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145
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1 /* This file is part of GCC.
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2
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3 GCC is free software; you can redistribute it and/or modify
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4 it under the terms of the GNU General Public License as published by
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5 the Free Software Foundation; either version 3, or (at your option)
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6 any later version.
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7
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8 GCC is distributed in the hope that it will be useful,
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9 but WITHOUT ANY WARRANTY; without even the implied warranty of
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10 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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11 GNU General Public License for more details.
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12
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13 You should have received a copy of the GNU General Public License
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14 along with GCC; see the file COPYING3. If not see
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15 <http://www.gnu.org/licenses/>. */
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16
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17 /* This file contains code aimed at optimizing function generated with the
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18 use of '-msave-restore. The goal is to identify cases where the call
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19 out to the save/restore routines are sub-optimal, and remove the calls
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20 in this case.
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21
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22 As GCC currently makes the choice between using or not using
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23 save/restore early on (during the gimple expand pass) once we have
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24 selected to use save/restore we are stuck with it. */
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25
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26 #define IN_TARGET_CODE 1
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27
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28 #include "config.h"
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29 #include "system.h"
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30 #include "coretypes.h"
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31 #include "tm.h"
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32 #include "rtl.h"
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33 #include "function.h"
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34 #include "memmodel.h"
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35 #include "emit-rtl.h"
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36 #include "target.h"
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37 #include "basic-block.h"
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38 #include "bitmap.h"
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39 #include "df.h"
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40 #include "tree.h"
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41 #include "expr.h"
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42 #include "cfg.h"
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43
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44 /* This file should be included last. */
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45 #include "hard-reg-set.h"
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46
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47 /* Look in the function prologue for a call to the save stub. Ensure that
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48 the instruction is as we expect (see detail below) and if the
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49 instruction matches return a pointer to it. Otherwise, return NULL.
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50
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51 We expect the function prologue to look like this:
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52
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53 (note NOTE_INSN_BASIC_BLOCK)
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54 (insn (parallel [
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55 (unspec_volatile [
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56 (const_int 2 [0x2])
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57 ] UNSPECV_GPR_SAVE)
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58 (clobber (reg:SI 5 t0))
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59 (clobber (reg:SI 6 t1))])
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60 (note NOTE_INSN_PROLOGUE_END)
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61
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62 Between the NOTE_INSN_BASIC_BLOCK and the GPR_SAVE insn we might find
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63 other notes of type NOTE_INSN_DELETED and/or NOTE_INSN_FUNCTION_BEG.
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64
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65 The parameter BODY is updated to point to the first instruction after
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66 the NOTE_INSN_PROLOGUE_END or will be updated to NULL if the prologue
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67 end note was not found. */
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68
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69 static rtx_insn *
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70 riscv_sr_match_prologue (rtx_insn **body)
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71 {
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72 rtx_insn *insn, *bb_note;
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73 *body = NULL;
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74
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75 /* Find the prologue end note. */
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76 for (insn = get_insns (); insn != NULL; insn = NEXT_INSN (insn))
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77 if (NOTE_P (insn) && NOTE_KIND (insn) == NOTE_INSN_PROLOGUE_END)
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78 {
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79 *body = NEXT_INSN (insn);
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80 break;
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81 }
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82
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83 /* If we don't have the prologue end note and at least one instruction
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84 before it, then this function doesn't have the structure we expect. */
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85 if (insn == NULL
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86 || PREV_INSN (insn) == NULL)
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87 return NULL;
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88
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89 /* The INSN is the end of prologue note, before this we expect to find
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90 one real instruction which makes the prologue, and before that we
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91 expect to find some number of notes for deleted instructions, the
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92 beginning of the function, and finally a basicblock beginning. The
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93 following loop checks that this assumption is true. */
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94 for (bb_note = PREV_INSN (PREV_INSN (insn));
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95 bb_note != NULL;
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96 bb_note = PREV_INSN (bb_note))
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97 {
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98 if (!NOTE_P (bb_note))
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99 return NULL;
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100 if (NOTE_KIND (bb_note) == NOTE_INSN_BASIC_BLOCK)
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101 break;
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102 if (NOTE_KIND (bb_note) != NOTE_INSN_DELETED
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103 && NOTE_KIND (bb_note) != NOTE_INSN_FUNCTION_BEG)
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104 return NULL;
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105 }
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106 if (bb_note == NULL)
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107 return NULL;
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108
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109 /* Set INSN to point to the actual interesting prologue instruction. */
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110 insn = PREV_INSN (insn);
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111 if (INSN_P (insn)
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112 && INSN_CODE (insn) == CODE_FOR_gpr_save
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113 /* Check this is a call to _riscv_save_0. */
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114 && GET_CODE (PATTERN (insn)) == PARALLEL
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115 && GET_CODE (XVECEXP (PATTERN (insn), 0, 0)) == UNSPEC_VOLATILE
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116 && (GET_CODE (XVECEXP (XVECEXP (PATTERN (insn), 0, 0), 0, 0))
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117 == CONST_INT)
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118 && INTVAL (XVECEXP (XVECEXP (PATTERN (insn), 0, 0), 0, 0)) == 2)
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119 return insn;
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120
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121 return NULL;
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122 }
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123
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124 /* Find the first instruction in the epilogue of the current function, and
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125 return a pointer to that instruction if, and only if, the epilogue has
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126 the correct structure that would allow us to optimize out the call to
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127 _riscv_restore_0. */
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128
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129 static rtx_insn *
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130 riscv_sr_match_epilogue (void)
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131 {
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132 /* Find the first instruction in the epilogue. */
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133 rtx_insn *insn, *start;
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134 for (insn = get_insns (); insn != NULL; insn = NEXT_INSN (insn))
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135 if (NOTE_P (insn) && NOTE_KIND (insn) == NOTE_INSN_EPILOGUE_BEG)
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136 {
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137 insn = NEXT_INSN (insn);
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138 break;
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139 }
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140 if (insn == NULL)
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141 return NULL;
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142
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143 /* At this point INSN is the first instruction in the epilogue. A
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144 standard epilogue (of the form we expect to handle) consists of the
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145 following instructions:
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146
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147 1. A stack_tiesi or stack_tiedi (for RV32 and RV64 respectively),
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148
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149 2. An optional use instruction for the register holding the return
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150 value. This will be missing in functions with no return value,
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151
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152 3. A gpr_restore instruction, and
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153
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154 4. A jump instruction of type gpr_restore_return. */
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155 start = insn;
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156 if (INSN_CODE (insn) != CODE_FOR_stack_tiesi
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157 && INSN_CODE (insn) != CODE_FOR_stack_tiedi)
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158 return NULL;
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159
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160 insn = NEXT_INSN (insn);
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161 if (INSN_P (insn) && GET_CODE (PATTERN (insn)) == USE)
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162 insn = NEXT_INSN (insn);
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163
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164 if (!INSN_P (insn) || INSN_CODE (insn) != CODE_FOR_gpr_restore)
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165 return NULL;
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166
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167 insn = NEXT_INSN (insn);
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168 if (!INSN_P (insn) || INSN_CODE (insn) != CODE_FOR_gpr_restore_return)
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169 return NULL;
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170
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171 return start;
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172 }
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173
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174 /* Helper for riscv_remove_unneeded_save_restore_calls. If we match the
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175 prologue instructions but not the epilogue then we might have the case
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176 where the epilogue has been optimized out due to a call to a no-return
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177 function. In this case we might be able to remove the prologue too -
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178 that's what this function does. PROLOGUE is the matched prolgoue
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179 instruction, by the time this function returns the progloue instruction
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180 may have been removed. */
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181
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182 static void
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183 check_for_no_return_call (rtx_insn *prologue)
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184 {
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185 /* Check to see if we have the following pattern:
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186
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187 PROLOGUE instruction
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188 NOTE_INSN_PROLOGUE_END
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189 A no-return call instruction
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190
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191 If we do, then we can remove the prologue instruction safely. Remember
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192 that we've already confirmed by this point that the prologue is a call
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193 to riscv_save_0. */
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194
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195 if (dump_file)
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196 fprintf (dump_file,
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197 "Prologue matched, checking for no-return epilogue.\n");
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198
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199 rtx_insn *tmp = NEXT_INSN (prologue);
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200 if (!NOTE_P (tmp) || NOTE_KIND (tmp) != NOTE_INSN_PROLOGUE_END)
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201 return;
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202
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203 /* Skip any extra notes in here, they're most likely just debug. */
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204 do
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205 {
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206 tmp = NEXT_INSN (tmp);
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207 }
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208 while (tmp != NULL && NOTE_P (tmp));
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209
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210 if (tmp == NULL || !INSN_P (tmp))
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211 return;
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212
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213 bool noreturn_p = find_reg_note (tmp, REG_NORETURN, NULL_RTX) != NULL_RTX;
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214 if (!CALL_P (tmp) || !noreturn_p)
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215 return;
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216
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217 if (dump_file)
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218 fprintf (dump_file,
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219 "Prologue call to riscv_save_0 followed by noreturn call, "
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220 "removing prologue.\n");
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221 remove_insn (prologue);
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222 }
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223
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224 /* Entry point called from riscv_reorg to remove some unneeded calls to
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225 the save and restore stubs. This should only be called when
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226 -msave-restore is in use.
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227
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228 We identify some simple cases where the function looks like this:
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229
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230 call t0,__riscv_save_0
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231 <other-code>
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232 call foo
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233 tail __riscv_restore_0
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234
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235 And transform it into something like this:
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236
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237 <other-code>
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238 tail foo
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239
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240 In the above examples, what can appear in <other-code> is pretty
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241 restricted; only caller saved registers can be touched, this prevents
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242 any additional calls (as they would write to 'ra'). */
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243
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244 void
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245 riscv_remove_unneeded_save_restore_calls (void)
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246 {
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247 /* Will point to the first instruction of the function body, after the
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248 prologue end note. */
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249 rtx_insn *body = NULL;
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250
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251 /* Should only be called with -msave-restore is in use. */
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252 gcc_assert (TARGET_SAVE_RESTORE);
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253
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254 /* Match the expected prologue and epilogue patterns. If either of these
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255 fail to match then we abandon our attempt to optimize this function. */
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256 rtx_insn *prologue_matched = riscv_sr_match_prologue (&body);
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257 if (prologue_matched == NULL || body == NULL)
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258 return;
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259
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260 rtx_insn *epilogue_matched = riscv_sr_match_epilogue ();
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261 if (epilogue_matched == NULL)
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262 {
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263 check_for_no_return_call (prologue_matched);
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264 return;
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265 }
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266
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267 if (dump_file)
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268 fprintf (dump_file,
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269 "Could be a candidate for save/restore removal\n");
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270
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271 /* We want to check which registers this function uses. */
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272 df_analyze ();
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273
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274 int call_count = 0;
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275 bool good_use = true;
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276 int epilogue_count = 0;
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277
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278 /* Now examine all of the instructions that make up this function, we're
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279 looking for call instructions and also double checking register usage
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280 while we're at it (see comments below). */
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281 basic_block bb;
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282 FOR_EACH_BB_FN (bb, cfun)
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283 {
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284 rtx_insn *insn;
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285
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286 FOR_BB_INSNS (bb, insn)
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287 {
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288 if (dump_file)
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289 fprintf (dump_file,
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290 "Block %d, Insn %d\n", bb->index, INSN_UID (insn));
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291
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292 /* If we scan the epilogue we will fall foul of our register
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293 usage check below (due to it's use of the return address), so
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294 once we spot we're at the epilogue, just skip the rest of this
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295 block. Scanning the prologue instructions again (if they
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296 match the expected pattern) is harmless. */
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297 if (NOTE_P (insn)
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298 && NOTE_KIND (insn) == NOTE_INSN_EPILOGUE_BEG)
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299 {
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300 ++epilogue_count;
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301 break;
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302 }
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303
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304 if (!INSN_P (insn))
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305 continue;
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306
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307 if (CALL_P (insn))
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308 ++call_count;
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309 else
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310 {
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311 df_ref use;
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312
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313 FOR_EACH_INSN_USE (use, insn)
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314 {
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315 /* If the function makes use of any registers that are
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316 callee saved then we should be saving them in this
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317 function, which would suggest that a call to the save
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318 and restore functions is required. This would seem to
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319 indicate that something has gone wrong above, as we
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320 should only get here if we are saving zero registers.
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321
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322 The one exception to this rule is the return address
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323 register used within a call instruction. We can
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324 optimize a single call within a function (by making it
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325 a tail call), so we skip call instructions here. */
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326 if (!call_used_regs[DF_REF_REGNO (use)])
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327 {
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328 if (dump_file)
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329 fprintf (dump_file,
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330 "Found unsupported use of callee saved "
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331 "register in instruction %d\n",
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332 INSN_UID (insn));
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333 good_use = false;
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334 break;
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335 }
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336 }
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337 if (!good_use)
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338 break;
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339 }
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340 }
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341 }
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342
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343 /* If we used any registers that would indicate a need for a call to a
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344 save/restore stub then don't optimize. */
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345 if (!good_use)
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346 return;
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347
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348 /* If this function has multiple epilogues, then for now we don't try to
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349 optimize it. */
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350 if (epilogue_count != 1)
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351 return;
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352
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353 /* We can only optimize functions containing a single call, any more
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354 would require us to add instructions to store the return address on
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355 the stack (and restore it before we return). We could do this in the
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356 future, but for now we don't. A single call can be transformed into
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357 a tail call reasonably easily. */
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358 if (call_count > 1)
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359 {
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360 if (dump_file)
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361 fprintf (dump_file,
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362 "Found too many call instructions\n");
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363 return;
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364 }
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365
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366 rtx_insn *epilogue_begin_note = PREV_INSN (epilogue_matched);
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367 gcc_assert (NOTE_P (epilogue_begin_note)
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368 && NOTE_KIND (epilogue_begin_note) == NOTE_INSN_EPILOGUE_BEG);
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369
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370 df_finish_pass (false);
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371
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372 /* Find the first instruction before the function epilogue. */
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373 rtx_insn *insn_before_epilogue;
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374 for (insn_before_epilogue = PREV_INSN (epilogue_begin_note);
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375 NOTE_P (insn_before_epilogue);
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376 insn_before_epilogue = PREV_INSN (insn_before_epilogue))
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377 ;
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378
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379 /* Leaf functions will not generate calls to the save/restore stubs, so
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380 there's no need for this optimization there. We know this function
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381 has no more than 1 call (checked above). To convert this single call
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382 into a tail call we rely on the call being the last thing before the
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383 epilogue. */
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384 if (GET_CODE (insn_before_epilogue) != CALL_INSN)
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385 return;
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386
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387 /* The last instruction in this block, just before the epilogue is a
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388 call. We can potentially change this call into a tail call. */
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389 rtx_insn *call = insn_before_epilogue;
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390
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391 /* Transform call in insn to a sibcall, this will only be done if the
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392 last thing in the function is a call. */
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393 rtx callpat = PATTERN (call);
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394 gcc_assert (GET_CODE (callpat) == PARALLEL);
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395
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396 /* Extract from CALLPAT the information we need to build the sibcall. */
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397 rtx target_call = NULL;
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398 rtx tmp_rtx = XVECEXP (callpat, 0, 0);
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399 rtx set_target = NULL;
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400 switch (GET_CODE (tmp_rtx))
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401 {
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402 case CALL:
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403 target_call = tmp_rtx;
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404 break;
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405
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406 case SET:
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407 {
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408 set_target = XEXP (tmp_rtx, 0);
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409 tmp_rtx = XEXP (tmp_rtx, 1);
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410 if (GET_CODE (tmp_rtx) != CALL)
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411 return;
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412 target_call = tmp_rtx;
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413 break;
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414 }
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415
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416 default:
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417 return;
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418 }
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419
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420 rtx target_mem = XEXP (target_call, 0);
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421 if (GET_CODE (target_mem) != MEM)
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422 return;
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423
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424 rtx target = XEXP (target_mem, 0);
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425 if (GET_CODE (target) != SYMBOL_REF && GET_CODE (target) != REG)
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426 return;
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427
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428 /* The sibcall instructions can only use a specific subset of
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429 registers, we're about to (possibly) move a call through a
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430 register from the function body and make it a sibcall. If we're
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431 not using an appropriate register then we can't make this change.
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432
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433 Maybe in some future iteration we could actually scan the
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434 function, find a suitable sibcall register, and switch over the
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435 registers. But we don't do that yet. */
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436 if (GET_CODE (target) == REG
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437 && !SIBCALL_REG_P (REGNO (target)))
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438 return;
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439
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440 rtx sibcall = NULL;
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441 if (set_target != NULL)
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442 sibcall
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443 = gen_sibcall_value_internal (set_target, target, const0_rtx);
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444 else
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445 sibcall = gen_sibcall_internal (target, const0_rtx);
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446
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447 rtx_insn *before_call = PREV_INSN (call);
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|
|
448 remove_insn (call);
|
|
|
449 rtx_insn *insn = emit_call_insn_after_setloc (sibcall, before_call,
|
|
|
450 INSN_LOCATION (call));
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|
|
451 REG_NOTES (insn) = REG_NOTES (call);
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|
|
452 SIBLING_CALL_P (insn) = 1;
|
|
|
453
|
|
|
454 /* Now update the prologue and epilogue to take account of the
|
|
|
455 changes within the function body. */
|
|
|
456 remove_insn (prologue_matched);
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|
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457 remove_insn (NEXT_INSN (NEXT_INSN (NEXT_INSN (epilogue_matched))));
|
|
|
458 remove_insn (NEXT_INSN (NEXT_INSN (epilogue_matched)));
|
|
|
459 remove_insn (NEXT_INSN (epilogue_matched));
|
|
|
460 remove_insn (epilogue_matched);
|
|
|
461
|
|
|
462 if (dump_file)
|
|
|
463 fprintf (dump_file,
|
|
|
464 "Save/restore successfully removed\n");
|
|
|
465 }
|
