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 attempts to flatten if-statements to conditional assignments for
  28  * GPUs with limited or no flow control support.
  29  *
  30  * It can't handle other control flow being inside of its block, such
  31  * as calls or loops.  Hopefully loop unrolling and inlining will take
  32  * care of those.
  33  *
  34  * Drivers for GPUs with no control flow support should simply call
  35  *
  36  *    lower_if_to_cond_assign(instructions)
  37  *
  38  * to attempt to flatten all if-statements.
  39  *
  40  * Some GPUs (such as i965 prior to gen6) do support control flow, but have a
  41  * maximum nesting depth N.  Drivers for such hardware can call
  42  *
  43  *    lower_if_to_cond_assign(instructions, N)
  44  *
  45  * to attempt to flatten any if-statements appearing at depth > N.
  46  */
  47
  48 #include "compiler/glsl_types.h"
  49 #include "ir.h"
  50 #include "util/set.h"
  51 #include "util/hash_table.h" /* Needed for the hashing functions */
  52
  53 namespace {
  54
  55 class ir_if_to_cond_assign_visitor : public ir_hierarchical_visitor {
  56 public:
  57    ir_if_to_cond_assign_visitor(unsigned max_depth)
  58    {
  59       this->progress = false;
  60       this->max_depth = max_depth;
  61       this->depth = 0;
  62
  63       this->condition_variables =
  64             _mesa_set_create(NULL, _mesa_hash_pointer,
  65                                     _mesa_key_pointer_equal);
  66    }
  67
  68    ~ir_if_to_cond_assign_visitor()
  69    {
  70       _mesa_set_destroy(this->condition_variables, NULL);
  71    }
  72
  73    ir_visitor_status visit_enter(ir_if *);
  74    ir_visitor_status visit_leave(ir_if *);
  75
  76    bool progress;
  77    unsigned max_depth;
  78    unsigned depth;
  79
  80    struct set *condition_variables;
  81 };
  82
  83 } /* anonymous namespace */
  84
  85 bool
  86 lower_if_to_cond_assign(exec_list *instructions, unsigned max_depth)
  87 {
  88    if (max_depth == UINT_MAX)
  89       return false;
  90
  91    ir_if_to_cond_assign_visitor v(max_depth);
  92
  93    visit_list_elements(&v, instructions);
  94
  95    return v.progress;
  96 }
  97
  98 void
  99 check_control_flow(ir_instruction *ir, void *data)
 100 {
 101    bool *found_control_flow = (bool *)data;
 102    switch (ir->ir_type) {
 103    case ir_type_call:
 104    case ir_type_discard:
 105    case ir_type_loop:
 106    case ir_type_loop_jump:
 107    case ir_type_return:
 108       *found_control_flow = true;
 109       break;
 110    default:
 111       break;
 112    }
 113 }
 114
 115 void
 116 move_block_to_cond_assign(void *mem_ctx,
 117                           ir_if *if_ir, ir_rvalue *cond_expr,
 118                           exec_list *instructions,
 119                           struct set *set)
 120 {
 121    foreach_in_list_safe(ir_instruction, ir, instructions) {
 122       if (ir->ir_type == ir_type_assignment) {
 123          ir_assignment *assign = (ir_assignment *)ir;
 124
 125          if (_mesa_set_search(set, assign) == NULL) {
 126             _mesa_set_add(set, assign);
 127
 128             /* If the LHS of the assignment is a condition variable that was
 129              * previously added, insert an additional assignment of false to
 130              * the variable.
 131              */
 132             const bool assign_to_cv =
 133                   _mesa_set_search(
 134                         set, assign->lhs->variable_referenced()) != NULL;
 135
 136             if (!assign->condition) {
 137           if (assign_to_cv) {
 138              assign->rhs =
 139                           new(mem_ctx) ir_expression(ir_binop_logic_and,
 140                                            glsl_type::bool_type,
 141                                            cond_expr->clone(mem_ctx, NULL),
 142                                            assign->rhs);
 143           } else {
 144              assign->condition = cond_expr->clone(mem_ctx, NULL);
 145                }
 146             } else {
 147                assign->condition =
 148              new(mem_ctx) ir_expression(ir_binop_logic_and,
 149                                         glsl_type::bool_type,
 150                                         cond_expr->clone(mem_ctx, NULL),
 151                                         assign->condition);
 152             }
 153          }
 154       }
 155
 156       /* Now, move from the if block to the block surrounding it. */
 157       ir->remove();
 158       if_ir->insert_before(ir);
 159    }
 160 }
 161
 162 ir_visitor_status
 163 ir_if_to_cond_assign_visitor::visit_enter(ir_if *ir)
 164 {
 165    (void) ir;
 166    this->depth++;
 167
 168    return visit_continue;
 169 }
 170
 171 ir_visitor_status
 172 ir_if_to_cond_assign_visitor::visit_leave(ir_if *ir)
 173 {
 174    /* Only flatten when beyond the GPU's maximum supported nesting depth. */
 175    if (this->depth-- <= this->max_depth)
 176       return visit_continue;
 177
 178    bool found_control_flow = false;
 179    ir_assignment *assign;
 180
 181    /* Check that both blocks don't contain anything we can't support. */
 182    foreach_in_list(ir_instruction, then_ir, &ir->then_instructions) {
 183       visit_tree(then_ir, check_control_flow, &found_control_flow);
 184    }
 185    foreach_in_list(ir_instruction, else_ir, &ir->else_instructions) {
 186       visit_tree(else_ir, check_control_flow, &found_control_flow);
 187    }
 188    if (found_control_flow)
 189       return visit_continue;
 190
 191    void *mem_ctx = ralloc_parent(ir);
 192
 193    /* Store the condition to a variable.  Move all of the instructions from
 194     * the then-clause of the if-statement.  Use the condition variable as a
 195     * condition for all assignments.
 196     */
 197    ir_variable *const then_var =
 198       new(mem_ctx) ir_variable(glsl_type::bool_type,
 199                                "if_to_cond_assign_then",
 200                                ir_var_temporary);
 201    ir->insert_before(then_var);
 202
 203    ir_dereference_variable *then_cond =
 204       new(mem_ctx) ir_dereference_variable(then_var);
 205
 206    assign = new(mem_ctx) ir_assignment(then_cond, ir->condition);
 207    ir->insert_before(assign);
 208
 209    move_block_to_cond_assign(mem_ctx, ir, then_cond,
 210                              &ir->then_instructions,
 211                              this->condition_variables);
 212
 213    /* Add the new condition variable to the hash table.  This allows us to
 214     * find this variable when lowering other (enclosing) if-statements.
 215     */
 216    _mesa_set_add(this->condition_variables, then_var);
 217
 218    /* If there are instructions in the else-clause, store the inverse of the
 219     * condition to a variable.  Move all of the instructions from the
 220     * else-clause if the if-statement.  Use the (inverse) condition variable
 221     * as a condition for all assignments.
 222     */
 223    if (!ir->else_instructions.is_empty()) {
 224       ir_variable *const else_var =
 225          new(mem_ctx) ir_variable(glsl_type::bool_type,
 226                                   "if_to_cond_assign_else",
 227                                   ir_var_temporary);
 228       ir->insert_before(else_var);
 229
 230       ir_dereference_variable *else_cond =
 231          new(mem_ctx) ir_dereference_variable(else_var);
 232
 233       ir_rvalue *inverse =
 234          new(mem_ctx) ir_expression(ir_unop_logic_not,
 235                                     then_cond->clone(mem_ctx, NULL));
 236
 237       assign = new(mem_ctx) ir_assignment(else_cond, inverse);
 238       ir->insert_before(assign);
 239
 240       move_block_to_cond_assign(mem_ctx, ir, else_cond,
 241                                 &ir->else_instructions,
 242                                 this->condition_variables);
 243
 244       /* Add the new condition variable to the hash table.  This allows us to
 245        * find this variable when lowering other (enclosing) if-statements.
 246        */
 247       _mesa_set_add(this->condition_variables, else_var);
 248    }
 249
 250    ir->remove();
 251
 252    this->progress = true;
 253
 254    return visit_continue;
 255 }