src/compiler/glsl/lower_if_to_cond_assign.cpp

   1 /*
   2  * Copyright © 2010 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
  21  * DEALINGS IN THE SOFTWARE.
  22  */
  23
  24 /**
  25  * \file lower_if_to_cond_assign.cpp
  26  *
  27  * This flattens if-statements to conditional assignments if:
  28  *
  29  * - the GPU has limited or no flow control support
  30  *   (controlled by max_depth)
  31  *
  32  * - small conditional branches are more expensive than conditional assignments
  33  *   (controlled by min_branch_cost, that's the cost for a branch to be
  34  *    preserved)
  35  *
  36  * It can't handle other control flow being inside of its block, such
  37  * as calls or loops.  Hopefully loop unrolling and inlining will take
  38  * care of those.
  39  *
  40  * Drivers for GPUs with no control flow support should simply call
  41  *
  42  *    lower_if_to_cond_assign(instructions)
  43  *
  44  * to attempt to flatten all if-statements.
  45  *
  46  * Some GPUs (such as i965 prior to gen6) do support control flow, but have a
  47  * maximum nesting depth N.  Drivers for such hardware can call
  48  *
  49  *    lower_if_to_cond_assign(instructions, N)
  50  *
  51  * to attempt to flatten any if-statements appearing at depth > N.
  52  */
  53
  54 #include "compiler/glsl_types.h"
  55 #include "ir.h"
  56 #include "util/set.h"
  57 #include "util/hash_table.h" /* Needed for the hashing functions */
  58 #include "main/macros.h" /* for MAX2 */
  59
  60 namespace {
  61
  62 class ir_if_to_cond_assign_visitor : public ir_hierarchical_visitor {
  63 public:
  64    ir_if_to_cond_assign_visitor(gl_shader_stage stage,
  65                                 unsigned max_depth,
  66                                 unsigned min_branch_cost)
  67    {
  68       this->progress = false;
  69       this->stage = stage;
  70       this->max_depth = max_depth;
  71       this->min_branch_cost = min_branch_cost;
  72       this->depth = 0;
  73
  74       this->condition_variables =
  75             _mesa_set_create(NULL, _mesa_hash_pointer,
  76                                     _mesa_key_pointer_equal);
  77    }
  78
  79    ~ir_if_to_cond_assign_visitor()
  80    {
  81       _mesa_set_destroy(this->condition_variables, NULL);
  82    }
  83
  84    ir_visitor_status visit_enter(ir_if *);
  85    ir_visitor_status visit_leave(ir_if *);
  86
  87    bool found_unsupported_op;
  88    bool found_expensive_op;
  89    bool found_dynamic_arrayref;
  90    bool is_then;
  91    bool progress;
  92    gl_shader_stage stage;
  93    unsigned then_cost;
  94    unsigned else_cost;
  95    unsigned min_branch_cost;
  96    unsigned max_depth;
  97    unsigned depth;
  98
  99    struct set *condition_variables;
 100 };
 101
 102 } /* anonymous namespace */
 103
 104 bool
 105 lower_if_to_cond_assign(gl_shader_stage stage, exec_list *instructions,
 106                         unsigned max_depth, unsigned min_branch_cost)
 107 {
 108    if (max_depth == UINT_MAX)
 109       return false;
 110
 111    ir_if_to_cond_assign_visitor v(stage, max_depth, min_branch_cost);
 112
 113    visit_list_elements(&v, instructions);
 114
 115    return v.progress;
 116 }
 117
 118 void
 119 check_ir_node(ir_instruction *ir, void *data)
 120 {
 121    ir_if_to_cond_assign_visitor *v = (ir_if_to_cond_assign_visitor *)data;
 122
 123    switch (ir->ir_type) {
 124    case ir_type_call:
 125    case ir_type_discard:
 126    case ir_type_loop:
 127    case ir_type_loop_jump:
 128    case ir_type_return:
 129    case ir_type_emit_vertex:
 130    case ir_type_end_primitive:
 131    case ir_type_barrier:
 132       v->found_unsupported_op = true;
 133       break;
 134
 135    case ir_type_dereference_variable: {
 136       ir_variable *var = ir->as_dereference_variable()->variable_referenced();
 137
 138       /* Lowering branches with TCS output accesses breaks many piglit tests,
 139        * so don't touch them for now.
 140        */
 141       if (v->stage == MESA_SHADER_TESS_CTRL &&
 142           var->data.mode == ir_var_shader_out)
 143          v->found_unsupported_op = true;
 144       break;
 145    }
 146
 147    /* SSBO, images, atomic counters are handled by ir_type_call */
 148    case ir_type_texture:
 149       v->found_expensive_op = true;
 150       break;
 151
 152    case ir_type_dereference_array: {
 153       ir_dereference_array *deref = ir->as_dereference_array();
 154
 155       if (deref->array_index->ir_type != ir_type_constant)
 156          v->found_dynamic_arrayref = true;
 157    } /* fall-through */
 158    case ir_type_expression:
 159    case ir_type_dereference_record:
 160       if (v->is_then)
 161          v->then_cost++;
 162       else
 163          v->else_cost++;
 164       break;
 165
 166    default:
 167       break;
 168    }
 169 }
 170
 171 void
 172 move_block_to_cond_assign(void *mem_ctx,
 173                           ir_if *if_ir, ir_rvalue *cond_expr,
 174                           exec_list *instructions,
 175                           struct set *set)
 176 {
 177    foreach_in_list_safe(ir_instruction, ir, instructions) {
 178       if (ir->ir_type == ir_type_assignment) {
 179          ir_assignment *assign = (ir_assignment *)ir;
 180
 181          if (_mesa_set_search(set, assign) == NULL) {
 182             _mesa_set_add(set, assign);
 183
 184             /* If the LHS of the assignment is a condition variable that was
 185              * previously added, insert an additional assignment of false to
 186              * the variable.
 187              */
 188             const bool assign_to_cv =
 189                   _mesa_set_search(
 190                         set, assign->lhs->variable_referenced()) != NULL;
 191
 192             if (!assign->condition) {
 193           if (assign_to_cv) {
 194              assign->rhs =
 195                           new(mem_ctx) ir_expression(ir_binop_logic_and,
 196                                            glsl_type::bool_type,
 197                                            cond_expr->clone(mem_ctx, NULL),
 198                                            assign->rhs);
 199           } else {
 200              assign->condition = cond_expr->clone(mem_ctx, NULL);
 201                }
 202             } else {
 203                assign->condition =
 204              new(mem_ctx) ir_expression(ir_binop_logic_and,
 205                                         glsl_type::bool_type,
 206                                         cond_expr->clone(mem_ctx, NULL),
 207                                         assign->condition);
 208             }
 209          }
 210       }
 211
 212       /* Now, move from the if block to the block surrounding it. */
 213       ir->remove();
 214       if_ir->insert_before(ir);
 215    }
 216 }
 217
 218 ir_visitor_status
 219 ir_if_to_cond_assign_visitor::visit_enter(ir_if *ir)
 220 {
 221    (void) ir;
 222    this->depth++;
 223
 224    return visit_continue;
 225 }
 226
 227 ir_visitor_status
 228 ir_if_to_cond_assign_visitor::visit_leave(ir_if *ir)
 229 {
 230    bool must_lower = this->depth-- > this->max_depth;
 231
 232    /* Only flatten when beyond the GPU's maximum supported nesting depth. */
 233    if (!must_lower && this->min_branch_cost == 0)
 234       return visit_continue;
 235
 236    this->found_unsupported_op = false;
 237    this->found_expensive_op = false;
 238    this->found_dynamic_arrayref = false;
 239    this->then_cost = 0;
 240    this->else_cost = 0;
 241
 242    ir_assignment *assign;
 243
 244    /* Check that both blocks don't contain anything we can't support. */
 245    this->is_then = true;
 246    foreach_in_list(ir_instruction, then_ir, &ir->then_instructions) {
 247       visit_tree(then_ir, check_ir_node, this);
 248    }
 249
 250    this->is_then = false;
 251    foreach_in_list(ir_instruction, else_ir, &ir->else_instructions) {
 252       visit_tree(else_ir, check_ir_node, this);
 253    }
 254
 255    if (this->found_unsupported_op)
 256       return visit_continue; /* can't handle inner unsupported opcodes */
 257
 258    /* Skip if the branch cost is high enough or if there's an expensive op.
 259     *
 260     * Also skip if non-constant array indices were encountered, since those
 261     * can be out-of-bounds for a not-taken branch, and so generating an
 262     * assignment would be incorrect. In the case of must_lower, it's up to the
 263     * backend to deal with any potential fall-out (perhaps by translating the
 264     * assignments to hardware-predicated moves).
 265     */
 266    if (!must_lower &&
 267        (this->found_expensive_op ||
 268         this->found_dynamic_arrayref ||
 269         MAX2(this->then_cost, this->else_cost) >= this->min_branch_cost))
 270       return visit_continue;
 271
 272    void *mem_ctx = ralloc_parent(ir);
 273
 274    /* Store the condition to a variable.  Move all of the instructions from
 275     * the then-clause of the if-statement.  Use the condition variable as a
 276     * condition for all assignments.
 277     */
 278    ir_variable *const then_var =
 279       new(mem_ctx) ir_variable(glsl_type::bool_type,
 280                                "if_to_cond_assign_then",
 281                                ir_var_temporary);
 282    ir->insert_before(then_var);
 283
 284    ir_dereference_variable *then_cond =
 285       new(mem_ctx) ir_dereference_variable(then_var);
 286
 287    assign = new(mem_ctx) ir_assignment(then_cond, ir->condition);
 288    ir->insert_before(assign);
 289
 290    move_block_to_cond_assign(mem_ctx, ir, then_cond,
 291                              &ir->then_instructions,
 292                              this->condition_variables);
 293
 294    /* Add the new condition variable to the hash table.  This allows us to
 295     * find this variable when lowering other (enclosing) if-statements.
 296     */
 297    _mesa_set_add(this->condition_variables, then_var);
 298
 299    /* If there are instructions in the else-clause, store the inverse of the
 300     * condition to a variable.  Move all of the instructions from the
 301     * else-clause if the if-statement.  Use the (inverse) condition variable
 302     * as a condition for all assignments.
 303     */
 304    if (!ir->else_instructions.is_empty()) {
 305       ir_variable *const else_var =
 306          new(mem_ctx) ir_variable(glsl_type::bool_type,
 307                                   "if_to_cond_assign_else",
 308                                   ir_var_temporary);
 309       ir->insert_before(else_var);
 310
 311       ir_dereference_variable *else_cond =
 312          new(mem_ctx) ir_dereference_variable(else_var);
 313
 314       ir_rvalue *inverse =
 315          new(mem_ctx) ir_expression(ir_unop_logic_not,
 316                                     then_cond->clone(mem_ctx, NULL));
 317
 318       assign = new(mem_ctx) ir_assignment(else_cond, inverse);
 319       ir->insert_before(assign);
 320
 321       move_block_to_cond_assign(mem_ctx, ir, else_cond,
 322                                 &ir->else_instructions,
 323                                 this->condition_variables);
 324
 325       /* Add the new condition variable to the hash table.  This allows us to
 326        * find this variable when lowering other (enclosing) if-statements.
 327        */
 328       _mesa_set_add(this->condition_variables, else_var);
 329    }
 330
 331    ir->remove();
 332
 333    this->progress = true;
 334
 335    return visit_continue;
 336 }