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