src/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 "glsl_types.h"
  49 #include "ir.h"
  50 #include "program/hash_table.h"
  51
  52 class ir_if_to_cond_assign_visitor : public ir_hierarchical_visitor {
  53 public:
  54    ir_if_to_cond_assign_visitor(unsigned max_depth)
  55    {
  56       this->progress = false;
  57       this->max_depth = max_depth;
  58       this->depth = 0;
  59
  60       this->condition_variables = hash_table_ctor(0, hash_table_pointer_hash,
  61                                                   hash_table_pointer_compare);
  62    }
  63
  64    ~ir_if_to_cond_assign_visitor()
  65    {
  66       hash_table_dtor(this->condition_variables);
  67    }
  68
  69    ir_visitor_status visit_enter(ir_if *);
  70    ir_visitor_status visit_leave(ir_if *);
  71
  72    bool progress;
  73    unsigned max_depth;
  74    unsigned depth;
  75
  76    struct hash_table *condition_variables;
  77 };
  78
  79 bool
  80 lower_if_to_cond_assign(exec_list *instructions, unsigned max_depth)
  81 {
  82    ir_if_to_cond_assign_visitor v(max_depth);
  83
  84    visit_list_elements(&v, instructions);
  85
  86    return v.progress;
  87 }
  88
  89 void
  90 check_control_flow(ir_instruction *ir, void *data)
  91 {
  92    bool *found_control_flow = (bool *)data;
  93    switch (ir->ir_type) {
  94    case ir_type_call:
  95    case ir_type_discard:
  96    case ir_type_loop:
  97    case ir_type_loop_jump:
  98    case ir_type_return:
  99       *found_control_flow = true;
 100       break;
 101    default:
 102       break;
 103    }
 104 }
 105
 106 void
 107 move_block_to_cond_assign(void *mem_ctx,
 108                           ir_if *if_ir, ir_rvalue *cond_expr,
 109                           exec_list *instructions,
 110                           struct hash_table *ht)
 111 {
 112    foreach_list_safe(node, instructions) {
 113       ir_instruction *ir = (ir_instruction *) node;
 114
 115       if (ir->ir_type == ir_type_assignment) {
 116          ir_assignment *assign = (ir_assignment *)ir;
 117
 118          if (hash_table_find(ht, assign) == NULL) {
 119             hash_table_insert(ht, assign, assign);
 120
 121             /* If the LHS of the assignment is a condition variable that was
 122              * previously added, insert an additional assignment of false to
 123              * the variable.
 124              */
 125             const bool assign_to_cv =
 126                hash_table_find(ht, assign->lhs->variable_referenced()) != NULL;
 127
 128             if (!assign->condition) {
 129                if (assign_to_cv) {
 130                   assign->rhs =
 131                      new(mem_ctx) ir_expression(ir_binop_logic_and,
 132                                                 glsl_type::bool_type,
 133                                                 cond_expr->clone(mem_ctx, NULL),
 134                                                 assign->rhs);
 135                } else {
 136                   assign->condition = cond_expr->clone(mem_ctx, NULL);
 137                }
 138             } else {
 139                assign->condition =
 140                   new(mem_ctx) ir_expression(ir_binop_logic_and,
 141                                              glsl_type::bool_type,
 142                                              cond_expr->clone(mem_ctx, NULL),
 143                                              assign->condition);
 144             }
 145          }
 146       }
 147
 148       /* Now, move from the if block to the block surrounding it. */
 149       ir->remove();
 150       if_ir->insert_before(ir);
 151    }
 152 }
 153
 154 ir_visitor_status
 155 ir_if_to_cond_assign_visitor::visit_enter(ir_if *ir)
 156 {
 157    (void) ir;
 158    this->depth++;
 159
 160    return visit_continue;
 161 }
 162
 163 ir_visitor_status
 164 ir_if_to_cond_assign_visitor::visit_leave(ir_if *ir)
 165 {
 166    /* Only flatten when beyond the GPU's maximum supported nesting depth. */
 167    if (this->depth-- <= this->max_depth)
 168       return visit_continue;
 169
 170    bool found_control_flow = false;
 171    ir_assignment *assign;
 172    ir_dereference_variable *deref;
 173
 174    /* Check that both blocks don't contain anything we can't support. */
 175    foreach_iter(exec_list_iterator, then_iter, ir->then_instructions) {
 176       ir_instruction *then_ir = (ir_instruction *)then_iter.get();
 177       visit_tree(then_ir, check_control_flow, &found_control_flow);
 178    }
 179    foreach_iter(exec_list_iterator, else_iter, ir->else_instructions) {
 180       ir_instruction *else_ir = (ir_instruction *)else_iter.get();
 181       visit_tree(else_ir, check_control_flow, &found_control_flow);
 182    }
 183    if (found_control_flow)
 184       return visit_continue;
 185
 186    void *mem_ctx = ralloc_parent(ir);
 187
 188    /* Store the condition to a variable.  Move all of the instructions from
 189     * the then-clause of the if-statement.  Use the condition variable as a
 190     * condition for all assignments.
 191     */
 192    ir_variable *const then_var =
 193       new(mem_ctx) ir_variable(glsl_type::bool_type,
 194                                "if_to_cond_assign_then",
 195                                ir_var_temporary);
 196    ir->insert_before(then_var);
 197
 198    ir_dereference_variable *then_cond =
 199       new(mem_ctx) ir_dereference_variable(then_var);
 200
 201    assign = new(mem_ctx) ir_assignment(then_cond, ir->condition);
 202    ir->insert_before(assign);
 203
 204    move_block_to_cond_assign(mem_ctx, ir, then_cond,
 205                              &ir->then_instructions,
 206                              this->condition_variables);
 207
 208    /* Add the new condition variable to the hash table.  This allows us to
 209     * find this variable when lowering other (enclosing) if-statements.
 210     */
 211    hash_table_insert(this->condition_variables, then_var, then_var);
 212
 213    /* If there are instructions in the else-clause, store the inverse of the
 214     * condition to a variable.  Move all of the instructions from the
 215     * else-clause if the if-statement.  Use the (inverse) condition variable
 216     * as a condition for all assignments.
 217     */
 218    if (!ir->else_instructions.is_empty()) {
 219       ir_variable *const else_var =
 220          new(mem_ctx) ir_variable(glsl_type::bool_type,
 221                                   "if_to_cond_assign_else",
 222                                   ir_var_temporary);
 223       ir->insert_before(else_var);
 224
 225       ir_dereference_variable *else_cond =
 226          new(mem_ctx) ir_dereference_variable(else_var);
 227
 228       ir_rvalue *inverse =
 229          new(mem_ctx) ir_expression(ir_unop_logic_not,
 230                                     then_cond->clone(mem_ctx, NULL));
 231
 232       assign = new(mem_ctx) ir_assignment(else_cond, inverse);
 233       ir->insert_before(assign);
 234
 235       move_block_to_cond_assign(mem_ctx, ir, else_cond,
 236                                 &ir->else_instructions,
 237                                 this->condition_variables);
 238
 239       /* Add the new condition variable to the hash table.  This allows us to
 240        * find this variable when lowering other (enclosing) if-statements.
 241        */
 242       hash_table_insert(this->condition_variables, else_var, else_var);
 243    }
 244
 245    ir->remove();
 246
 247    this->progress = true;
 248
 249    return visit_continue;
 250 }