src/intel/compiler/brw_cfg.cpp

   1 /*
   2  * Copyright © 2012 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  *    Eric Anholt <eric@anholt.net>
  25  *
  26  */
  27
  28 #include "brw_cfg.h"
  29 #include "brw_shader.h"
  30
  31 /** @file brw_cfg.cpp
  32  *
  33  * Walks the shader instructions generated and creates a set of basic
  34  * blocks with successor/predecessor edges connecting them.
  35  */
  36
  37 using namespace brw;
  38
  39 static bblock_t *
  40 pop_stack(exec_list *list)
  41 {
  42    bblock_link *link = (bblock_link *)list->get_tail();
  43    bblock_t *block = link->block;
  44    link->link.remove();
  45
  46    return block;
  47 }
  48
  49 static exec_node *
  50 link(void *mem_ctx, bblock_t *block, enum bblock_link_kind kind)
  51 {
  52    bblock_link *l = new(mem_ctx) bblock_link(block, kind);
  53    return &l->link;
  54 }
  55
  56 void
  57 push_stack(exec_list *list, void *mem_ctx, bblock_t *block)
  58 {
  59    /* The kind of the link is immaterial, but we need to provide one since
  60     * this is (ab)using the edge data structure in order to implement a stack.
  61     */
  62    list->push_tail(link(mem_ctx, block, bblock_link_logical));
  63 }
  64
  65 bblock_t::bblock_t(cfg_t *cfg) :
  66    cfg(cfg), idom(NULL), start_ip(0), end_ip(0), num(0), cycle_count(0)
  67 {
  68    instructions.make_empty();
  69    parents.make_empty();
  70    children.make_empty();
  71 }
  72
  73 void
  74 bblock_t::add_successor(void *mem_ctx, bblock_t *successor,
  75                         enum bblock_link_kind kind)
  76 {
  77    successor->parents.push_tail(::link(mem_ctx, this, kind));
  78    children.push_tail(::link(mem_ctx, successor, kind));
  79 }
  80
  81 bool
  82 bblock_t::is_predecessor_of(const bblock_t *block,
  83                             enum bblock_link_kind kind) const
  84 {
  85    foreach_list_typed_safe (bblock_link, parent, link, &block->parents) {
  86       if (parent->block == this && parent->kind <= kind) {
  87          return true;
  88       }
  89    }
  90
  91    return false;
  92 }
  93
  94 bool
  95 bblock_t::is_successor_of(const bblock_t *block,
  96                           enum bblock_link_kind kind) const
  97 {
  98    foreach_list_typed_safe (bblock_link, child, link, &block->children) {
  99       if (child->block == this && child->kind <= kind) {
 100          return true;
 101       }
 102    }
 103
 104    return false;
 105 }
 106
 107 static bool
 108 ends_block(const backend_instruction *inst)
 109 {
 110    enum opcode op = inst->opcode;
 111
 112    return op == BRW_OPCODE_IF ||
 113           op == BRW_OPCODE_ELSE ||
 114           op == BRW_OPCODE_CONTINUE ||
 115           op == BRW_OPCODE_BREAK ||
 116           op == BRW_OPCODE_DO ||
 117           op == BRW_OPCODE_WHILE;
 118 }
 119
 120 static bool
 121 starts_block(const backend_instruction *inst)
 122 {
 123    enum opcode op = inst->opcode;
 124
 125    return op == BRW_OPCODE_DO ||
 126           op == BRW_OPCODE_ENDIF;
 127 }
 128
 129 bool
 130 bblock_t::can_combine_with(const bblock_t *that) const
 131 {
 132    if ((const bblock_t *)this->link.next != that)
 133       return false;
 134
 135    if (ends_block(this->end()) ||
 136        starts_block(that->start()))
 137       return false;
 138
 139    return true;
 140 }
 141
 142 void
 143 bblock_t::combine_with(bblock_t *that)
 144 {
 145    assert(this->can_combine_with(that));
 146    foreach_list_typed (bblock_link, link, link, &that->parents) {
 147       assert(link->block == this);
 148    }
 149
 150    this->end_ip = that->end_ip;
 151    this->instructions.append_list(&that->instructions);
 152
 153    this->cfg->remove_block(that);
 154 }
 155
 156 void
 157 bblock_t::dump(backend_shader *s) const
 158 {
 159    int ip = this->start_ip;
 160    foreach_inst_in_block(backend_instruction, inst, this) {
 161       fprintf(stderr, "%5d: ", ip);
 162       s->dump_instruction(inst);
 163       ip++;
 164    }
 165 }
 166
 167 cfg_t::cfg_t(exec_list *instructions)
 168 {
 169    mem_ctx = ralloc_context(NULL);
 170    block_list.make_empty();
 171    blocks = NULL;
 172    num_blocks = 0;
 173    cycle_count = 0;
 174
 175    bblock_t *cur = NULL;
 176    int ip = 0;
 177
 178    bblock_t *entry = new_block();
 179    bblock_t *cur_if = NULL;    /**< BB ending with IF. */
 180    bblock_t *cur_else = NULL;  /**< BB ending with ELSE. */
 181    bblock_t *cur_endif = NULL; /**< BB starting with ENDIF. */
 182    bblock_t *cur_do = NULL;    /**< BB starting with DO. */
 183    bblock_t *cur_while = NULL; /**< BB immediately following WHILE. */
 184    exec_list if_stack, else_stack, do_stack, while_stack;
 185    bblock_t *next;
 186
 187    set_next_block(&cur, entry, ip);
 188
 189    foreach_in_list_safe(backend_instruction, inst, instructions) {
 190       /* set_next_block wants the post-incremented ip */
 191       ip++;
 192
 193       inst->exec_node::remove();
 194
 195       switch (inst->opcode) {
 196       case BRW_OPCODE_IF:
 197          cur->instructions.push_tail(inst);
 198
 199          /* Push our information onto a stack so we can recover from
 200           * nested ifs.
 201           */
 202          push_stack(&if_stack, mem_ctx, cur_if);
 203          push_stack(&else_stack, mem_ctx, cur_else);
 204
 205          cur_if = cur;
 206          cur_else = NULL;
 207          cur_endif = NULL;
 208
 209          /* Set up our immediately following block, full of "then"
 210           * instructions.
 211           */
 212          next = new_block();
 213          cur_if->add_successor(mem_ctx, next, bblock_link_logical);
 214
 215          set_next_block(&cur, next, ip);
 216          break;
 217
 218       case BRW_OPCODE_ELSE:
 219          cur->instructions.push_tail(inst);
 220
 221          cur_else = cur;
 222
 223          next = new_block();
 224          assert(cur_if != NULL);
 225          cur_if->add_successor(mem_ctx, next, bblock_link_logical);
 226          cur_else->add_successor(mem_ctx, next, bblock_link_physical);
 227
 228          set_next_block(&cur, next, ip);
 229          break;
 230
 231       case BRW_OPCODE_ENDIF: {
 232          if (cur->instructions.is_empty()) {
 233             /* New block was just created; use it. */
 234             cur_endif = cur;
 235          } else {
 236             cur_endif = new_block();
 237
 238             cur->add_successor(mem_ctx, cur_endif, bblock_link_logical);
 239
 240             set_next_block(&cur, cur_endif, ip - 1);
 241          }
 242
 243          cur->instructions.push_tail(inst);
 244
 245          if (cur_else) {
 246             cur_else->add_successor(mem_ctx, cur_endif, bblock_link_logical);
 247          } else {
 248             assert(cur_if != NULL);
 249             cur_if->add_successor(mem_ctx, cur_endif, bblock_link_logical);
 250          }
 251
 252          assert(cur_if->end()->opcode == BRW_OPCODE_IF);
 253          assert(!cur_else || cur_else->end()->opcode == BRW_OPCODE_ELSE);
 254
 255          /* Pop the stack so we're in the previous if/else/endif */
 256          cur_if = pop_stack(&if_stack);
 257          cur_else = pop_stack(&else_stack);
 258          break;
 259       }
 260       case BRW_OPCODE_DO:
 261          /* Push our information onto a stack so we can recover from
 262           * nested loops.
 263           */
 264          push_stack(&do_stack, mem_ctx, cur_do);
 265          push_stack(&while_stack, mem_ctx, cur_while);
 266
 267          /* Set up the block just after the while.  Don't know when exactly
 268           * it will start, yet.
 269           */
 270          cur_while = new_block();
 271
 272          if (cur->instructions.is_empty()) {
 273             /* New block was just created; use it. */
 274             cur_do = cur;
 275          } else {
 276             cur_do = new_block();
 277
 278             cur->add_successor(mem_ctx, cur_do, bblock_link_logical);
 279
 280             set_next_block(&cur, cur_do, ip - 1);
 281          }
 282
 283          cur->instructions.push_tail(inst);
 284
 285          /* Represent divergent execution of the loop as a pair of alternative
 286           * edges coming out of the DO instruction: For any physical iteration
 287           * of the loop a given logical thread can either start off enabled
 288           * (which is represented as the "next" successor), or disabled (if it
 289           * has reached a non-uniform exit of the loop during a previous
 290           * iteration, which is represented as the "cur_while" successor).
 291           *
 292           * The disabled edge will be taken by the logical thread anytime we
 293           * arrive at the DO instruction through a back-edge coming from a
 294           * conditional exit of the loop where divergent control flow started.
 295           *
 296           * This guarantees that there is a control-flow path from any
 297           * divergence point of the loop into the convergence point
 298           * (immediately past the WHILE instruction) such that it overlaps the
 299           * whole IP region of divergent control flow (potentially the whole
 300           * loop) *and* doesn't imply the execution of any instructions part
 301           * of the loop (since the corresponding execution mask bit will be
 302           * disabled for a diverging thread).
 303           *
 304           * This way we make sure that any variables that are live throughout
 305           * the region of divergence for an inactive logical thread are also
 306           * considered to interfere with any other variables assigned by
 307           * active logical threads within the same physical region of the
 308           * program, since otherwise we would risk cross-channel data
 309           * corruption.
 310           */
 311          next = new_block();
 312          cur->add_successor(mem_ctx, next, bblock_link_logical);
 313          cur->add_successor(mem_ctx, cur_while, bblock_link_physical);
 314          set_next_block(&cur, next, ip);
 315          break;
 316
 317       case BRW_OPCODE_CONTINUE:
 318          cur->instructions.push_tail(inst);
 319
 320          /* A conditional CONTINUE may start a region of divergent control
 321           * flow until the start of the next loop iteration (*not* until the
 322           * end of the loop which is why the successor is not the top-level
 323           * divergence point at cur_do).  The live interval of any variable
 324           * extending through a CONTINUE edge is guaranteed to overlap the
 325           * whole region of divergent execution, because any variable live-out
 326           * at the CONTINUE instruction will also be live-in at the top of the
 327           * loop, and therefore also live-out at the bottom-most point of the
 328           * loop which is reachable from the top (since a control flow path
 329           * exists from a definition of the variable through this CONTINUE
 330           * instruction, the top of the loop, the (reachable) bottom of the
 331           * loop, the top of the loop again, into a use of the variable).
 332           */
 333          assert(cur_do != NULL);
 334          cur->add_successor(mem_ctx, cur_do->next(), bblock_link_logical);
 335
 336          next = new_block();
 337          if (inst->predicate)
 338             cur->add_successor(mem_ctx, next, bblock_link_logical);
 339          else
 340             cur->add_successor(mem_ctx, next, bblock_link_physical);
 341
 342          set_next_block(&cur, next, ip);
 343          break;
 344
 345       case BRW_OPCODE_BREAK:
 346          cur->instructions.push_tail(inst);
 347
 348          /* A conditional BREAK instruction may start a region of divergent
 349           * control flow until the end of the loop if the condition is
 350           * non-uniform, in which case the loop will execute additional
 351           * iterations with the present channel disabled.  We model this as a
 352           * control flow path from the divergence point to the convergence
 353           * point that overlaps the whole IP range of the loop and skips over
 354           * the execution of any other instructions part of the loop.
 355           *
 356           * See the DO case for additional explanation.
 357           */
 358          assert(cur_do != NULL);
 359          cur->add_successor(mem_ctx, cur_do, bblock_link_physical);
 360          cur->add_successor(mem_ctx, cur_while, bblock_link_logical);
 361
 362          next = new_block();
 363          if (inst->predicate)
 364             cur->add_successor(mem_ctx, next, bblock_link_logical);
 365
 366          set_next_block(&cur, next, ip);
 367          break;
 368
 369       case BRW_OPCODE_WHILE:
 370          cur->instructions.push_tail(inst);
 371
 372          assert(cur_do != NULL && cur_while != NULL);
 373
 374          /* A conditional WHILE instruction may start a region of divergent
 375           * control flow until the end of the loop, just like the BREAK
 376           * instruction.  See the BREAK case for more details.  OTOH an
 377           * unconditional WHILE instruction is non-divergent (just like an
 378           * unconditional CONTINUE), and will necessarily lead to the
 379           * execution of an additional iteration of the loop for all enabled
 380           * channels, so we may skip over the divergence point at the top of
 381           * the loop to keep the CFG as unambiguous as possible.
 382           */
 383          if (inst->predicate) {
 384             cur->add_successor(mem_ctx, cur_do, bblock_link_logical);
 385          } else {
 386             cur->add_successor(mem_ctx, cur_do->next(), bblock_link_logical);
 387          }
 388
 389          set_next_block(&cur, cur_while, ip);
 390
 391          /* Pop the stack so we're in the previous loop */
 392          cur_do = pop_stack(&do_stack);
 393          cur_while = pop_stack(&while_stack);
 394          break;
 395
 396       default:
 397          cur->instructions.push_tail(inst);
 398          break;
 399       }
 400    }
 401
 402    cur->end_ip = ip - 1;
 403
 404    make_block_array();
 405 }
 406
 407 cfg_t::~cfg_t()
 408 {
 409    ralloc_free(mem_ctx);
 410 }
 411
 412 void
 413 cfg_t::remove_block(bblock_t *block)
 414 {
 415    foreach_list_typed_safe (bblock_link, predecessor, link, &block->parents) {
 416       /* Remove block from all of its predecessors' successor lists. */
 417       foreach_list_typed_safe (bblock_link, successor, link,
 418                                &predecessor->block->children) {
 419          if (block == successor->block) {
 420             successor->link.remove();
 421             ralloc_free(successor);
 422          }
 423       }
 424
 425       /* Add removed-block's successors to its predecessors' successor lists. */
 426       foreach_list_typed (bblock_link, successor, link, &block->children) {
 427          if (!successor->block->is_successor_of(predecessor->block,
 428                                                 successor->kind)) {
 429             predecessor->block->children.push_tail(link(mem_ctx,
 430                                                         successor->block,
 431                                                         successor->kind));
 432          }
 433       }
 434    }
 435
 436    foreach_list_typed_safe (bblock_link, successor, link, &block->children) {
 437       /* Remove block from all of its childrens' parents lists. */
 438       foreach_list_typed_safe (bblock_link, predecessor, link,
 439                                &successor->block->parents) {
 440          if (block == predecessor->block) {
 441             predecessor->link.remove();
 442             ralloc_free(predecessor);
 443          }
 444       }
 445
 446       /* Add removed-block's predecessors to its successors' predecessor lists. */
 447       foreach_list_typed (bblock_link, predecessor, link, &block->parents) {
 448          if (!predecessor->block->is_predecessor_of(successor->block,
 449                                                     predecessor->kind)) {
 450             successor->block->parents.push_tail(link(mem_ctx,
 451                                                      predecessor->block,
 452                                                      predecessor->kind));
 453          }
 454       }
 455    }
 456
 457    block->link.remove();
 458
 459    for (int b = block->num; b < this->num_blocks - 1; b++) {
 460       this->blocks[b] = this->blocks[b + 1];
 461       this->blocks[b]->num = b;
 462    }
 463
 464    this->blocks[this->num_blocks - 1]->num = this->num_blocks - 2;
 465    this->num_blocks--;
 466 }
 467
 468 bblock_t *
 469 cfg_t::new_block()
 470 {
 471    bblock_t *block = new(mem_ctx) bblock_t(this);
 472
 473    return block;
 474 }
 475
 476 void
 477 cfg_t::set_next_block(bblock_t **cur, bblock_t *block, int ip)
 478 {
 479    if (*cur) {
 480       (*cur)->end_ip = ip - 1;
 481    }
 482
 483    block->start_ip = ip;
 484    block->num = num_blocks++;
 485    block_list.push_tail(&block->link);
 486    *cur = block;
 487 }
 488
 489 void
 490 cfg_t::make_block_array()
 491 {
 492    blocks = ralloc_array(mem_ctx, bblock_t *, num_blocks);
 493
 494    int i = 0;
 495    foreach_block (block, this) {
 496       blocks[i++] = block;
 497    }
 498    assert(i == num_blocks);
 499 }
 500
 501 void
 502 cfg_t::dump(backend_shader *s)
 503 {
 504    const idom_tree *idom = (s ? &s->idom_analysis.require() : NULL);
 505
 506    foreach_block (block, this) {
 507       if (block->idom)
 508          fprintf(stderr, "START B%d IDOM(B%d)", block->num, block->idom->num);
 509       else
 510          fprintf(stderr, "START B%d IDOM(none)", block->num);
 511
 512       foreach_list_typed(bblock_link, link, link, &block->parents) {
 513          fprintf(stderr, " <%cB%d",
 514                  link->kind == bblock_link_logical ? '-' : '~',
 515                  link->block->num);
 516       }
 517       fprintf(stderr, "\n");
 518       if (s != NULL)
 519          block->dump(s);
 520       fprintf(stderr, "END B%d", block->num);
 521       foreach_list_typed(bblock_link, link, link, &block->children) {
 522          fprintf(stderr, " %c>B%d",
 523                  link->kind == bblock_link_logical ? '-' : '~',
 524                  link->block->num);
 525       }
 526       fprintf(stderr, "\n");
 527    }
 528 }
 529
 530 /* Calculates the immediate dominator of each block, according to "A Simple,
 531  * Fast Dominance Algorithm" by Keith D. Cooper, Timothy J. Harvey, and Ken
 532  * Kennedy.
 533  *
 534  * The authors claim that for control flow graphs of sizes normally encountered
 535  * (less than 1000 nodes) that this algorithm is significantly faster than
 536  * others like Lengauer-Tarjan.
 537  */
 538 idom_tree::idom_tree(const backend_shader *s)
 539 {
 540    foreach_block(block, s->cfg) {
 541       block->idom = NULL;
 542    }
 543    s->cfg->blocks[0]->idom = s->cfg->blocks[0];
 544
 545    bool changed;
 546    do {
 547       changed = false;
 548
 549       foreach_block(block, s->cfg) {
 550          if (block->num == 0)
 551             continue;
 552
 553          bblock_t *new_idom = NULL;
 554          foreach_list_typed(bblock_link, parent, link, &block->parents) {
 555             if (parent->block->idom) {
 556                if (new_idom == NULL) {
 557                   new_idom = parent->block;
 558                } else if (parent->block->idom != NULL) {
 559                   new_idom = intersect(parent->block, new_idom);
 560                }
 561             }
 562          }
 563
 564          if (block->idom != new_idom) {
 565             block->idom = new_idom;
 566             changed = true;
 567          }
 568       }
 569    } while (changed);
 570 }
 571
 572 bblock_t *
 573 idom_tree::intersect(bblock_t *b1, bblock_t *b2) const
 574 {
 575    /* Note, the comparisons here are the opposite of what the paper says
 576     * because we index blocks from beginning -> end (i.e. reverse post-order)
 577     * instead of post-order like they assume.
 578     */
 579    while (b1->num != b2->num) {
 580       while (b1->num > b2->num)
 581          b1 = b1->idom;
 582       while (b2->num > b1->num)
 583          b2 = b2->idom;
 584    }
 585    assert(b1);
 586    return b1;
 587 }
 588
 589 void
 590 idom_tree::dump(const backend_shader *s) const
 591 {
 592    printf("digraph DominanceTree {\n");
 593    foreach_block(block, s->cfg) {
 594       if (block->idom) {
 595          printf("\t%d -> %d\n", block->idom->num, block->num);
 596       }
 597    }
 598    printf("}\n");
 599 }
 600
 601 void
 602 cfg_t::dump_cfg()
 603 {
 604    printf("digraph CFG {\n");
 605    for (int b = 0; b < num_blocks; b++) {
 606       bblock_t *block = this->blocks[b];
 607
 608       foreach_list_typed_safe (bblock_link, child, link, &block->children) {
 609          printf("\t%d -> %d\n", b, child->block->num);
 610       }
 611    }
 612    printf("}\n");
 613 }