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