Mercurial > hg > CbC > CbC_gcc
annotate gcc/tree-ssa-dse.c @ 55:77e2b8dfacca gcc-4.4.5
update it from 4.4.3 to 4.5.0
| author | ryoma <e075725@ie.u-ryukyu.ac.jp> |
|---|---|
| date | Fri, 12 Feb 2010 23:39:51 +0900 |
| parents | a06113de4d67 |
| children | b7f97abdc517 |
| rev | line source |
|---|---|
| 0 | 1 /* Dead store elimination |
| 2 Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009 | |
| 3 Free Software Foundation, Inc. | |
| 4 | |
| 5 This file is part of GCC. | |
| 6 | |
| 7 GCC is free software; you can redistribute it and/or modify | |
| 8 it under the terms of the GNU General Public License as published by | |
| 9 the Free Software Foundation; either version 3, or (at your option) | |
| 10 any later version. | |
| 11 | |
| 12 GCC is distributed in the hope that it will be useful, | |
| 13 but WITHOUT ANY WARRANTY; without even the implied warranty of | |
| 14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the | |
| 15 GNU General Public License for more details. | |
| 16 | |
| 17 You should have received a copy of the GNU General Public License | |
| 18 along with GCC; see the file COPYING3. If not see | |
| 19 <http://www.gnu.org/licenses/>. */ | |
| 20 | |
| 21 #include "config.h" | |
| 22 #include "system.h" | |
| 23 #include "coretypes.h" | |
| 24 #include "tm.h" | |
| 25 #include "ggc.h" | |
| 26 #include "tree.h" | |
| 27 #include "rtl.h" | |
| 28 #include "tm_p.h" | |
| 29 #include "basic-block.h" | |
| 30 #include "timevar.h" | |
| 31 #include "diagnostic.h" | |
| 32 #include "tree-flow.h" | |
| 33 #include "tree-pass.h" | |
| 34 #include "tree-dump.h" | |
| 35 #include "domwalk.h" | |
| 36 #include "flags.h" | |
| 37 #include "langhooks.h" | |
| 38 | |
| 39 /* This file implements dead store elimination. | |
| 40 | |
| 41 A dead store is a store into a memory location which will later be | |
| 42 overwritten by another store without any intervening loads. In this | |
| 43 case the earlier store can be deleted. | |
| 44 | |
| 45 In our SSA + virtual operand world we use immediate uses of virtual | |
| 46 operands to detect dead stores. If a store's virtual definition | |
| 47 is used precisely once by a later store to the same location which | |
|
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48 post dominates the first store, then the first store is dead. |
| 0 | 49 |
| 50 The single use of the store's virtual definition ensures that | |
| 51 there are no intervening aliased loads and the requirement that | |
| 52 the second load post dominate the first ensures that if the earlier | |
| 53 store executes, then the later stores will execute before the function | |
| 54 exits. | |
| 55 | |
| 56 It may help to think of this as first moving the earlier store to | |
| 57 the point immediately before the later store. Again, the single | |
| 58 use of the virtual definition and the post-dominance relationship | |
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59 ensure that such movement would be safe. Clearly if there are |
| 0 | 60 back to back stores, then the second is redundant. |
| 61 | |
| 62 Reviewing section 10.7.2 in Morgan's "Building an Optimizing Compiler" | |
| 63 may also help in understanding this code since it discusses the | |
| 64 relationship between dead store and redundant load elimination. In | |
| 65 fact, they are the same transformation applied to different views of | |
| 66 the CFG. */ | |
| 67 | |
| 68 | |
| 69 struct dse_global_data | |
| 70 { | |
| 71 /* This is the global bitmap for store statements. | |
| 72 | |
| 73 Each statement has a unique ID. When we encounter a store statement | |
| 74 that we want to record, set the bit corresponding to the statement's | |
| 75 unique ID in this bitmap. */ | |
| 76 bitmap stores; | |
| 77 }; | |
| 78 | |
| 79 /* We allocate a bitmap-per-block for stores which are encountered | |
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80 during the scan of that block. This allows us to restore the |
| 0 | 81 global bitmap of stores when we finish processing a block. */ |
| 82 struct dse_block_local_data | |
| 83 { | |
| 84 bitmap stores; | |
| 85 }; | |
| 86 | |
| 87 static bool gate_dse (void); | |
| 88 static unsigned int tree_ssa_dse (void); | |
| 89 static void dse_initialize_block_local_data (struct dom_walk_data *, | |
| 90 basic_block, | |
| 91 bool); | |
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92 static void dse_enter_block (struct dom_walk_data *, basic_block); |
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93 static void dse_leave_block (struct dom_walk_data *, basic_block); |
| 0 | 94 static void record_voperand_set (bitmap, bitmap *, unsigned int); |
| 95 | |
| 96 /* Returns uid of statement STMT. */ | |
| 97 | |
| 98 static unsigned | |
| 99 get_stmt_uid (gimple stmt) | |
| 100 { | |
| 101 if (gimple_code (stmt) == GIMPLE_PHI) | |
| 102 return SSA_NAME_VERSION (gimple_phi_result (stmt)) | |
| 103 + gimple_stmt_max_uid (cfun); | |
| 104 | |
| 105 return gimple_uid (stmt); | |
| 106 } | |
| 107 | |
| 108 /* Set bit UID in bitmaps GLOBAL and *LOCAL, creating *LOCAL as needed. */ | |
| 109 | |
| 110 static void | |
| 111 record_voperand_set (bitmap global, bitmap *local, unsigned int uid) | |
| 112 { | |
| 113 /* Lazily allocate the bitmap. Note that we do not get a notification | |
| 114 when the block local data structures die, so we allocate the local | |
| 115 bitmap backed by the GC system. */ | |
| 116 if (*local == NULL) | |
| 117 *local = BITMAP_GGC_ALLOC (); | |
| 118 | |
| 119 /* Set the bit in the local and global bitmaps. */ | |
| 120 bitmap_set_bit (*local, uid); | |
| 121 bitmap_set_bit (global, uid); | |
| 122 } | |
| 123 | |
| 124 /* Initialize block local data structures. */ | |
| 125 | |
| 126 static void | |
| 127 dse_initialize_block_local_data (struct dom_walk_data *walk_data, | |
| 128 basic_block bb ATTRIBUTE_UNUSED, | |
| 129 bool recycled) | |
| 130 { | |
| 131 struct dse_block_local_data *bd | |
| 132 = (struct dse_block_local_data *) | |
| 133 VEC_last (void_p, walk_data->block_data_stack); | |
| 134 | |
| 135 /* If we are given a recycled block local data structure, ensure any | |
| 136 bitmap associated with the block is cleared. */ | |
| 137 if (recycled) | |
| 138 { | |
| 139 if (bd->stores) | |
| 140 bitmap_clear (bd->stores); | |
| 141 } | |
| 142 } | |
| 143 | |
| 144 /* A helper of dse_optimize_stmt. | |
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145 Given a GIMPLE_ASSIGN in STMT, find a candidate statement *USE_STMT that |
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146 may prove STMT to be dead. |
| 0 | 147 Return TRUE if the above conditions are met, otherwise FALSE. */ |
| 148 | |
| 149 static bool | |
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150 dse_possible_dead_store_p (gimple stmt, gimple *use_stmt) |
| 0 | 151 { |
| 152 gimple temp; | |
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153 unsigned cnt = 0; |
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154 |
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155 *use_stmt = NULL; |
| 0 | 156 |
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157 /* Find the first dominated statement that clobbers (part of) the |
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158 memory stmt stores to with no intermediate statement that may use |
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159 part of the memory stmt stores. That is, find a store that may |
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160 prove stmt to be a dead store. */ |
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161 temp = stmt; |
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162 do |
| 0 | 163 { |
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164 gimple prev, use_stmt; |
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165 imm_use_iterator ui; |
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166 bool fail = false; |
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167 tree defvar; |
| 0 | 168 |
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169 /* Limit stmt walking to be linear in the number of possibly |
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170 dead stores. */ |
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171 if (++cnt > 256) |
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172 return false; |
| 0 | 173 |
| 174 if (gimple_code (temp) == GIMPLE_PHI) | |
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175 defvar = PHI_RESULT (temp); |
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176 else |
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177 defvar = gimple_vdef (temp); |
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178 prev = temp; |
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179 temp = NULL; |
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180 FOR_EACH_IMM_USE_STMT (use_stmt, ui, defvar) |
| 0 | 181 { |
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182 cnt++; |
| 0 | 183 |
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184 /* In simple cases we can look through PHI nodes, but we |
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185 have to be careful with loops and with memory references |
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186 containing operands that are also operands of PHI nodes. |
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187 See gcc.c-torture/execute/20051110-*.c. */ |
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188 if (gimple_code (use_stmt) == GIMPLE_PHI) |
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189 { |
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190 if (temp |
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191 /* We can look through PHIs to post-dominated regions |
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192 without worrying if the use not also dominates prev |
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193 (in which case it would be a loop PHI with the use |
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194 in a latch block). */ |
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195 || gimple_bb (prev) == gimple_bb (use_stmt) |
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196 || !dominated_by_p (CDI_POST_DOMINATORS, |
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197 gimple_bb (prev), gimple_bb (use_stmt)) |
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198 || dominated_by_p (CDI_DOMINATORS, |
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199 gimple_bb (prev), gimple_bb (use_stmt))) |
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200 { |
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201 fail = true; |
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202 BREAK_FROM_IMM_USE_STMT (ui); |
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203 } |
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204 temp = use_stmt; |
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205 } |
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206 /* If the statement is a use the store is not dead. */ |
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207 else if (ref_maybe_used_by_stmt_p (use_stmt, |
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208 gimple_assign_lhs (stmt))) |
| 0 | 209 { |
| 210 fail = true; | |
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211 BREAK_FROM_IMM_USE_STMT (ui); |
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212 } |
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213 /* If this is a store, remember it or bail out if we have |
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214 multiple ones (the will be in different CFG parts then). */ |
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215 else if (gimple_vdef (use_stmt)) |
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216 { |
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217 if (temp) |
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218 { |
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219 fail = true; |
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220 BREAK_FROM_IMM_USE_STMT (ui); |
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221 } |
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222 temp = use_stmt; |
| 0 | 223 } |
| 224 } | |
| 225 | |
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226 if (fail) |
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227 return false; |
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228 |
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229 /* If we didn't find any definition this means the store is dead |
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230 if it isn't a store to global reachable memory. In this case |
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231 just pretend the stmt makes itself dead. Otherwise fail. */ |
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232 if (!temp) |
| 0 | 233 { |
|
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234 if (is_hidden_global_store (stmt)) |
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235 return false; |
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236 |
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237 temp = stmt; |
| 0 | 238 break; |
| 239 } | |
| 240 } | |
|
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241 /* We deliberately stop on clobbering statements and not only on |
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242 killing ones to make walking cheaper. Otherwise we can just |
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243 continue walking until both stores have equal reference trees. */ |
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244 while (!stmt_may_clobber_ref_p (temp, gimple_assign_lhs (stmt))); |
| 0 | 245 |
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246 if (!is_gimple_assign (temp)) |
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247 return false; |
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248 |
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249 *use_stmt = temp; |
| 0 | 250 |
| 251 return true; | |
| 252 } | |
| 253 | |
| 254 | |
| 255 /* Attempt to eliminate dead stores in the statement referenced by BSI. | |
| 256 | |
| 257 A dead store is a store into a memory location which will later be | |
| 258 overwritten by another store without any intervening loads. In this | |
| 259 case the earlier store can be deleted. | |
| 260 | |
| 261 In our SSA + virtual operand world we use immediate uses of virtual | |
| 262 operands to detect dead stores. If a store's virtual definition | |
| 263 is used precisely once by a later store to the same location which | |
| 264 post dominates the first store, then the first store is dead. */ | |
| 265 | |
| 266 static void | |
|
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267 dse_optimize_stmt (struct dse_global_data *dse_gd, |
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268 struct dse_block_local_data *bd, |
| 0 | 269 gimple_stmt_iterator gsi) |
| 270 { | |
| 271 gimple stmt = gsi_stmt (gsi); | |
| 272 | |
| 273 /* If this statement has no virtual defs, then there is nothing | |
| 274 to do. */ | |
|
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275 if (!gimple_vdef (stmt)) |
| 0 | 276 return; |
| 277 | |
| 278 /* We know we have virtual definitions. If this is a GIMPLE_ASSIGN | |
| 279 that's not also a function call, then record it into our table. */ | |
| 280 if (is_gimple_call (stmt) && gimple_call_fndecl (stmt)) | |
| 281 return; | |
| 282 | |
| 283 if (gimple_has_volatile_ops (stmt)) | |
| 284 return; | |
| 285 | |
| 286 if (is_gimple_assign (stmt)) | |
| 287 { | |
| 288 gimple use_stmt; | |
| 289 | |
|
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290 record_voperand_set (dse_gd->stores, &bd->stores, gimple_uid (stmt)); |
| 0 | 291 |
|
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292 if (!dse_possible_dead_store_p (stmt, &use_stmt)) |
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293 return; |
| 0 | 294 |
| 295 /* If we have precisely one immediate use at this point and the | |
| 296 stores are to the same memory location or there is a chain of | |
| 297 virtual uses from stmt and the stmt which stores to that same | |
| 298 memory location, then we may have found redundant store. */ | |
|
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299 if (bitmap_bit_p (dse_gd->stores, get_stmt_uid (use_stmt)) |
| 0 | 300 && operand_equal_p (gimple_assign_lhs (stmt), |
|
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301 gimple_assign_lhs (use_stmt), 0)) |
| 0 | 302 { |
| 303 /* If use_stmt is or might be a nop assignment, e.g. for | |
| 304 struct { ... } S a, b, *p; ... | |
| 305 b = a; b = b; | |
| 306 or | |
| 307 b = a; b = *p; where p might be &b, | |
| 308 or | |
| 309 *p = a; *p = b; where p might be &b, | |
| 310 or | |
| 311 *p = *u; *p = *v; where p might be v, then USE_STMT | |
| 312 acts as a use as well as definition, so store in STMT | |
| 313 is not dead. */ | |
|
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314 if (stmt != use_stmt |
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315 && !is_gimple_reg (gimple_assign_rhs1 (use_stmt)) |
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316 && !is_gimple_min_invariant (gimple_assign_rhs1 (use_stmt)) |
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317 /* ??? Should {} be invariant? */ |
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318 && gimple_assign_rhs_code (use_stmt) != CONSTRUCTOR |
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319 && refs_may_alias_p (gimple_assign_lhs (use_stmt), |
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320 gimple_assign_rhs1 (use_stmt))) |
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321 return; |
| 0 | 322 |
| 323 if (dump_file && (dump_flags & TDF_DETAILS)) | |
| 324 { | |
| 325 fprintf (dump_file, " Deleted dead store '"); | |
| 326 print_gimple_stmt (dump_file, gsi_stmt (gsi), dump_flags, 0); | |
| 327 fprintf (dump_file, "'\n"); | |
| 328 } | |
| 329 | |
| 330 /* Then we need to fix the operand of the consuming stmt. */ | |
|
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331 unlink_stmt_vdef (stmt); |
| 0 | 332 |
| 333 /* Remove the dead store. */ | |
| 334 gsi_remove (&gsi, true); | |
| 335 | |
| 336 /* And release any SSA_NAMEs set in this statement back to the | |
| 337 SSA_NAME manager. */ | |
| 338 release_defs (stmt); | |
| 339 } | |
| 340 } | |
| 341 } | |
| 342 | |
| 343 /* Record that we have seen the PHIs at the start of BB which correspond | |
| 344 to virtual operands. */ | |
| 345 static void | |
|
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346 dse_record_phi (struct dse_global_data *dse_gd, |
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347 struct dse_block_local_data *bd, |
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348 gimple phi) |
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349 { |
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350 if (!is_gimple_reg (gimple_phi_result (phi))) |
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351 record_voperand_set (dse_gd->stores, &bd->stores, get_stmt_uid (phi)); |
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352 } |
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353 |
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354 static void |
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355 dse_enter_block (struct dom_walk_data *walk_data, basic_block bb) |
| 0 | 356 { |
| 357 struct dse_block_local_data *bd | |
| 358 = (struct dse_block_local_data *) | |
| 359 VEC_last (void_p, walk_data->block_data_stack); | |
| 360 struct dse_global_data *dse_gd | |
| 361 = (struct dse_global_data *) walk_data->global_data; | |
| 362 gimple_stmt_iterator gsi; | |
| 363 | |
|
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364 for (gsi = gsi_last (bb_seq (bb)); !gsi_end_p (gsi); gsi_prev (&gsi)) |
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365 dse_optimize_stmt (dse_gd, bd, gsi); |
| 0 | 366 for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi)) |
|
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367 dse_record_phi (dse_gd, bd, gsi_stmt (gsi)); |
| 0 | 368 } |
| 369 | |
| 370 static void | |
|
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371 dse_leave_block (struct dom_walk_data *walk_data, |
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372 basic_block bb ATTRIBUTE_UNUSED) |
| 0 | 373 { |
| 374 struct dse_block_local_data *bd | |
| 375 = (struct dse_block_local_data *) | |
| 376 VEC_last (void_p, walk_data->block_data_stack); | |
| 377 struct dse_global_data *dse_gd | |
| 378 = (struct dse_global_data *) walk_data->global_data; | |
| 379 bitmap stores = dse_gd->stores; | |
| 380 unsigned int i; | |
| 381 bitmap_iterator bi; | |
| 382 | |
| 383 /* Unwind the stores noted in this basic block. */ | |
| 384 if (bd->stores) | |
| 385 EXECUTE_IF_SET_IN_BITMAP (bd->stores, 0, i, bi) | |
| 386 { | |
| 387 bitmap_clear_bit (stores, i); | |
| 388 } | |
| 389 } | |
| 390 | |
| 391 /* Main entry point. */ | |
| 392 | |
| 393 static unsigned int | |
| 394 tree_ssa_dse (void) | |
| 395 { | |
| 396 struct dom_walk_data walk_data; | |
| 397 struct dse_global_data dse_gd; | |
| 398 | |
| 399 renumber_gimple_stmt_uids (); | |
| 400 | |
| 401 /* We might consider making this a property of each pass so that it | |
| 402 can be [re]computed on an as-needed basis. Particularly since | |
| 403 this pass could be seen as an extension of DCE which needs post | |
| 404 dominators. */ | |
| 405 calculate_dominance_info (CDI_POST_DOMINATORS); | |
|
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406 calculate_dominance_info (CDI_DOMINATORS); |
| 0 | 407 |
| 408 /* Dead store elimination is fundamentally a walk of the post-dominator | |
| 409 tree and a backwards walk of statements within each block. */ | |
| 410 walk_data.dom_direction = CDI_POST_DOMINATORS; | |
| 411 walk_data.initialize_block_local_data = dse_initialize_block_local_data; | |
|
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412 walk_data.before_dom_children = dse_enter_block; |
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413 walk_data.after_dom_children = dse_leave_block; |
| 0 | 414 |
| 415 walk_data.block_local_data_size = sizeof (struct dse_block_local_data); | |
| 416 | |
| 417 /* This is the main hash table for the dead store elimination pass. */ | |
| 418 dse_gd.stores = BITMAP_ALLOC (NULL); | |
| 419 walk_data.global_data = &dse_gd; | |
| 420 | |
| 421 /* Initialize the dominator walker. */ | |
| 422 init_walk_dominator_tree (&walk_data); | |
| 423 | |
| 424 /* Recursively walk the dominator tree. */ | |
| 425 walk_dominator_tree (&walk_data, EXIT_BLOCK_PTR); | |
| 426 | |
| 427 /* Finalize the dominator walker. */ | |
| 428 fini_walk_dominator_tree (&walk_data); | |
| 429 | |
| 430 /* Release the main bitmap. */ | |
| 431 BITMAP_FREE (dse_gd.stores); | |
| 432 | |
| 433 /* For now, just wipe the post-dominator information. */ | |
| 434 free_dominance_info (CDI_POST_DOMINATORS); | |
| 435 return 0; | |
| 436 } | |
| 437 | |
| 438 static bool | |
| 439 gate_dse (void) | |
| 440 { | |
| 441 return flag_tree_dse != 0; | |
| 442 } | |
| 443 | |
|
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444 struct gimple_opt_pass pass_dse = |
| 0 | 445 { |
| 446 { | |
| 447 GIMPLE_PASS, | |
| 448 "dse", /* name */ | |
| 449 gate_dse, /* gate */ | |
| 450 tree_ssa_dse, /* execute */ | |
| 451 NULL, /* sub */ | |
| 452 NULL, /* next */ | |
| 453 0, /* static_pass_number */ | |
| 454 TV_TREE_DSE, /* tv_id */ | |
|
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455 PROP_cfg | PROP_ssa, /* properties_required */ |
| 0 | 456 0, /* properties_provided */ |
| 457 0, /* properties_destroyed */ | |
| 458 0, /* todo_flags_start */ | |
| 459 TODO_dump_func | |
| 460 | TODO_ggc_collect | |
| 461 | TODO_verify_ssa /* todo_flags_finish */ | |
| 462 } | |
| 463 }; | |
| 464 |