src/compiler/nir/nir_opt_gcm.c

   1 /*
   2  * Copyright © 2014 Intel Corporation
   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  *    Jason Ekstrand (jason@jlekstrand.net)
  25  *
  26  */
  27
  28 #include "nir.h"
  29 #include "nir_instr_set.h"
  30
  31 /*
  32  * Implements Global Code Motion.  A description of GCM can be found in
  33  * "Global Code Motion; Global Value Numbering" by Cliff Click.
  34  * Unfortunately, the algorithm presented in the paper is broken in a
  35  * number of ways.  The algorithm used here differs substantially from the
  36  * one in the paper but it is, in my opinion, much easier to read and
  37  * verify correcness.
  38  */
  39
  40 struct gcm_block_info {
  41    /* Number of loops this block is inside */
  42    unsigned loop_depth;
  43
  44    /* The last instruction inserted into this block.  This is used as we
  45     * traverse the instructions and insert them back into the program to
  46     * put them in the right order.
  47     */
  48    nir_instr *last_instr;
  49 };
  50
  51 /* Flags used in the instr->pass_flags field for various instruction states */
  52 enum {
  53    GCM_INSTR_PINNED =            (1 << 0),
  54    GCM_INSTR_SCHEDULED_EARLY =   (1 << 1),
  55    GCM_INSTR_SCHEDULED_LATE =    (1 << 2),
  56    GCM_INSTR_PLACED =            (1 << 3),
  57 };
  58
  59 struct gcm_state {
  60    nir_function_impl *impl;
  61    nir_instr *instr;
  62
  63    /* The list of non-pinned instructions.  As we do the late scheduling,
  64     * we pull non-pinned instructions out of their blocks and place them in
  65     * this list.  This saves us from having linked-list problems when we go
  66     * to put instructions back in their blocks.
  67     */
  68    struct exec_list instrs;
  69
  70    struct gcm_block_info *blocks;
  71 };
  72
  73 /* Recursively walks the CFG and builds the block_info structure */
  74 static void
  75 gcm_build_block_info(struct exec_list *cf_list, struct gcm_state *state,
  76                      unsigned loop_depth)
  77 {
  78    foreach_list_typed(nir_cf_node, node, node, cf_list) {
  79       switch (node->type) {
  80       case nir_cf_node_block: {
  81          nir_block *block = nir_cf_node_as_block(node);
  82          state->blocks[block->index].loop_depth = loop_depth;
  83          break;
  84       }
  85       case nir_cf_node_if: {
  86          nir_if *if_stmt = nir_cf_node_as_if(node);
  87          gcm_build_block_info(&if_stmt->then_list, state, loop_depth);
  88          gcm_build_block_info(&if_stmt->else_list, state, loop_depth);
  89          break;
  90       }
  91       case nir_cf_node_loop: {
  92          nir_loop *loop = nir_cf_node_as_loop(node);
  93          gcm_build_block_info(&loop->body, state, loop_depth + 1);
  94          break;
  95       }
  96       default:
  97          unreachable("Invalid CF node type");
  98       }
  99    }
 100 }
 101
 102 /* Walks the instruction list and marks immovable instructions as pinned
 103  *
 104  * This function also serves to initialize the instr->pass_flags field.
 105  * After this is completed, all instructions' pass_flags fields will be set
 106  * to either GCM_INSTR_PINNED or 0.
 107  */
 108 static bool
 109 gcm_pin_instructions_block(nir_block *block, struct gcm_state *state)
 110 {
 111    nir_foreach_instr_safe(instr, block) {
 112       switch (instr->type) {
 113       case nir_instr_type_alu:
 114          switch (nir_instr_as_alu(instr)->op) {
 115          case nir_op_fddx:
 116          case nir_op_fddy:
 117          case nir_op_fddx_fine:
 118          case nir_op_fddy_fine:
 119          case nir_op_fddx_coarse:
 120          case nir_op_fddy_coarse:
 121             /* These can only go in uniform control flow; pin them for now */
 122             instr->pass_flags = GCM_INSTR_PINNED;
 123             break;
 124
 125          default:
 126             instr->pass_flags = 0;
 127             break;
 128          }
 129          break;
 130
 131       case nir_instr_type_tex:
 132          switch (nir_instr_as_tex(instr)->op) {
 133          case nir_texop_tex:
 134          case nir_texop_txb:
 135          case nir_texop_lod:
 136             /* These two take implicit derivatives so they need to be pinned */
 137             instr->pass_flags = GCM_INSTR_PINNED;
 138             break;
 139
 140          default:
 141             instr->pass_flags = 0;
 142             break;
 143          }
 144          break;
 145
 146       case nir_instr_type_load_const:
 147          instr->pass_flags = 0;
 148          break;
 149
 150       case nir_instr_type_intrinsic: {
 151          const nir_intrinsic_info *info =
 152             &nir_intrinsic_infos[nir_instr_as_intrinsic(instr)->intrinsic];
 153
 154          if ((info->flags & NIR_INTRINSIC_CAN_ELIMINATE) &&
 155              (info->flags & NIR_INTRINSIC_CAN_REORDER)) {
 156             instr->pass_flags = 0;
 157          } else {
 158             instr->pass_flags = GCM_INSTR_PINNED;
 159          }
 160          break;
 161       }
 162
 163       case nir_instr_type_jump:
 164       case nir_instr_type_ssa_undef:
 165       case nir_instr_type_phi:
 166          instr->pass_flags = GCM_INSTR_PINNED;
 167          break;
 168
 169       default:
 170          unreachable("Invalid instruction type in GCM");
 171       }
 172
 173       if (!(instr->pass_flags & GCM_INSTR_PINNED)) {
 174          /* If this is an unpinned instruction, go ahead and pull it out of
 175           * the program and put it on the instrs list.  This has a couple
 176           * of benifits.  First, it makes the scheduling algorithm more
 177           * efficient because we can avoid walking over basic blocks and
 178           * pinned instructions.  Second, it keeps us from causing linked
 179           * list confusion when we're trying to put everything in its
 180           * proper place at the end of the pass.
 181           *
 182           * Note that we don't use nir_instr_remove here because that also
 183           * cleans up uses and defs and we want to keep that information.
 184           */
 185          exec_node_remove(&instr->node);
 186          exec_list_push_tail(&state->instrs, &instr->node);
 187       }
 188    }
 189
 190    return true;
 191 }
 192
 193 static void
 194 gcm_schedule_early_instr(nir_instr *instr, struct gcm_state *state);
 195
 196 /** Update an instructions schedule for the given source
 197  *
 198  * This function is called iteratively as we walk the sources of an
 199  * instruction.  It ensures that the given source instruction has been
 200  * scheduled and then update this instruction's block if the source
 201  * instruction is lower down the tree.
 202  */
 203 static bool
 204 gcm_schedule_early_src(nir_src *src, void *void_state)
 205 {
 206    struct gcm_state *state = void_state;
 207    nir_instr *instr = state->instr;
 208
 209    assert(src->is_ssa);
 210
 211    gcm_schedule_early_instr(src->ssa->parent_instr, void_state);
 212
 213    /* While the index isn't a proper dominance depth, it does have the
 214     * property that if A dominates B then A->index <= B->index.  Since we
 215     * know that this instruction must have been dominated by all of its
 216     * sources at some point (even if it's gone through value-numbering),
 217     * all of the sources must lie on the same branch of the dominance tree.
 218     * Therefore, we can just go ahead and just compare indices.
 219     */
 220    if (instr->block->index < src->ssa->parent_instr->block->index)
 221       instr->block = src->ssa->parent_instr->block;
 222
 223    /* We need to restore the state instruction because it may have been
 224     * changed through the gcm_schedule_early_instr call above.  Since we
 225     * may still be iterating through sources and future calls to
 226     * gcm_schedule_early_src for the same instruction will still need it.
 227     */
 228    state->instr = instr;
 229
 230    return true;
 231 }
 232
 233 /** Schedules an instruction early
 234  *
 235  * This function performs a recursive depth-first search starting at the
 236  * given instruction and proceeding through the sources to schedule
 237  * instructions as early as they can possibly go in the dominance tree.
 238  * The instructions are "scheduled" by updating their instr->block field.
 239  */
 240 static void
 241 gcm_schedule_early_instr(nir_instr *instr, struct gcm_state *state)
 242 {
 243    if (instr->pass_flags & GCM_INSTR_SCHEDULED_EARLY)
 244       return;
 245
 246    instr->pass_flags |= GCM_INSTR_SCHEDULED_EARLY;
 247
 248    /* Pinned instructions are already scheduled so we don't need to do
 249     * anything.  Also, bailing here keeps us from ever following the
 250     * sources of phi nodes which can be back-edges.
 251     */
 252    if (instr->pass_flags & GCM_INSTR_PINNED)
 253       return;
 254
 255    /* Start with the instruction at the top.  As we iterate over the
 256     * sources, it will get moved down as needed.
 257     */
 258    instr->block = nir_start_block(state->impl);
 259    state->instr = instr;
 260
 261    nir_foreach_src(instr, gcm_schedule_early_src, state);
 262 }
 263
 264 static void
 265 gcm_schedule_late_instr(nir_instr *instr, struct gcm_state *state);
 266
 267 /** Schedules the instruction associated with the given SSA def late
 268  *
 269  * This function works by first walking all of the uses of the given SSA
 270  * definition, ensuring that they are scheduled, and then computing the LCA
 271  * (least common ancestor) of its uses.  It then schedules this instruction
 272  * as close to the LCA as possible while trying to stay out of loops.
 273  */
 274 static bool
 275 gcm_schedule_late_def(nir_ssa_def *def, void *void_state)
 276 {
 277    struct gcm_state *state = void_state;
 278
 279    nir_block *lca = NULL;
 280
 281    nir_foreach_use(use_src, def) {
 282       nir_instr *use_instr = use_src->parent_instr;
 283
 284       gcm_schedule_late_instr(use_instr, state);
 285
 286       /* Phi instructions are a bit special.  SSA definitions don't have to
 287        * dominate the sources of the phi nodes that use them; instead, they
 288        * have to dominate the predecessor block corresponding to the phi
 289        * source.  We handle this by looking through the sources, finding
 290        * any that are usingg this SSA def, and using those blocks instead
 291        * of the one the phi lives in.
 292        */
 293       if (use_instr->type == nir_instr_type_phi) {
 294          nir_phi_instr *phi = nir_instr_as_phi(use_instr);
 295
 296          nir_foreach_phi_src(phi_src, phi) {
 297             if (phi_src->src.ssa == def)
 298                lca = nir_dominance_lca(lca, phi_src->pred);
 299          }
 300       } else {
 301          lca = nir_dominance_lca(lca, use_instr->block);
 302       }
 303    }
 304
 305    nir_foreach_if_use(use_src, def) {
 306       nir_if *if_stmt = use_src->parent_if;
 307
 308       /* For if statements, we consider the block to be the one immediately
 309        * preceding the if CF node.
 310        */
 311       nir_block *pred_block =
 312          nir_cf_node_as_block(nir_cf_node_prev(&if_stmt->cf_node));
 313
 314       lca = nir_dominance_lca(lca, pred_block);
 315    }
 316
 317    /* Some instructions may never be used.  We'll just leave them scheduled
 318     * early and let dead code clean them up.
 319     */
 320    if (lca == NULL)
 321       return true;
 322
 323    /* We know have the LCA of all of the uses.  If our invariants hold,
 324     * this is dominated by the block that we chose when scheduling early.
 325     * We now walk up the dominance tree and pick the lowest block that is
 326     * as far outside loops as we can get.
 327     */
 328    nir_block *best = lca;
 329    while (lca != def->parent_instr->block) {
 330       assert(lca);
 331       if (state->blocks[lca->index].loop_depth <
 332           state->blocks[best->index].loop_depth)
 333          best = lca;
 334       lca = lca->imm_dom;
 335    }
 336    def->parent_instr->block = best;
 337
 338    return true;
 339 }
 340
 341 /** Schedules an instruction late
 342  *
 343  * This function performs a depth-first search starting at the given
 344  * instruction and proceeding through its uses to schedule instructions as
 345  * late as they can reasonably go in the dominance tree.  The instructions
 346  * are "scheduled" by updating their instr->block field.
 347  *
 348  * The name of this function is actually a bit of a misnomer as it doesn't
 349  * schedule them "as late as possible" as the paper implies.  Instead, it
 350  * first finds the lates possible place it can schedule the instruction and
 351  * then possibly schedules it earlier than that.  The actual location is as
 352  * far down the tree as we can go while trying to stay out of loops.
 353  */
 354 static void
 355 gcm_schedule_late_instr(nir_instr *instr, struct gcm_state *state)
 356 {
 357    if (instr->pass_flags & GCM_INSTR_SCHEDULED_LATE)
 358       return;
 359
 360    instr->pass_flags |= GCM_INSTR_SCHEDULED_LATE;
 361
 362    /* Pinned instructions are already scheduled so we don't need to do
 363     * anything.  Also, bailing here keeps us from ever following phi nodes
 364     * which can be back-edges.
 365     */
 366    if (instr->pass_flags & GCM_INSTR_PINNED)
 367       return;
 368
 369    nir_foreach_ssa_def(instr, gcm_schedule_late_def, state);
 370 }
 371
 372 static void
 373 gcm_place_instr(nir_instr *instr, struct gcm_state *state);
 374
 375 static bool
 376 gcm_place_instr_def(nir_ssa_def *def, void *state)
 377 {
 378    nir_foreach_use(use_src, def)
 379       gcm_place_instr(use_src->parent_instr, state);
 380
 381    return false;
 382 }
 383
 384 /** Places an instrution back into the program
 385  *
 386  * The earlier passes of GCM simply choose blocks for each instruction and
 387  * otherwise leave them alone.  This pass actually places the instructions
 388  * into their chosen blocks.
 389  *
 390  * To do so, we use a standard post-order depth-first search linearization
 391  * algorithm.  We walk over the uses of the given instruction and ensure
 392  * that they are placed and then place this instruction.  Because we are
 393  * working on multiple blocks at a time, we keep track of the last inserted
 394  * instruction per-block in the state structure's block_info array.  When
 395  * we insert an instruction in a block we insert it before the last
 396  * instruction inserted in that block rather than the last instruction
 397  * inserted globally.
 398  */
 399 static void
 400 gcm_place_instr(nir_instr *instr, struct gcm_state *state)
 401 {
 402    if (instr->pass_flags & GCM_INSTR_PLACED)
 403       return;
 404
 405    instr->pass_flags |= GCM_INSTR_PLACED;
 406
 407    /* Phi nodes are our once source of back-edges.  Since right now we are
 408     * only doing scheduling within blocks, we don't need to worry about
 409     * them since they are always at the top.  Just skip them completely.
 410     */
 411    if (instr->type == nir_instr_type_phi) {
 412       assert(instr->pass_flags & GCM_INSTR_PINNED);
 413       return;
 414    }
 415
 416    nir_foreach_ssa_def(instr, gcm_place_instr_def, state);
 417
 418    if (instr->pass_flags & GCM_INSTR_PINNED) {
 419       /* Pinned instructions have an implicit dependence on the pinned
 420        * instructions that come after them in the block.  Since the pinned
 421        * instructions will naturally "chain" together, we only need to
 422        * explicitly visit one of them.
 423        */
 424       for (nir_instr *after = nir_instr_next(instr);
 425            after;
 426            after = nir_instr_next(after)) {
 427          if (after->pass_flags & GCM_INSTR_PINNED) {
 428             gcm_place_instr(after, state);
 429             break;
 430          }
 431       }
 432    }
 433
 434    struct gcm_block_info *block_info = &state->blocks[instr->block->index];
 435    if (!(instr->pass_flags & GCM_INSTR_PINNED)) {
 436       exec_node_remove(&instr->node);
 437
 438       if (block_info->last_instr) {
 439          exec_node_insert_node_before(&block_info->last_instr->node,
 440                                       &instr->node);
 441       } else {
 442          /* Schedule it at the end of the block */
 443          nir_instr *jump_instr = nir_block_last_instr(instr->block);
 444          if (jump_instr && jump_instr->type == nir_instr_type_jump) {
 445             exec_node_insert_node_before(&jump_instr->node, &instr->node);
 446          } else {
 447             exec_list_push_tail(&instr->block->instr_list, &instr->node);
 448          }
 449       }
 450    }
 451
 452    block_info->last_instr = instr;
 453 }
 454
 455 static bool
 456 opt_gcm_impl(nir_function_impl *impl, bool value_number)
 457 {
 458    struct gcm_state state;
 459
 460    state.impl = impl;
 461    state.instr = NULL;
 462    exec_list_make_empty(&state.instrs);
 463    state.blocks = rzalloc_array(NULL, struct gcm_block_info, impl->num_blocks);
 464
 465    nir_metadata_require(impl, nir_metadata_block_index |
 466                               nir_metadata_dominance);
 467
 468    gcm_build_block_info(&impl->body, &state, 0);
 469
 470    nir_foreach_block(block, impl) {
 471       gcm_pin_instructions_block(block, &state);
 472    }
 473
 474    bool progress = false;
 475    if (value_number) {
 476       struct set *gvn_set = nir_instr_set_create(NULL);
 477       foreach_list_typed_safe(nir_instr, instr, node, &state.instrs) {
 478          if (nir_instr_set_add_or_rewrite(gvn_set, instr)) {
 479             nir_instr_remove(instr);
 480             progress = true;
 481          }
 482       }
 483       nir_instr_set_destroy(gvn_set);
 484    }
 485
 486    foreach_list_typed(nir_instr, instr, node, &state.instrs)
 487       gcm_schedule_early_instr(instr, &state);
 488
 489    foreach_list_typed(nir_instr, instr, node, &state.instrs)
 490       gcm_schedule_late_instr(instr, &state);
 491
 492    while (!exec_list_is_empty(&state.instrs)) {
 493       nir_instr *instr = exec_node_data(nir_instr,
 494                                         state.instrs.tail_sentinel.prev, node);
 495       gcm_place_instr(instr, &state);
 496    }
 497
 498    ralloc_free(state.blocks);
 499
 500    nir_metadata_preserve(impl, nir_metadata_block_index |
 501                                nir_metadata_dominance);
 502
 503    return progress;
 504 }
 505
 506 bool
 507 nir_opt_gcm(nir_shader *shader, bool value_number)
 508 {
 509    bool progress = false;
 510
 511    nir_foreach_function(function, shader) {
 512       if (function->impl)
 513          progress |= opt_gcm_impl(function->impl, value_number);
 514    }
 515
 516    return progress;
 517 }