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(gl_shader_stage stage,
  58                                 unsigned max_depth)
  59    {
  60       this->progress = false;
  61       this->stage = stage;
  62       this->max_depth = max_depth;
  63       this->depth = 0;
  64
  65       this->condition_variables =
  66             _mesa_set_create(NULL, _mesa_hash_pointer,
  67                                     _mesa_key_pointer_equal);
  68    }
  69
  70    ~ir_if_to_cond_assign_visitor()
  71    {
  72       _mesa_set_destroy(this->condition_variables, NULL);
  73    }
  74
  75    ir_visitor_status visit_enter(ir_if *);
  76    ir_visitor_status visit_leave(ir_if *);
  77
  78    bool found_unsupported_op;
  79    bool progress;
  80    gl_shader_stage stage;
  81    unsigned max_depth;
  82    unsigned depth;
  83
  84    struct set *condition_variables;
  85 };
  86
  87 } /* anonymous namespace */
  88
  89 bool
  90 lower_if_to_cond_assign(gl_shader_stage stage, exec_list *instructions,
  91                         unsigned max_depth)
  92 {
  93    if (max_depth == UINT_MAX)
  94       return false;
  95
  96    ir_if_to_cond_assign_visitor v(stage, max_depth);
  97
  98    visit_list_elements(&v, instructions);
  99
 100    return v.progress;
 101 }
 102
 103 void
 104 check_ir_node(ir_instruction *ir, void *data)
 105 {
 106    ir_if_to_cond_assign_visitor *v = (ir_if_to_cond_assign_visitor *)data;
 107
 108    switch (ir->ir_type) {
 109    case ir_type_call:
 110    case ir_type_discard:
 111    case ir_type_loop:
 112    case ir_type_loop_jump:
 113    case ir_type_return:
 114    case ir_type_emit_vertex:
 115    case ir_type_end_primitive:
 116    case ir_type_barrier:
 117       v->found_unsupported_op = true;
 118       break;
 119
 120    case ir_type_dereference_variable: {
 121       ir_variable *var = ir->as_dereference_variable()->variable_referenced();
 122
 123       /* Lowering branches with TCS output accesses breaks many piglit tests,
 124        * so don't touch them for now.
 125        */
 126       if (v->stage == MESA_SHADER_TESS_CTRL &&
 127           var->data.mode == ir_var_shader_out)
 128          v->found_unsupported_op = true;
 129       break;
 130    }
 131
 132    default:
 133       break;
 134    }
 135 }
 136
 137 void
 138 move_block_to_cond_assign(void *mem_ctx,
 139                           ir_if *if_ir, ir_rvalue *cond_expr,
 140                           exec_list *instructions,
 141                           struct set *set)
 142 {
 143    foreach_in_list_safe(ir_instruction, ir, instructions) {
 144       if (ir->ir_type == ir_type_assignment) {
 145          ir_assignment *assign = (ir_assignment *)ir;
 146
 147          if (_mesa_set_search(set, assign) == NULL) {
 148             _mesa_set_add(set, assign);
 149
 150             /* If the LHS of the assignment is a condition variable that was
 151              * previously added, insert an additional assignment of false to
 152              * the variable.
 153              */
 154             const bool assign_to_cv =
 155                   _mesa_set_search(
 156                         set, assign->lhs->variable_referenced()) != NULL;
 157
 158             if (!assign->condition) {
 159           if (assign_to_cv) {
 160              assign->rhs =
 161                           new(mem_ctx) ir_expression(ir_binop_logic_and,
 162                                            glsl_type::bool_type,
 163                                            cond_expr->clone(mem_ctx, NULL),
 164                                            assign->rhs);
 165           } else {
 166              assign->condition = cond_expr->clone(mem_ctx, NULL);
 167                }
 168             } else {
 169                assign->condition =
 170              new(mem_ctx) ir_expression(ir_binop_logic_and,
 171                                         glsl_type::bool_type,
 172                                         cond_expr->clone(mem_ctx, NULL),
 173                                         assign->condition);
 174             }
 175          }
 176       }
 177
 178       /* Now, move from the if block to the block surrounding it. */
 179       ir->remove();
 180       if_ir->insert_before(ir);
 181    }
 182 }
 183
 184 ir_visitor_status
 185 ir_if_to_cond_assign_visitor::visit_enter(ir_if *ir)
 186 {
 187    (void) ir;
 188    this->depth++;
 189
 190    return visit_continue;
 191 }
 192
 193 ir_visitor_status
 194 ir_if_to_cond_assign_visitor::visit_leave(ir_if *ir)
 195 {
 196    /* Only flatten when beyond the GPU's maximum supported nesting depth. */
 197    if (this->depth-- <= this->max_depth)
 198       return visit_continue;
 199
 200    this->found_unsupported_op = false;
 201
 202    ir_assignment *assign;
 203
 204    /* Check that both blocks don't contain anything we can't support. */
 205    foreach_in_list(ir_instruction, then_ir, &ir->then_instructions) {
 206       visit_tree(then_ir, check_ir_node, this);
 207    }
 208    foreach_in_list(ir_instruction, else_ir, &ir->else_instructions) {
 209       visit_tree(else_ir, check_ir_node, this);
 210    }
 211    if (this->found_unsupported_op)
 212       return visit_continue; /* can't handle inner unsupported opcodes */
 213
 214    void *mem_ctx = ralloc_parent(ir);
 215
 216    /* Store the condition to a variable.  Move all of the instructions from
 217     * the then-clause of the if-statement.  Use the condition variable as a
 218     * condition for all assignments.
 219     */
 220    ir_variable *const then_var =
 221       new(mem_ctx) ir_variable(glsl_type::bool_type,
 222                                "if_to_cond_assign_then",
 223                                ir_var_temporary);
 224    ir->insert_before(then_var);
 225
 226    ir_dereference_variable *then_cond =
 227       new(mem_ctx) ir_dereference_variable(then_var);
 228
 229    assign = new(mem_ctx) ir_assignment(then_cond, ir->condition);
 230    ir->insert_before(assign);
 231
 232    move_block_to_cond_assign(mem_ctx, ir, then_cond,
 233                              &ir->then_instructions,
 234                              this->condition_variables);
 235
 236    /* Add the new condition variable to the hash table.  This allows us to
 237     * find this variable when lowering other (enclosing) if-statements.
 238     */
 239    _mesa_set_add(this->condition_variables, then_var);
 240
 241    /* If there are instructions in the else-clause, store the inverse of the
 242     * condition to a variable.  Move all of the instructions from the
 243     * else-clause if the if-statement.  Use the (inverse) condition variable
 244     * as a condition for all assignments.
 245     */
 246    if (!ir->else_instructions.is_empty()) {
 247       ir_variable *const else_var =
 248          new(mem_ctx) ir_variable(glsl_type::bool_type,
 249                                   "if_to_cond_assign_else",
 250                                   ir_var_temporary);
 251       ir->insert_before(else_var);
 252
 253       ir_dereference_variable *else_cond =
 254          new(mem_ctx) ir_dereference_variable(else_var);
 255
 256       ir_rvalue *inverse =
 257          new(mem_ctx) ir_expression(ir_unop_logic_not,
 258                                     then_cond->clone(mem_ctx, NULL));
 259
 260       assign = new(mem_ctx) ir_assignment(else_cond, inverse);
 261       ir->insert_before(assign);
 262
 263       move_block_to_cond_assign(mem_ctx, ir, else_cond,
 264                                 &ir->else_instructions,
 265                                 this->condition_variables);
 266
 267       /* Add the new condition variable to the hash table.  This allows us to
 268        * find this variable when lowering other (enclosing) if-statements.
 269        */
 270       _mesa_set_add(this->condition_variables, else_var);
 271    }
 272
 273    ir->remove();
 274
 275    this->progress = true;
 276
 277    return visit_continue;
 278 }