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 "program/hash_table.h"
  51
  52 namespace {
  53
  54 class ir_if_to_cond_assign_visitor : public ir_hierarchical_visitor {
  55 public:
  56    ir_if_to_cond_assign_visitor(unsigned max_depth)
  57    {
  58       this->progress = false;
  59       this->max_depth = max_depth;
  60       this->depth = 0;
  61
  62       this->condition_variables = hash_table_ctor(0, hash_table_pointer_hash,
  63                                                   hash_table_pointer_compare);
  64    }
  65
  66    ~ir_if_to_cond_assign_visitor()
  67    {
  68       hash_table_dtor(this->condition_variables);
  69    }
  70
  71    ir_visitor_status visit_enter(ir_if *);
  72    ir_visitor_status visit_leave(ir_if *);
  73
  74    bool progress;
  75    unsigned max_depth;
  76    unsigned depth;
  77
  78    struct hash_table *condition_variables;
  79 };
  80
  81 } /* anonymous namespace */
  82
  83 bool
  84 lower_if_to_cond_assign(exec_list *instructions, unsigned max_depth)
  85 {
  86    if (max_depth == UINT_MAX)
  87       return false;
  88
  89    ir_if_to_cond_assign_visitor v(max_depth);
  90
  91    visit_list_elements(&v, instructions);
  92
  93    return v.progress;
  94 }
  95
  96 void
  97 check_control_flow(ir_instruction *ir, void *data)
  98 {
  99    bool *found_control_flow = (bool *)data;
 100    switch (ir->ir_type) {
 101    case ir_type_call:
 102    case ir_type_discard:
 103    case ir_type_loop:
 104    case ir_type_loop_jump:
 105    case ir_type_return:
 106       *found_control_flow = true;
 107       break;
 108    default:
 109       break;
 110    }
 111 }
 112
 113 void
 114 move_block_to_cond_assign(void *mem_ctx,
 115                           ir_if *if_ir, ir_rvalue *cond_expr,
 116                           exec_list *instructions,
 117                           struct hash_table *ht)
 118 {
 119    foreach_in_list_safe(ir_instruction, ir, instructions) {
 120       if (ir->ir_type == ir_type_assignment) {
 121          ir_assignment *assign = (ir_assignment *)ir;
 122
 123          if (hash_table_find(ht, assign) == NULL) {
 124             hash_table_insert(ht, assign, assign);
 125
 126             /* If the LHS of the assignment is a condition variable that was
 127              * previously added, insert an additional assignment of false to
 128              * the variable.
 129              */
 130             const bool assign_to_cv =
 131                hash_table_find(ht, assign->lhs->variable_referenced()) != NULL;
 132
 133             if (!assign->condition) {
 134                if (assign_to_cv) {
 135                   assign->rhs =
 136                      new(mem_ctx) ir_expression(ir_binop_logic_and,
 137                                                 glsl_type::bool_type,
 138                                                 cond_expr->clone(mem_ctx, NULL),
 139                                                 assign->rhs);
 140                } else {
 141                   assign->condition = cond_expr->clone(mem_ctx, NULL);
 142                }
 143             } else {
 144                assign->condition =
 145                   new(mem_ctx) ir_expression(ir_binop_logic_and,
 146                                              glsl_type::bool_type,
 147                                              cond_expr->clone(mem_ctx, NULL),
 148                                              assign->condition);
 149             }
 150          }
 151       }
 152
 153       /* Now, move from the if block to the block surrounding it. */
 154       ir->remove();
 155       if_ir->insert_before(ir);
 156    }
 157 }
 158
 159 ir_visitor_status
 160 ir_if_to_cond_assign_visitor::visit_enter(ir_if *ir)
 161 {
 162    (void) ir;
 163    this->depth++;
 164
 165    return visit_continue;
 166 }
 167
 168 ir_visitor_status
 169 ir_if_to_cond_assign_visitor::visit_leave(ir_if *ir)
 170 {
 171    /* Only flatten when beyond the GPU's maximum supported nesting depth. */
 172    if (this->depth-- <= this->max_depth)
 173       return visit_continue;
 174
 175    bool found_control_flow = false;
 176    ir_assignment *assign;
 177
 178    /* Check that both blocks don't contain anything we can't support. */
 179    foreach_in_list(ir_instruction, then_ir, &ir->then_instructions) {
 180       visit_tree(then_ir, check_control_flow, &found_control_flow);
 181    }
 182    foreach_in_list(ir_instruction, else_ir, &ir->else_instructions) {
 183       visit_tree(else_ir, check_control_flow, &found_control_flow);
 184    }
 185    if (found_control_flow)
 186       return visit_continue;
 187
 188    void *mem_ctx = ralloc_parent(ir);
 189
 190    /* Store the condition to a variable.  Move all of the instructions from
 191     * the then-clause of the if-statement.  Use the condition variable as a
 192     * condition for all assignments.
 193     */
 194    ir_variable *const then_var =
 195       new(mem_ctx) ir_variable(glsl_type::bool_type,
 196                                "if_to_cond_assign_then",
 197                                ir_var_temporary);
 198    ir->insert_before(then_var);
 199
 200    ir_dereference_variable *then_cond =
 201       new(mem_ctx) ir_dereference_variable(then_var);
 202
 203    assign = new(mem_ctx) ir_assignment(then_cond, ir->condition);
 204    ir->insert_before(assign);
 205
 206    move_block_to_cond_assign(mem_ctx, ir, then_cond,
 207                              &ir->then_instructions,
 208                              this->condition_variables);
 209
 210    /* Add the new condition variable to the hash table.  This allows us to
 211     * find this variable when lowering other (enclosing) if-statements.
 212     */
 213    hash_table_insert(this->condition_variables, then_var, then_var);
 214
 215    /* If there are instructions in the else-clause, store the inverse of the
 216     * condition to a variable.  Move all of the instructions from the
 217     * else-clause if the if-statement.  Use the (inverse) condition variable
 218     * as a condition for all assignments.
 219     */
 220    if (!ir->else_instructions.is_empty()) {
 221       ir_variable *const else_var =
 222          new(mem_ctx) ir_variable(glsl_type::bool_type,
 223                                   "if_to_cond_assign_else",
 224                                   ir_var_temporary);
 225       ir->insert_before(else_var);
 226
 227       ir_dereference_variable *else_cond =
 228          new(mem_ctx) ir_dereference_variable(else_var);
 229
 230       ir_rvalue *inverse =
 231          new(mem_ctx) ir_expression(ir_unop_logic_not,
 232                                     then_cond->clone(mem_ctx, NULL));
 233
 234       assign = new(mem_ctx) ir_assignment(else_cond, inverse);
 235       ir->insert_before(assign);
 236
 237       move_block_to_cond_assign(mem_ctx, ir, else_cond,
 238                                 &ir->else_instructions,
 239                                 this->condition_variables);
 240
 241       /* Add the new condition variable to the hash table.  This allows us to
 242        * find this variable when lowering other (enclosing) if-statements.
 243        */
 244       hash_table_insert(this->condition_variables, else_var, else_var);
 245    }
 246
 247    ir->remove();
 248
 249    this->progress = true;
 250
 251    return visit_continue;
 252 }