src/compiler/glsl/opt_constant_propagation.cpp

   1 /*
   2  * Copyright © 2010 Intel Corporation
   3  *
   4  * Permission is hereby granted, free of charge, to any person obtaining a
   5  * constant 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, constant, 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 constantright 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 CONSTANTRIGHT 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
  21  * DEALINGS IN THE SOFTWARE.
  22  */
  23
  24 /**
  25  * \file opt_constant_propagation.cpp
  26  *
  27  * Tracks assignments of constants to channels of variables, and
  28  * usage of those constant channels with direct usage of the constants.
  29  *
  30  * This can lead to constant folding and algebraic optimizations in
  31  * those later expressions, while causing no increase in instruction
  32  * count (due to constants being generally free to load from a
  33  * constant push buffer or as instruction immediate values) and
  34  * possibly reducing register pressure.
  35  */
  36
  37 #include "ir.h"
  38 #include "ir_visitor.h"
  39 #include "ir_rvalue_visitor.h"
  40 #include "ir_basic_block.h"
  41 #include "ir_optimization.h"
  42 #include "compiler/glsl_types.h"
  43 #include "util/hash_table.h"
  44
  45 namespace {
  46
  47 class acp_entry : public exec_node
  48 {
  49 public:
  50    /* override operator new from exec_node */
  51    DECLARE_LINEAR_ZALLOC_CXX_OPERATORS(acp_entry)
  52
  53    acp_entry(ir_variable *var, unsigned write_mask, ir_constant *constant)
  54    {
  55       assert(var);
  56       assert(constant);
  57       this->var = var;
  58       this->write_mask = write_mask;
  59       this->constant = constant;
  60       this->initial_values = write_mask;
  61    }
  62
  63    acp_entry(const acp_entry *src)
  64    {
  65       this->var = src->var;
  66       this->write_mask = src->write_mask;
  67       this->constant = src->constant;
  68       this->initial_values = src->initial_values;
  69    }
  70
  71    ir_variable *var;
  72    ir_constant *constant;
  73    unsigned write_mask;
  74
  75    /** Mask of values initially available in the constant. */
  76    unsigned initial_values;
  77 };
  78
  79
  80 class ir_constant_propagation_visitor : public ir_rvalue_visitor {
  81 public:
  82    ir_constant_propagation_visitor()
  83    {
  84       progress = false;
  85       killed_all = false;
  86       mem_ctx = ralloc_context(0);
  87       this->lin_ctx = linear_alloc_parent(this->mem_ctx, 0);
  88       this->acp = new(mem_ctx) exec_list;
  89       this->kills = _mesa_pointer_hash_table_create(mem_ctx);
  90    }
  91    ~ir_constant_propagation_visitor()
  92    {
  93       ralloc_free(mem_ctx);
  94    }
  95
  96    virtual ir_visitor_status visit_enter(class ir_loop *);
  97    virtual ir_visitor_status visit_enter(class ir_function_signature *);
  98    virtual ir_visitor_status visit_enter(class ir_function *);
  99    virtual ir_visitor_status visit_leave(class ir_assignment *);
 100    virtual ir_visitor_status visit_enter(class ir_call *);
 101    virtual ir_visitor_status visit_enter(class ir_if *);
 102
 103    void add_constant(ir_assignment *ir);
 104    void constant_folding(ir_rvalue **rvalue);
 105    void constant_propagation(ir_rvalue **rvalue);
 106    void kill(ir_variable *ir, unsigned write_mask);
 107    void handle_if_block(exec_list *instructions, hash_table *kills, bool *killed_all);
 108    void handle_loop(class ir_loop *, bool keep_acp);
 109    void handle_rvalue(ir_rvalue **rvalue);
 110
 111    /** List of acp_entry: The available constants to propagate */
 112    exec_list *acp;
 113
 114    /**
 115     * Hash table of killed entries: maps variables to the mask of killed channels.
 116     */
 117    hash_table *kills;
 118
 119    bool progress;
 120
 121    bool killed_all;
 122
 123    void *mem_ctx;
 124    void *lin_ctx;
 125 };
 126
 127
 128 void
 129 ir_constant_propagation_visitor::constant_folding(ir_rvalue **rvalue)
 130 {
 131    if (this->in_assignee || *rvalue == NULL)
 132       return;
 133
 134    if (ir_constant_fold(rvalue))
 135       this->progress = true;
 136
 137    ir_dereference_variable *var_ref = (*rvalue)->as_dereference_variable();
 138    if (var_ref && !var_ref->type->is_array()) {
 139       ir_constant *constant =
 140          var_ref->constant_expression_value(ralloc_parent(var_ref));
 141       if (constant) {
 142          *rvalue = constant;
 143          this->progress = true;
 144       }
 145    }
 146 }
 147
 148 void
 149 ir_constant_propagation_visitor::constant_propagation(ir_rvalue **rvalue) {
 150
 151    if (this->in_assignee || !*rvalue)
 152       return;
 153
 154    const glsl_type *type = (*rvalue)->type;
 155    if (!type->is_scalar() && !type->is_vector())
 156       return;
 157
 158    ir_swizzle *swiz = NULL;
 159    ir_dereference_variable *deref = (*rvalue)->as_dereference_variable();
 160    if (!deref) {
 161       swiz = (*rvalue)->as_swizzle();
 162       if (!swiz)
 163          return;
 164
 165       deref = swiz->val->as_dereference_variable();
 166       if (!deref)
 167          return;
 168    }
 169
 170    ir_constant_data data;
 171    memset(&data, 0, sizeof(data));
 172
 173    for (unsigned int i = 0; i < type->components(); i++) {
 174       int channel;
 175       acp_entry *found = NULL;
 176
 177       if (swiz) {
 178          switch (i) {
 179          case 0: channel = swiz->mask.x; break;
 180          case 1: channel = swiz->mask.y; break;
 181          case 2: channel = swiz->mask.z; break;
 182          case 3: channel = swiz->mask.w; break;
 183          default: assert(!"shouldn't be reached"); channel = 0; break;
 184          }
 185       } else {
 186          channel = i;
 187       }
 188
 189       foreach_in_list(acp_entry, entry, this->acp) {
 190          if (entry->var == deref->var && entry->write_mask & (1 << channel)) {
 191             found = entry;
 192             break;
 193          }
 194       }
 195
 196       if (!found)
 197          return;
 198
 199       int rhs_channel = 0;
 200       for (int j = 0; j < 4; j++) {
 201          if (j == channel)
 202             break;
 203          if (found->initial_values & (1 << j))
 204             rhs_channel++;
 205       }
 206
 207       switch (type->base_type) {
 208       case GLSL_TYPE_FLOAT:
 209          data.f[i] = found->constant->value.f[rhs_channel];
 210          break;
 211       case GLSL_TYPE_FLOAT16:
 212          data.f16[i] = found->constant->value.f16[rhs_channel];
 213          break;
 214       case GLSL_TYPE_DOUBLE:
 215          data.d[i] = found->constant->value.d[rhs_channel];
 216          break;
 217       case GLSL_TYPE_INT:
 218          data.i[i] = found->constant->value.i[rhs_channel];
 219          break;
 220       case GLSL_TYPE_UINT:
 221          data.u[i] = found->constant->value.u[rhs_channel];
 222          break;
 223       case GLSL_TYPE_INT16:
 224          data.i16[i] = found->constant->value.i16[rhs_channel];
 225          break;
 226       case GLSL_TYPE_UINT16:
 227          data.u16[i] = found->constant->value.u16[rhs_channel];
 228          break;
 229       case GLSL_TYPE_BOOL:
 230          data.b[i] = found->constant->value.b[rhs_channel];
 231          break;
 232       case GLSL_TYPE_UINT64:
 233          data.u64[i] = found->constant->value.u64[rhs_channel];
 234          break;
 235       case GLSL_TYPE_INT64:
 236          data.i64[i] = found->constant->value.i64[rhs_channel];
 237          break;
 238       default:
 239          assert(!"not reached");
 240          break;
 241       }
 242    }
 243
 244    *rvalue = new(ralloc_parent(deref)) ir_constant(type, &data);
 245    this->progress = true;
 246 }
 247
 248 void
 249 ir_constant_propagation_visitor::handle_rvalue(ir_rvalue **rvalue)
 250 {
 251    constant_propagation(rvalue);
 252    constant_folding(rvalue);
 253 }
 254
 255 ir_visitor_status
 256 ir_constant_propagation_visitor::visit_enter(ir_function_signature *ir)
 257 {
 258    /* Treat entry into a function signature as a completely separate
 259     * block.  Any instructions at global scope will be shuffled into
 260     * main() at link time, so they're irrelevant to us.
 261     */
 262    exec_list *orig_acp = this->acp;
 263    hash_table *orig_kills = this->kills;
 264    bool orig_killed_all = this->killed_all;
 265
 266    this->acp = new(mem_ctx) exec_list;
 267    this->kills = _mesa_pointer_hash_table_create(mem_ctx);
 268    this->killed_all = false;
 269
 270    visit_list_elements(this, &ir->body);
 271
 272    this->kills = orig_kills;
 273    this->acp = orig_acp;
 274    this->killed_all = orig_killed_all;
 275
 276    return visit_continue_with_parent;
 277 }
 278
 279 ir_visitor_status
 280 ir_constant_propagation_visitor::visit_leave(ir_assignment *ir)
 281 {
 282   constant_folding(&ir->rhs);
 283
 284    if (this->in_assignee)
 285       return visit_continue;
 286
 287    unsigned kill_mask = ir->write_mask;
 288    if (ir->lhs->as_dereference_array()) {
 289       /* The LHS of the assignment uses an array indexing operator (e.g. v[i]
 290        * = ...;).  Since we only try to constant propagate vectors and
 291        * scalars, this means that either (a) array indexing is being used to
 292        * select a vector component, or (b) the variable in question is neither
 293        * a scalar or a vector, so we don't care about it.  In the former case,
 294        * we want to kill the whole vector, since in general we can't predict
 295        * which vector component will be selected by array indexing.  In the
 296        * latter case, it doesn't matter what we do, so go ahead and kill the
 297        * whole variable anyway.
 298        *
 299        * Note that if the array index is constant (e.g. v[2] = ...;), we could
 300        * in principle be smarter, but we don't need to, because a future
 301        * optimization pass will convert it to a simple assignment with the
 302        * correct mask.
 303        */
 304       kill_mask = ~0;
 305    }
 306    kill(ir->lhs->variable_referenced(), kill_mask);
 307
 308    add_constant(ir);
 309
 310    return visit_continue;
 311 }
 312
 313 ir_visitor_status
 314 ir_constant_propagation_visitor::visit_enter(ir_function *ir)
 315 {
 316    (void) ir;
 317    return visit_continue;
 318 }
 319
 320 ir_visitor_status
 321 ir_constant_propagation_visitor::visit_enter(ir_call *ir)
 322 {
 323    /* Do constant propagation on call parameters, but skip any out params */
 324    foreach_two_lists(formal_node, &ir->callee->parameters,
 325                      actual_node, &ir->actual_parameters) {
 326       ir_variable *sig_param = (ir_variable *) formal_node;
 327       ir_rvalue *param = (ir_rvalue *) actual_node;
 328       if (sig_param->data.mode != ir_var_function_out
 329           && sig_param->data.mode != ir_var_function_inout) {
 330          ir_rvalue *new_param = param;
 331          handle_rvalue(&new_param);
 332          if (new_param != param)
 333             param->replace_with(new_param);
 334          else
 335             param->accept(this);
 336       }
 337    }
 338
 339    /* Since we're unlinked, we don't (necssarily) know the side effects of
 340     * this call.  So kill all copies.
 341     */
 342    acp->make_empty();
 343    this->killed_all = true;
 344
 345    return visit_continue_with_parent;
 346 }
 347
 348 void
 349 ir_constant_propagation_visitor::handle_if_block(exec_list *instructions, hash_table *kills, bool *killed_all)
 350 {
 351    exec_list *orig_acp = this->acp;
 352    hash_table *orig_kills = this->kills;
 353    bool orig_killed_all = this->killed_all;
 354
 355    this->acp = new(mem_ctx) exec_list;
 356    this->kills = kills;
 357    this->killed_all = false;
 358
 359    /* Populate the initial acp with a constant of the original */
 360    foreach_in_list(acp_entry, a, orig_acp) {
 361       this->acp->push_tail(new(this->lin_ctx) acp_entry(a));
 362    }
 363
 364    visit_list_elements(this, instructions);
 365
 366    *killed_all = this->killed_all;
 367    this->kills = orig_kills;
 368    this->acp = orig_acp;
 369    this->killed_all = orig_killed_all;
 370 }
 371
 372 ir_visitor_status
 373 ir_constant_propagation_visitor::visit_enter(ir_if *ir)
 374 {
 375    ir->condition->accept(this);
 376    handle_rvalue(&ir->condition);
 377
 378    hash_table *new_kills = _mesa_pointer_hash_table_create(mem_ctx);
 379    bool then_killed_all = false;
 380    bool else_killed_all = false;
 381
 382    handle_if_block(&ir->then_instructions, new_kills, &then_killed_all);
 383    handle_if_block(&ir->else_instructions, new_kills, &else_killed_all);
 384
 385    if (then_killed_all || else_killed_all) {
 386       acp->make_empty();
 387       killed_all = true;
 388    } else {
 389       hash_table_foreach(new_kills, htk)
 390          kill((ir_variable *) htk->key, (uintptr_t) htk->data);
 391    }
 392
 393    _mesa_hash_table_destroy(new_kills, NULL);
 394
 395    /* handle_if_block() already descended into the children. */
 396    return visit_continue_with_parent;
 397 }
 398
 399 void
 400 ir_constant_propagation_visitor::handle_loop(ir_loop *ir, bool keep_acp)
 401 {
 402    exec_list *orig_acp = this->acp;
 403    hash_table *orig_kills = this->kills;
 404    bool orig_killed_all = this->killed_all;
 405
 406    this->acp = new(mem_ctx) exec_list;
 407    this->kills = _mesa_pointer_hash_table_create(mem_ctx);
 408    this->killed_all = false;
 409
 410    if (keep_acp) {
 411       foreach_in_list(acp_entry, a, orig_acp) {
 412          this->acp->push_tail(new(this->lin_ctx) acp_entry(a));
 413       }
 414    }
 415
 416    visit_list_elements(this, &ir->body_instructions);
 417
 418    if (this->killed_all) {
 419       orig_acp->make_empty();
 420    }
 421
 422    hash_table *new_kills = this->kills;
 423    this->kills = orig_kills;
 424    this->acp = orig_acp;
 425    this->killed_all = this->killed_all || orig_killed_all;
 426
 427    hash_table_foreach(new_kills, htk) {
 428       kill((ir_variable *) htk->key, (uintptr_t) htk->data);
 429    }
 430 }
 431
 432 ir_visitor_status
 433 ir_constant_propagation_visitor::visit_enter(ir_loop *ir)
 434 {
 435    /* Make a conservative first pass over the loop with an empty ACP set.
 436     * This also removes any killed entries from the original ACP set.
 437     */
 438    handle_loop(ir, false);
 439
 440    /* Then, run it again with the real ACP set, minus any killed entries.
 441     * This takes care of propagating values from before the loop into it.
 442     */
 443    handle_loop(ir, true);
 444
 445    /* already descended into the children. */
 446    return visit_continue_with_parent;
 447 }
 448
 449 void
 450 ir_constant_propagation_visitor::kill(ir_variable *var, unsigned write_mask)
 451 {
 452    assert(var != NULL);
 453
 454    /* We don't track non-vectors. */
 455    if (!var->type->is_vector() && !var->type->is_scalar())
 456       return;
 457
 458    /* Remove any entries currently in the ACP for this kill. */
 459    foreach_in_list_safe(acp_entry, entry, this->acp) {
 460       if (entry->var == var) {
 461          entry->write_mask &= ~write_mask;
 462          if (entry->write_mask == 0)
 463             entry->remove();
 464       }
 465    }
 466
 467    /* Add this writemask of the variable to the hash table of killed
 468     * variables in this block.
 469     */
 470    hash_entry *kill_hash_entry = _mesa_hash_table_search(this->kills, var);
 471    if (kill_hash_entry) {
 472       uintptr_t new_write_mask = ((uintptr_t) kill_hash_entry->data) | write_mask;
 473       kill_hash_entry->data = (void *) new_write_mask;
 474       return;
 475    }
 476    /* Not already in the hash table.  Make new entry. */
 477    _mesa_hash_table_insert(this->kills, var, (void *) uintptr_t(write_mask));
 478 }
 479
 480 /**
 481  * Adds an entry to the available constant list if it's a plain assignment
 482  * of a variable to a variable.
 483  */
 484 void
 485 ir_constant_propagation_visitor::add_constant(ir_assignment *ir)
 486 {
 487    acp_entry *entry;
 488
 489    if (ir->condition)
 490       return;
 491
 492    if (!ir->write_mask)
 493       return;
 494
 495    ir_dereference_variable *deref = ir->lhs->as_dereference_variable();
 496    ir_constant *constant = ir->rhs->as_constant();
 497
 498    if (!deref || !constant)
 499       return;
 500
 501    /* Only do constant propagation on vectors.  Constant matrices,
 502     * arrays, or structures would require more work elsewhere.
 503     */
 504    if (!deref->var->type->is_vector() && !deref->var->type->is_scalar())
 505       return;
 506
 507    /* We can't do copy propagation on buffer variables, since the underlying
 508     * memory storage is shared across multiple threads we can't be sure that
 509     * the variable value isn't modified between this assignment and the next
 510     * instruction where its value is read.
 511     */
 512    if (deref->var->data.mode == ir_var_shader_storage ||
 513        deref->var->data.mode == ir_var_shader_shared)
 514       return;
 515
 516    entry = new(this->lin_ctx) acp_entry(deref->var, ir->write_mask, constant);
 517    this->acp->push_tail(entry);
 518 }
 519
 520 } /* unnamed namespace */
 521
 522 /**
 523  * Does a constant propagation pass on the code present in the instruction stream.
 524  */
 525 bool
 526 do_constant_propagation(exec_list *instructions)
 527 {
 528    ir_constant_propagation_visitor v;
 529
 530    visit_list_elements(&v, instructions);
 531
 532    return v.progress;
 533 }