src/compiler/nir/nir_opt_dead_cf.c

   1 /*
   2  * Copyright © 2014 Connor Abbott
   3  *
   4  * Permission is hereby granted, free of charge, to any person obtaining a
   5  * copy of this software and associated documentation files (the "Software"),
   6  * to deal in the Software without restriction, including without limitation
   7  * the rights to use, copy, modify, merge, publish, distribute, sublicense,
   8  * and/or sell copies of the Software, and to permit persons to whom the
   9  * Software is furnished to do so, subject to the following conditions:
  10  *
  11  * The above copyright notice and this permission notice (including the next
  12  * paragraph) shall be included in all copies or substantial portions of the
  13  * Software.
  14  *
  15  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  16  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  17  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
  18  * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  19  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
  20  * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
  21  * IN THE SOFTWARE.
  22  *
  23  * Authors:
  24  *    Connor Abbott (cwabbott0@gmail.com)
  25  *
  26  */
  27
  28 #include "nir.h"
  29 #include "nir_control_flow.h"
  30
  31 /*
  32  * This file implements an optimization that deletes statically
  33  * unreachable/dead code. In NIR, one way this can happen is when an if
  34  * statement has a constant condition:
  35  *
  36  * if (true) {
  37  *    ...
  38  * }
  39  *
  40  * We delete the if statement and paste the contents of the always-executed
  41  * branch into the surrounding control flow, possibly removing more code if
  42  * the branch had a jump at the end.
  43  *
  44  * Another way is that control flow can end in a jump so that code after it
  45  * never gets executed. In particular, this can happen after optimizing
  46  * something like:
  47  *
  48  * if (true) {
  49  *    ...
  50  *    break;
  51  * }
  52  * ...
  53  *
  54  * We also consider the case where both branches of an if end in a jump, e.g.:
  55  *
  56  * if (...) {
  57  *    break;
  58  * } else {
  59  *    continue;
  60  * }
  61  * ...
  62  *
  63  * Finally, we also handle removing useless loops and ifs, i.e. loops and ifs
  64  * with no side effects and without any definitions that are used
  65  * elsewhere. This case is a little different from the first two in that the
  66  * code is actually run (it just never does anything), but there are similar
  67  * issues with needing to be careful with restarting after deleting the
  68  * cf_node (see dead_cf_list()) so this is a convenient place to remove them.
  69  */
  70
  71 static void
  72 remove_after_cf_node(nir_cf_node *node)
  73 {
  74    nir_cf_node *end = node;
  75    while (!nir_cf_node_is_last(end))
  76       end = nir_cf_node_next(end);
  77
  78    nir_cf_list list;
  79    nir_cf_extract(&list, nir_after_cf_node(node), nir_after_cf_node(end));
  80    nir_cf_delete(&list);
  81 }
  82
  83 static void
  84 opt_constant_if(nir_if *if_stmt, bool condition)
  85 {
  86    /* First, we need to remove any phi nodes after the if by rewriting uses to
  87     * point to the correct source.
  88     */
  89    nir_block *after = nir_cf_node_as_block(nir_cf_node_next(&if_stmt->cf_node));
  90    nir_block *last_block = condition ? nir_if_last_then_block(if_stmt)
  91                                      : nir_if_last_else_block(if_stmt);
  92
  93    nir_foreach_instr_safe(instr, after) {
  94       if (instr->type != nir_instr_type_phi)
  95          break;
  96
  97       nir_phi_instr *phi = nir_instr_as_phi(instr);
  98       nir_ssa_def *def = NULL;
  99       nir_foreach_phi_src(phi_src, phi) {
 100          if (phi_src->pred != last_block)
 101             continue;
 102
 103          assert(phi_src->src.is_ssa);
 104          def = phi_src->src.ssa;
 105       }
 106
 107       assert(def);
 108       assert(phi->dest.is_ssa);
 109       nir_ssa_def_rewrite_uses(&phi->dest.ssa, nir_src_for_ssa(def));
 110       nir_instr_remove(instr);
 111    }
 112
 113    /* The control flow list we're about to paste in may include a jump at the
 114     * end, and in that case we have to delete the rest of the control flow
 115     * list after the if since it's unreachable and the validator will balk if
 116     * we don't.
 117     */
 118
 119    if (!exec_list_is_empty(&last_block->instr_list)) {
 120       nir_instr *last_instr = nir_block_last_instr(last_block);
 121       if (last_instr->type == nir_instr_type_jump)
 122          remove_after_cf_node(&if_stmt->cf_node);
 123    }
 124
 125    /* Finally, actually paste in the then or else branch and delete the if. */
 126    struct exec_list *cf_list = condition ? &if_stmt->then_list
 127                                          : &if_stmt->else_list;
 128
 129    nir_cf_list list;
 130    nir_cf_list_extract(&list, cf_list);
 131    nir_cf_reinsert(&list, nir_after_cf_node(&if_stmt->cf_node));
 132    nir_cf_node_remove(&if_stmt->cf_node);
 133 }
 134
 135 static bool
 136 def_only_used_in_cf_node(nir_ssa_def *def, void *_node)
 137 {
 138    nir_cf_node *node = _node;
 139    assert(node->type == nir_cf_node_loop || node->type == nir_cf_node_if);
 140
 141    nir_block *before = nir_cf_node_as_block(nir_cf_node_prev(node));
 142    nir_block *after = nir_cf_node_as_block(nir_cf_node_next(node));
 143
 144    nir_foreach_use(use, def) {
 145       /* Because NIR is structured, we can easily determine whether or not a
 146        * value escapes a CF node by looking at the block indices of its uses
 147        * to see if they lie outside the bounds of the CF node.
 148        *
 149        * Note: Normally, the uses of a phi instruction are considered to be
 150        * used in the block that is the predecessor of the phi corresponding to
 151        * that use.  If we were computing liveness or something similar, that
 152        * would mean a special case here for phis.  However, we're trying here
 153        * to determine if the SSA def ever escapes the loop.  If it's used by a
 154        * phi that lives outside the loop then it doesn't matter if the
 155        * corresponding predecessor is inside the loop or not because the value
 156        * can go through the phi into the outside world and escape the loop.
 157        */
 158       if (use->parent_instr->block->index <= before->index ||
 159           use->parent_instr->block->index >= after->index)
 160          return false;
 161    }
 162
 163    /* Same check for if-condition uses */
 164    nir_foreach_if_use(use, def) {
 165       nir_block *use_block =
 166          nir_cf_node_as_block(nir_cf_node_prev(&use->parent_if->cf_node));
 167
 168       if (use_block->index <= before->index ||
 169           use_block->index >= after->index)
 170          return false;
 171    }
 172
 173    return true;
 174 }
 175
 176 /*
 177  * Test if a loop node is dead. Such nodes are dead if:
 178  *
 179  * 1) It has no side effects (i.e. intrinsics which could possibly affect the
 180  * state of the program aside from producing an SSA value, indicated by a lack
 181  * of NIR_INTRINSIC_CAN_ELIMINATE).
 182  *
 183  * 2) It has no phi instructions after it, since those indicate values inside
 184  * the node being used after the node.
 185  *
 186  * 3) None of the values defined inside the node is used outside the node,
 187  * i.e. none of the definitions that dominate the node exit are used outside.
 188  *
 189  * If those conditions hold, then the node is dead and can be deleted.
 190  */
 191
 192 static bool
 193 node_is_dead(nir_cf_node *node)
 194 {
 195    assert(node->type == nir_cf_node_loop);
 196
 197    nir_block *after = nir_cf_node_as_block(nir_cf_node_next(node));
 198
 199    /* Quick check if there are any phis that follow this CF node.  If there
 200     * are, then we automatically know it isn't dead.
 201     */
 202    if (!exec_list_is_empty(&after->instr_list) &&
 203        nir_block_first_instr(after)->type == nir_instr_type_phi)
 204       return false;
 205
 206    nir_function_impl *impl = nir_cf_node_get_function(node);
 207    nir_metadata_require(impl, nir_metadata_block_index);
 208
 209    nir_foreach_block_in_cf_node(block, node) {
 210       bool inside_loop = node->type == nir_cf_node_loop;
 211       for (nir_cf_node *n = &block->cf_node;
 212            !inside_loop && n != node; n = n->parent) {
 213          if (n->type == nir_cf_node_loop)
 214             inside_loop = true;
 215       }
 216
 217       nir_foreach_instr(instr, block) {
 218          if (instr->type == nir_instr_type_call)
 219             return false;
 220
 221          /* Return instructions can cause us to skip over other side-effecting
 222           * instructions after the loop, so consider them to have side effects
 223           * here.
 224           *
 225           * When the block is not inside a loop, break and continue might also
 226           * cause a skip.
 227           */
 228          if (instr->type == nir_instr_type_jump &&
 229              (!inside_loop || nir_instr_as_jump(instr)->type == nir_jump_return))
 230             return false;
 231
 232          if (instr->type == nir_instr_type_intrinsic) {
 233             nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);
 234             if (!(nir_intrinsic_infos[intrin->intrinsic].flags &
 235                 NIR_INTRINSIC_CAN_ELIMINATE))
 236                return false;
 237          }
 238
 239          if (!nir_foreach_ssa_def(instr, def_only_used_in_cf_node, node))
 240             return false;
 241       }
 242    }
 243
 244    return true;
 245 }
 246
 247 static bool
 248 dead_cf_block(nir_block *block)
 249 {
 250    nir_if *following_if = nir_block_get_following_if(block);
 251    if (following_if) {
 252       if (!nir_src_is_const(following_if->condition))
 253          return false;
 254
 255       opt_constant_if(following_if, nir_src_as_bool(following_if->condition));
 256       return true;
 257    }
 258
 259    nir_loop *following_loop = nir_block_get_following_loop(block);
 260    if (!following_loop)
 261       return false;
 262
 263    if (!node_is_dead(&following_loop->cf_node))
 264       return false;
 265
 266    nir_cf_node_remove(&following_loop->cf_node);
 267    return true;
 268 }
 269
 270 static bool
 271 dead_cf_list(struct exec_list *list, bool *list_ends_in_jump)
 272 {
 273    bool progress = false;
 274    *list_ends_in_jump = false;
 275
 276    nir_cf_node *prev = NULL;
 277
 278    foreach_list_typed(nir_cf_node, cur, node, list) {
 279       switch (cur->type) {
 280       case nir_cf_node_block: {
 281          nir_block *block = nir_cf_node_as_block(cur);
 282          if (dead_cf_block(block)) {
 283             /* We just deleted the if or loop after this block, so we may have
 284              * deleted the block before or after it -- which one is an
 285              * implementation detail. Therefore, to recover the place we were
 286              * at, we have to use the previous cf_node.
 287              */
 288
 289             if (prev) {
 290                cur = nir_cf_node_next(prev);
 291             } else {
 292                cur = exec_node_data(nir_cf_node, exec_list_get_head(list),
 293                                     node);
 294             }
 295
 296             block = nir_cf_node_as_block(cur);
 297
 298             progress = true;
 299          }
 300
 301          if (nir_block_ends_in_jump(block)) {
 302             *list_ends_in_jump = true;
 303
 304             if (!exec_node_is_tail_sentinel(cur->node.next)) {
 305                remove_after_cf_node(cur);
 306                return true;
 307             }
 308          }
 309
 310          break;
 311       }
 312
 313       case nir_cf_node_if: {
 314          nir_if *if_stmt = nir_cf_node_as_if(cur);
 315          bool then_ends_in_jump, else_ends_in_jump;
 316          progress |= dead_cf_list(&if_stmt->then_list, &then_ends_in_jump);
 317          progress |= dead_cf_list(&if_stmt->else_list, &else_ends_in_jump);
 318
 319          if (then_ends_in_jump && else_ends_in_jump) {
 320             *list_ends_in_jump = true;
 321             nir_block *next = nir_cf_node_as_block(nir_cf_node_next(cur));
 322             if (!exec_list_is_empty(&next->instr_list) ||
 323                 !exec_node_is_tail_sentinel(next->cf_node.node.next)) {
 324                remove_after_cf_node(cur);
 325                return true;
 326             }
 327          }
 328
 329          break;
 330       }
 331
 332       case nir_cf_node_loop: {
 333          nir_loop *loop = nir_cf_node_as_loop(cur);
 334          bool dummy;
 335          progress |= dead_cf_list(&loop->body, &dummy);
 336
 337          nir_block *next = nir_cf_node_as_block(nir_cf_node_next(cur));
 338          if (next->predecessors->entries == 0 &&
 339              (!exec_list_is_empty(&next->instr_list) ||
 340              !exec_node_is_tail_sentinel(next->cf_node.node.next))) {
 341             remove_after_cf_node(cur);
 342             return true;
 343          }
 344          break;
 345       }
 346
 347       default:
 348          unreachable("unknown cf node type");
 349       }
 350
 351       prev = cur;
 352    }
 353
 354    return progress;
 355 }
 356
 357 static bool
 358 opt_dead_cf_impl(nir_function_impl *impl)
 359 {
 360    bool dummy;
 361    bool progress = dead_cf_list(&impl->body, &dummy);
 362
 363    if (progress) {
 364       nir_metadata_preserve(impl, nir_metadata_none);
 365
 366       /* The CF manipulation code called by this pass is smart enough to keep
 367        * from breaking any SSA use/def chains by replacing any uses of removed
 368        * instructions with SSA undefs.  However, it's not quite smart enough
 369        * to always preserve the dominance properties.  In particular, if you
 370        * remove the one break from a loop, stuff in the loop may still be used
 371        * outside the loop even though there's no path between the two.  We can
 372        * easily fix these issues by calling nir_repair_ssa which will ensure
 373        * that the dominance properties hold.
 374        */
 375       nir_repair_ssa_impl(impl);
 376    } else {
 377       nir_metadata_preserve(impl, nir_metadata_all);
 378    }
 379
 380    return progress;
 381 }
 382
 383 bool
 384 nir_opt_dead_cf(nir_shader *shader)
 385 {
 386    bool progress = false;
 387
 388    nir_foreach_function(function, shader)
 389       if (function->impl)
 390          progress |= opt_dead_cf_impl(function->impl);
 391
 392    return progress;
 393 }