cgraph_node * cn = dyn_cast <cgraph_node *> (symbol2))
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100 {
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145
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101 if (cn->inlined_to)
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102 symbol2 = cn->inlined_to;
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103 }
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104
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105 /* The actual merge operation. */
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106
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107 tree *val2 = map.get (symbol2);
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108
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109 if (val2 && *val2 != newgroup)
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110 {
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111 if (!newgroup)
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112 newgroup = *val2;
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113 else
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114 newgroup = error_mark_node;
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115 }
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116 }
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117
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118 /* If we analyze function, walk also callers. */
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119
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120 cgraph_node *cnode = dyn_cast <cgraph_node *> (symbol);
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121
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122 if (cnode)
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123 for (struct cgraph_edge * edge = cnode->callers;
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124 edge && newgroup != error_mark_node; edge = edge->next_caller)
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125 {
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126 struct symtab_node *symbol2 = edge->caller;
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127
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128 if (cgraph_node * cn = dyn_cast <cgraph_node *> (symbol2))
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129 {
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130 /* Thunks cannot call across section boundary. */
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131 if (cn->thunk.thunk_p)
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132 newgroup = propagate_comdat_group (symbol2, newgroup, map);
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133 /* If we see inline clone, its comdat group actually
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134 corresponds to the comdat group of the function it
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135 is inlined to. */
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136 if (cn->inlined_to)
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137 symbol2 = cn->inlined_to;
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138 }
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139
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140 /* The actual merge operation. */
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141
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142 tree *val2 = map.get (symbol2);
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143
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144 if (val2 && *val2 != newgroup)
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145 {
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146 if (!newgroup)
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147 newgroup = *val2;
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148 else
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149 newgroup = error_mark_node;
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150 }
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151 }
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152 return newgroup;
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153 }
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154
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155
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156 /* Add all references of SYMBOL that are defined into queue started by FIRST
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157 and linked by AUX pointer (unless they are already enqueued).
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158 Walk recursively inlined functions. */
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159
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160 void
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161 enqueue_references (symtab_node **first,
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162 symtab_node *symbol)
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163 {
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164 int i;
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165 struct ipa_ref *ref = NULL;
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166
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167 for (i = 0; symbol->iterate_reference (i, ref); i++)
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168 {
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169 symtab_node *node = ref->referred->ultimate_alias_target ();
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170
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171 /* Always keep thunks in same sections as target function. */
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172 if (is_a <cgraph_node *>(node))
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173 node = dyn_cast <cgraph_node *> (node)->function_symbol ();
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174 if (!node->aux && node->definition)
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175 {
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176 node->aux = *first;
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177 *first = node;
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178 }
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179 }
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180
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181 if (cgraph_node *cnode = dyn_cast <cgraph_node *> (symbol))
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182 {
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183 struct cgraph_edge *edge;
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184
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185 for (edge = cnode->callees; edge; edge = edge->next_callee)
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186 if (!edge->inline_failed)
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187 enqueue_references (first, edge->callee);
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188 else
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189 {
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190 symtab_node *node = edge->callee->ultimate_alias_target ();
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191
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192 /* Always keep thunks in same sections as target function. */
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193 if (is_a <cgraph_node *>(node))
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194 node = dyn_cast <cgraph_node *> (node)->function_symbol ();
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195 if (!node->aux && node->definition)
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196 {
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197 node->aux = *first;
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198 *first = node;
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199 }
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200 }
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201 }
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202 }
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203
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204 /* Set comdat group of SYMBOL to GROUP.
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205 Callback for for_node_and_aliases. */
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206
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207 bool
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208 set_comdat_group (symtab_node *symbol,
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209 void *head_p)
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210 {
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211 symtab_node *head = (symtab_node *)head_p;
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212
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213 gcc_assert (!symbol->get_comdat_group ());
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131
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214 if (symbol->real_symbol_p ())
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215 {
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216 symbol->set_comdat_group (head->get_comdat_group ());
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217 symbol->add_to_same_comdat_group (head);
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218 }
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219 return false;
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220 }
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221
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222 /* Set comdat group of SYMBOL to GROUP.
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223 Callback for for_node_thunks_and_aliases. */
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224
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225 bool
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226 set_comdat_group_1 (cgraph_node *symbol,
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227 void *head_p)
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228 {
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229 return set_comdat_group (symbol, head_p);
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230 }
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231
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232 /* The actual pass with the main dataflow loop. */
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233
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234 static unsigned int
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235 ipa_comdats (void)
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236 {
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237 hash_map<symtab_node *, tree> map (251);
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238 hash_map<tree, symtab_node *> comdat_head_map (251);
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239 symtab_node *symbol;
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240 bool comdat_group_seen = false;
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241 symtab_node *first = (symtab_node *) (void *) 1;
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242 tree group;
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243
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244 /* Start the dataflow by assigning comdat group to symbols that are in comdat
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245 groups already. All other externally visible symbols must stay, we use
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246 ERROR_MARK_NODE as bottom for the propagation. */
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247
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248 FOR_EACH_DEFINED_SYMBOL (symbol)
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249 if (!symbol->real_symbol_p ())
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250 ;
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251 else if ((group = symbol->get_comdat_group ()) != NULL)
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252 {
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253 map.put (symbol, group);
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254 comdat_head_map.put (group, symbol);
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255 comdat_group_seen = true;
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256
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257 /* Mark the symbol so we won't waste time visiting it for dataflow. */
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258 symbol->aux = (symtab_node *) (void *) 1;
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259 }
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260 /* See symbols that cannot be privatized to comdats; that is externally
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261 visible symbols or otherwise used ones. We also do not want to mangle
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262 user section names. */
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263 else if (symbol->externally_visible
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264 || symbol->force_output
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265 || symbol->used_from_other_partition
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266 || TREE_THIS_VOLATILE (symbol->decl)
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267 || symbol->get_section ()
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268 || (TREE_CODE (symbol->decl) == FUNCTION_DECL
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269 && (DECL_STATIC_CONSTRUCTOR (symbol->decl)
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270 || DECL_STATIC_DESTRUCTOR (symbol->decl))))
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271 {
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272 symtab_node *target = symbol->ultimate_alias_target ();
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273
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274 /* Always keep thunks in same sections as target function. */
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275 if (is_a <cgraph_node *>(target))
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276 target = dyn_cast <cgraph_node *> (target)->function_symbol ();
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277 map.put (target, error_mark_node);
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278
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279 /* Mark the symbol so we won't waste time visiting it for dataflow. */
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280 symbol->aux = (symtab_node *) (void *) 1;
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281 }
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282 else
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283 {
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284 /* Enqueue symbol for dataflow. */
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285 symbol->aux = first;
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286 first = symbol;
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287 }
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288
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289 if (!comdat_group_seen)
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290 {
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291 FOR_EACH_DEFINED_SYMBOL (symbol)
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292 symbol->aux = NULL;
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293 return 0;
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294 }
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295
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296 /* The actual dataflow. */
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297
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298 while (first != (void *) 1)
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299 {
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300 tree group = NULL;
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301 tree newgroup, *val;
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302
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303 symbol = first;
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304 first = (symtab_node *)first->aux;
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305
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306 /* Get current lattice value of SYMBOL. */
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307 val = map.get (symbol);
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308 if (val)
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309 group = *val;
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310
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311 /* If it is bottom, there is nothing to do; do not clear AUX
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312 so we won't re-queue the symbol. */
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313 if (group == error_mark_node)
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314 continue;
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315
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316 newgroup = propagate_comdat_group (symbol, group, map);
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317
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318 /* If nothing changed, proceed to next symbol. */
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319 if (newgroup == group)
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320 {
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321 symbol->aux = NULL;
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322 continue;
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323 }
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324
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325 /* Update lattice value and enqueue all references for re-visiting. */
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326 gcc_assert (newgroup);
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327 if (val)
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328 *val = newgroup;
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329 else
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330 map.put (symbol, newgroup);
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331 enqueue_references (&first, symbol);
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332
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333 /* We may need to revisit the symbol unless it is BOTTOM. */
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334 if (newgroup != error_mark_node)
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335 symbol->aux = NULL;
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336 }
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337
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338 /* Finally assign symbols to the sections. */
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339
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340 FOR_EACH_DEFINED_SYMBOL (symbol)
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341 {
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342 struct cgraph_node *fun;
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343 symbol->aux = NULL;
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344 if (!symbol->get_comdat_group ()
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345 && !symbol->alias
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346 && (!(fun = dyn_cast <cgraph_node *> (symbol))
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347 || !fun->thunk.thunk_p)
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348 && symbol->real_symbol_p ())
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349 {
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350 tree *val = map.get (symbol);
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351
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352 /* A NULL here means that SYMBOL is unreachable in the definition
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353 of ipa-comdats. Either ipa-comdats is wrong about this or someone
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354 forgot to cleanup and remove unreachable functions earlier. */
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355 gcc_assert (val);
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356
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357 tree group = *val;
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358
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359 if (group == error_mark_node)
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360 continue;
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361 if (dump_file)
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362 {
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363 fprintf (dump_file, "Localizing symbol\n");
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364 symbol->dump (dump_file);
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365 fprintf (dump_file, "To group: %s\n", IDENTIFIER_POINTER (group));
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366 }
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367 if (is_a <cgraph_node *> (symbol))
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368 dyn_cast <cgraph_node *>(symbol)->call_for_symbol_thunks_and_aliases
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369 (set_comdat_group_1,
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370 *comdat_head_map.get (group),
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371 true);
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372 else
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373 symbol->call_for_symbol_and_aliases
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374 (set_comdat_group,
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375 *comdat_head_map.get (group),
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376 true);
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377 }
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378 }
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379 return 0;
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380 }
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381
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382 namespace {
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383
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384 const pass_data pass_data_ipa_comdats =
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385 {
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386 IPA_PASS, /* type */
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387 "comdats", /* name */
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388 OPTGROUP_NONE, /* optinfo_flags */
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389 TV_IPA_COMDATS, /* tv_id */
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390 0, /* properties_required */
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391 0, /* properties_provided */
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392 0, /* properties_destroyed */
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393 0, /* todo_flags_start */
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394 0, /* todo_flags_finish */
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395 };
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396
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397 class pass_ipa_comdats : public ipa_opt_pass_d
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398 {
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399 public:
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400 pass_ipa_comdats (gcc::context *ctxt)
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401 : ipa_opt_pass_d (pass_data_ipa_comdats, ctxt,
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402 NULL, /* generate_summary */
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403 NULL, /* write_summary */
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404 NULL, /* read_summary */
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405 NULL, /* write_optimization_summary */
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406 NULL, /* read_optimization_summary */
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407 NULL, /* stmt_fixup */
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408 0, /* function_transform_todo_flags_start */
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409 NULL, /* function_transform */
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410 NULL) /* variable_transform */
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411 {}
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412
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413 /* opt_pass methods: */
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414 virtual bool gate (function *);
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415 virtual unsigned int execute (function *) { return ipa_comdats (); }
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416
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417 }; // class pass_ipa_comdats
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418
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419 bool
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420 pass_ipa_comdats::gate (function *)
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421 {
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422 return HAVE_COMDAT_GROUP;
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423 }
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424
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425 } // anon namespace
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426
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427 ipa_opt_pass_d *
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428 make_pass_ipa_comdats (gcc::context *ctxt)
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429 {
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430 return new pass_ipa_comdats (ctxt);
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431 }
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