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 = _mesa_pointer_set_create(NULL);
  75    }
  76
  77    ~ir_if_to_cond_assign_visitor()
  78    {
  79       _mesa_set_destroy(this->condition_variables, NULL);
  80    }
  81
  82    ir_visitor_status visit_enter(ir_if *);
  83    ir_visitor_status visit_leave(ir_if *);
  84
  85    bool found_unsupported_op;
  86    bool found_expensive_op;
  87    bool found_dynamic_arrayref;
  88    bool is_then;
  89    bool progress;
  90    gl_shader_stage stage;
  91    unsigned then_cost;
  92    unsigned else_cost;
  93    unsigned min_branch_cost;
  94    unsigned max_depth;
  95    unsigned depth;
  96
  97    struct set *condition_variables;
  98 };
  99
 100 } /* anonymous namespace */
 101
 102 bool
 103 lower_if_to_cond_assign(gl_shader_stage stage, exec_list *instructions,
 104                         unsigned max_depth, unsigned min_branch_cost)
 105 {
 106    if (max_depth == UINT_MAX)
 107       return false;
 108
 109    ir_if_to_cond_assign_visitor v(stage, max_depth, min_branch_cost);
 110
 111    visit_list_elements(&v, instructions);
 112
 113    return v.progress;
 114 }
 115
 116 static void
 117 check_ir_node(ir_instruction *ir, void *data)
 118 {
 119    ir_if_to_cond_assign_visitor *v = (ir_if_to_cond_assign_visitor *)data;
 120
 121    switch (ir->ir_type) {
 122    case ir_type_call:
 123    case ir_type_discard:
 124    case ir_type_loop:
 125    case ir_type_loop_jump:
 126    case ir_type_return:
 127    case ir_type_emit_vertex:
 128    case ir_type_end_primitive:
 129    case ir_type_barrier:
 130       v->found_unsupported_op = true;
 131       break;
 132
 133    case ir_type_dereference_variable: {
 134       ir_variable *var = ir->as_dereference_variable()->variable_referenced();
 135
 136       /* Lowering branches with TCS output accesses breaks many piglit tests,
 137        * so don't touch them for now.
 138        */
 139       if (v->stage == MESA_SHADER_TESS_CTRL &&
 140           var->data.mode == ir_var_shader_out)
 141          v->found_unsupported_op = true;
 142       break;
 143    }
 144
 145    /* SSBO, images, atomic counters are handled by ir_type_call */
 146    case ir_type_texture:
 147       v->found_expensive_op = true;
 148       break;
 149
 150    case ir_type_dereference_array: {
 151       ir_dereference_array *deref = ir->as_dereference_array();
 152
 153       if (deref->array_index->ir_type != ir_type_constant)
 154          v->found_dynamic_arrayref = true;
 155    } /* fall-through */
 156    case ir_type_expression:
 157    case ir_type_dereference_record:
 158       if (v->is_then)
 159          v->then_cost++;
 160       else
 161          v->else_cost++;
 162       break;
 163
 164    default:
 165       break;
 166    }
 167 }
 168
 169 static void
 170 move_block_to_cond_assign(void *mem_ctx,
 171                           ir_if *if_ir, ir_rvalue *cond_expr,
 172                           exec_list *instructions,
 173                           struct set *set)
 174 {
 175    foreach_in_list_safe(ir_instruction, ir, instructions) {
 176       if (ir->ir_type == ir_type_assignment) {
 177          ir_assignment *assign = (ir_assignment *)ir;
 178
 179          if (_mesa_set_search(set, assign) == NULL) {
 180             _mesa_set_add(set, assign);
 181
 182             /* If the LHS of the assignment is a condition variable that was
 183              * previously added, insert an additional assignment of false to
 184              * the variable.
 185              */
 186             const bool assign_to_cv =
 187                _mesa_set_search(
 188                   set, assign->lhs->variable_referenced()) != NULL;
 189
 190             if (!assign->condition) {
 191                if (assign_to_cv) {
 192                   assign->rhs =
 193                      new(mem_ctx) ir_expression(ir_binop_logic_and,
 194                                                 glsl_type::bool_type,
 195                                                 cond_expr->clone(mem_ctx, NULL),
 196                                                 assign->rhs);
 197                } else {
 198                   assign->condition = cond_expr->clone(mem_ctx, NULL);
 199                }
 200             } else {
 201                assign->condition =
 202                   new(mem_ctx) ir_expression(ir_binop_logic_and,
 203                                              glsl_type::bool_type,
 204                                              cond_expr->clone(mem_ctx, NULL),
 205                                              assign->condition);
 206             }
 207          }
 208       }
 209
 210       /* Now, move from the if block to the block surrounding it. */
 211       ir->remove();
 212       if_ir->insert_before(ir);
 213    }
 214 }
 215
 216 ir_visitor_status
 217 ir_if_to_cond_assign_visitor::visit_enter(ir_if *)
 218 {
 219    this->depth++;
 220
 221    return visit_continue;
 222 }
 223
 224 ir_visitor_status
 225 ir_if_to_cond_assign_visitor::visit_leave(ir_if *ir)
 226 {
 227    bool must_lower = this->depth-- > this->max_depth;
 228
 229    /* Only flatten when beyond the GPU's maximum supported nesting depth. */
 230    if (!must_lower && this->min_branch_cost == 0)
 231       return visit_continue;
 232
 233    this->found_unsupported_op = false;
 234    this->found_expensive_op = false;
 235    this->found_dynamic_arrayref = false;
 236    this->then_cost = 0;
 237    this->else_cost = 0;
 238
 239    ir_assignment *assign;
 240
 241    /* Check that both blocks don't contain anything we can't support. */
 242    this->is_then = true;
 243    foreach_in_list(ir_instruction, then_ir, &ir->then_instructions) {
 244       visit_tree(then_ir, check_ir_node, this);
 245    }
 246
 247    this->is_then = false;
 248    foreach_in_list(ir_instruction, else_ir, &ir->else_instructions) {
 249       visit_tree(else_ir, check_ir_node, this);
 250    }
 251
 252    if (this->found_unsupported_op)
 253       return visit_continue; /* can't handle inner unsupported opcodes */
 254
 255    /* Skip if the branch cost is high enough or if there's an expensive op.
 256     *
 257     * Also skip if non-constant array indices were encountered, since those
 258     * can be out-of-bounds for a not-taken branch, and so generating an
 259     * assignment would be incorrect. In the case of must_lower, it's up to the
 260     * backend to deal with any potential fall-out (perhaps by translating the
 261     * assignments to hardware-predicated moves).
 262     */
 263    if (!must_lower &&
 264        (this->found_expensive_op ||
 265         this->found_dynamic_arrayref ||
 266         MAX2(this->then_cost, this->else_cost) >= this->min_branch_cost))
 267       return visit_continue;
 268
 269    void *mem_ctx = ralloc_parent(ir);
 270
 271    /* Store the condition to a variable.  Move all of the instructions from
 272     * the then-clause of the if-statement.  Use the condition variable as a
 273     * condition for all assignments.
 274     */
 275    ir_variable *const then_var =
 276       new(mem_ctx) ir_variable(glsl_type::bool_type,
 277                                "if_to_cond_assign_then",
 278                                ir_var_temporary);
 279    ir->insert_before(then_var);
 280
 281    ir_dereference_variable *then_cond =
 282       new(mem_ctx) ir_dereference_variable(then_var);
 283
 284    assign = new(mem_ctx) ir_assignment(then_cond, ir->condition);
 285    ir->insert_before(assign);
 286
 287    move_block_to_cond_assign(mem_ctx, ir, then_cond,
 288                              &ir->then_instructions,
 289                              this->condition_variables);
 290
 291    /* Add the new condition variable to the hash table.  This allows us to
 292     * find this variable when lowering other (enclosing) if-statements.
 293     */
 294    _mesa_set_add(this->condition_variables, then_var);
 295
 296    /* If there are instructions in the else-clause, store the inverse of the
 297     * condition to a variable.  Move all of the instructions from the
 298     * else-clause if the if-statement.  Use the (inverse) condition variable
 299     * as a condition for all assignments.
 300     */
 301    if (!ir->else_instructions.is_empty()) {
 302       ir_variable *const else_var =
 303          new(mem_ctx) ir_variable(glsl_type::bool_type,
 304                                   "if_to_cond_assign_else",
 305                                   ir_var_temporary);
 306       ir->insert_before(else_var);
 307
 308       ir_dereference_variable *else_cond =
 309          new(mem_ctx) ir_dereference_variable(else_var);
 310
 311       ir_rvalue *inverse =
 312          new(mem_ctx) ir_expression(ir_unop_logic_not,
 313                                     then_cond->clone(mem_ctx, NULL));
 314
 315       assign = new(mem_ctx) ir_assignment(else_cond, inverse);
 316       ir->insert_before(assign);
 317
 318       move_block_to_cond_assign(mem_ctx, ir, else_cond,
 319                                 &ir->else_instructions,
 320                                 this->condition_variables);
 321
 322       /* Add the new condition variable to the hash table.  This allows us to
 323        * find this variable when lowering other (enclosing) if-statements.
 324        */
 325       _mesa_set_add(this->condition_variables, else_var);
 326    }
 327
 328    ir->remove();
 329
 330    this->progress = true;
 331
 332    return visit_continue;
 333 }