src/glsl/opt_discard_simplification.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 opt_discard_simplification.cpp
  26  *
  27  * This pass simplifies if-statements and loops containing unconditional
  28  * discards.
  29  *
  30  * Case 1: Both branches contain unconditional discards:
  31  * -----------------------------------------------------
  32  *
  33  *    if (cond) {
  34  *       s1;
  35  *       discard;
  36  *       s2;
  37  *    } else {
  38  *       s3;
  39  *       discard;
  40  *       s4;
  41  *    }
  42  *
  43  * becomes:
  44  *
  45  *    discard
  46  *
  47  * Case 2: The "then" clause contains an unconditional discard:
  48  * ------------------------------------------------------------
  49  *
  50  *    if (cond) {
  51  *       s1;
  52  *       discard;
  53  *       s2;
  54  *    } else {
  55  *       s3;
  56  *    }
  57  *
  58  * becomes:
  59  *
  60  *    if (cond) {
  61  *       discard;
  62  *    } else {
  63  *       s3;
  64  *    }
  65  *
  66  * Case 3: The "else" clause contains an unconditional discard:
  67  * ------------------------------------------------------------
  68  *
  69  *    if (cond) {
  70  *       s1;
  71  *    } else {
  72  *       s2;
  73  *       discard;
  74  *       s3;
  75  *    }
  76  *
  77  * becomes:
  78  *
  79  *    if (cond) {
  80  *       s1;
  81  *    } else {
  82  *       discard;
  83  *    }
  84  */
  85
  86 #include "glsl_types.h"
  87 #include "ir.h"
  88
  89 class discard_simplifier : public ir_hierarchical_visitor {
  90 public:
  91    discard_simplifier()
  92    {
  93       this->progress = false;
  94    }
  95
  96    ir_visitor_status visit_enter(ir_if *);
  97    ir_visitor_status visit_enter(ir_loop *);
  98    ir_visitor_status visit_enter(ir_assignment *);
  99
 100    bool progress;
 101 };
 102
 103 static ir_discard *
 104 find_unconditional_discard(exec_list &instructions)
 105 {
 106    foreach_list(n, &instructions) {
 107       ir_discard *ir = ((ir_instruction *) n)->as_discard();
 108       if (ir != NULL && ir->condition == NULL)
 109          return ir;
 110    }
 111    return NULL;
 112 }
 113
 114 static bool
 115 is_only_instruction(ir_discard *discard)
 116 {
 117    return (discard->prev->is_head_sentinel() &&
 118            discard->next->is_tail_sentinel());
 119 }
 120
 121 /* We only care about the top level instructions, so don't descend
 122  * into expressions.
 123  */
 124 ir_visitor_status
 125 discard_simplifier::visit_enter(ir_assignment *ir)
 126 {
 127    return visit_continue_with_parent;
 128 }
 129
 130 ir_visitor_status
 131 discard_simplifier::visit_enter(ir_if *ir)
 132 {
 133    ir_discard *then_discard = find_unconditional_discard(ir->then_instructions);
 134    ir_discard *else_discard = find_unconditional_discard(ir->else_instructions);
 135
 136    if (then_discard == NULL && else_discard == NULL)
 137       return visit_continue;
 138
 139    /* If both branches result in discard, replace whole if with discard. */
 140    if (then_discard != NULL && else_discard != NULL) {
 141       this->progress = true;
 142       ir->replace_with(then_discard);
 143       return visit_continue_with_parent;
 144    }
 145
 146    /* Otherwise, one branch has a discard. */
 147    if (then_discard != NULL && !is_only_instruction(then_discard)) {
 148       this->progress = true;
 149       ir->then_instructions.make_empty();
 150       ir->then_instructions.push_tail(then_discard);
 151    } else if (else_discard != NULL && !is_only_instruction(else_discard)) {
 152       this->progress = true;
 153       ir->else_instructions.make_empty();
 154       ir->else_instructions.push_tail(else_discard);
 155    }
 156
 157    visit_list_elements(this, &ir->then_instructions);
 158    return visit_continue_with_parent;
 159 }
 160
 161 ir_visitor_status
 162 discard_simplifier::visit_enter(ir_loop *ir)
 163 {
 164    ir_discard *discard = find_unconditional_discard(ir->body_instructions);
 165
 166    if (discard) {
 167       ir->replace_with(discard);
 168       return visit_continue_with_parent;
 169    }
 170
 171    return visit_continue;
 172 }
 173
 174 bool
 175 do_discard_simplification(exec_list *instructions)
 176 {
 177    /* Look for a top-level unconditional discard */
 178    ir_discard *discard = find_unconditional_discard(*instructions);
 179    if (discard != NULL) {
 180       instructions->make_empty();
 181       instructions->push_tail(discard);
 182       return true;
 183    }
 184
 185    discard_simplifier v;
 186
 187    visit_list_elements(&v, instructions);
 188
 189    return v.progress;
 190 }