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
  51 class ir_if_to_cond_assign_visitor : public ir_hierarchical_visitor {
  52 public:
  53    ir_if_to_cond_assign_visitor(unsigned max_depth)
  54    {
  55       this->progress = false;
  56       this->max_depth = max_depth;
  57       this->depth = 0;
  58    }
  59
  60    ir_visitor_status visit_enter(ir_if *);
  61    ir_visitor_status visit_leave(ir_if *);
  62
  63    bool progress;
  64    unsigned max_depth;
  65    unsigned depth;
  66 };
  67
  68 bool
  69 lower_if_to_cond_assign(exec_list *instructions, unsigned max_depth)
  70 {
  71    ir_if_to_cond_assign_visitor v(max_depth);
  72
  73    visit_list_elements(&v, instructions);
  74
  75    return v.progress;
  76 }
  77
  78 void
  79 check_control_flow(ir_instruction *ir, void *data)
  80 {
  81    bool *found_control_flow = (bool *)data;
  82    switch (ir->ir_type) {
  83    case ir_type_call:
  84    case ir_type_discard:
  85    case ir_type_loop:
  86    case ir_type_loop_jump:
  87    case ir_type_return:
  88       *found_control_flow = true;
  89       break;
  90    default:
  91       break;
  92    }
  93 }
  94
  95 void
  96 move_block_to_cond_assign(void *mem_ctx,
  97                           ir_if *if_ir, ir_variable *cond_var, bool then)
  98 {
  99    exec_list *instructions;
 100
 101    if (then) {
 102       instructions = &if_ir->then_instructions;
 103    } else {
 104       instructions = &if_ir->else_instructions;
 105    }
 106
 107    foreach_iter(exec_list_iterator, iter, *instructions) {
 108       ir_instruction *ir = (ir_instruction *)iter.get();
 109
 110       if (ir->ir_type == ir_type_assignment) {
 111          ir_assignment *assign = (ir_assignment *)ir;
 112          ir_rvalue *cond_expr;
 113          ir_dereference *deref = new(mem_ctx) ir_dereference_variable(cond_var);
 114
 115          if (then) {
 116             cond_expr = deref;
 117          } else {
 118             cond_expr = new(mem_ctx) ir_expression(ir_unop_logic_not,
 119                                                    glsl_type::bool_type,
 120                                                    deref,
 121                                                    NULL);
 122          }
 123
 124          if (!assign->condition) {
 125             assign->condition = cond_expr;
 126          } else {
 127             assign->condition = new(mem_ctx) ir_expression(ir_binop_logic_and,
 128                                                            glsl_type::bool_type,
 129                                                            cond_expr,
 130                                                            assign->condition);
 131          }
 132       }
 133
 134       /* Now, move from the if block to the block surrounding it. */
 135       ir->remove();
 136       if_ir->insert_before(ir);
 137    }
 138 }
 139
 140 ir_visitor_status
 141 ir_if_to_cond_assign_visitor::visit_enter(ir_if *ir)
 142 {
 143    (void) ir;
 144    this->depth++;
 145    return visit_continue;
 146 }
 147
 148 ir_visitor_status
 149 ir_if_to_cond_assign_visitor::visit_leave(ir_if *ir)
 150 {
 151    /* Only flatten when beyond the GPU's maximum supported nesting depth. */
 152    if (this->depth <= this->max_depth)
 153       return visit_continue;
 154
 155    this->depth--;
 156
 157    bool found_control_flow = false;
 158    ir_variable *cond_var;
 159    ir_assignment *assign;
 160    ir_dereference_variable *deref;
 161
 162    /* Check that both blocks don't contain anything we can't support. */
 163    foreach_iter(exec_list_iterator, then_iter, ir->then_instructions) {
 164       ir_instruction *then_ir = (ir_instruction *)then_iter.get();
 165       visit_tree(then_ir, check_control_flow, &found_control_flow);
 166    }
 167    foreach_iter(exec_list_iterator, else_iter, ir->else_instructions) {
 168       ir_instruction *else_ir = (ir_instruction *)else_iter.get();
 169       visit_tree(else_ir, check_control_flow, &found_control_flow);
 170    }
 171    if (found_control_flow)
 172       return visit_continue;
 173
 174    void *mem_ctx = ralloc_parent(ir);
 175
 176    /* Store the condition to a variable so the assignment conditions are
 177     * simpler.
 178     */
 179    cond_var = new(mem_ctx) ir_variable(glsl_type::bool_type,
 180                                        "if_to_cond_assign_condition",
 181                                        ir_var_temporary);
 182    ir->insert_before(cond_var);
 183
 184    deref = new(mem_ctx) ir_dereference_variable(cond_var);
 185    assign = new(mem_ctx) ir_assignment(deref,
 186                                        ir->condition, NULL);
 187    ir->insert_before(assign);
 188
 189    /* Now, move all of the instructions out of the if blocks, putting
 190     * conditions on assignments.
 191     */
 192    move_block_to_cond_assign(mem_ctx, ir, cond_var, true);
 193    move_block_to_cond_assign(mem_ctx, ir, cond_var, false);
 194
 195    ir->remove();
 196
 197    this->progress = true;
 198
 199    return visit_continue;
 200 }