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_instruction *ir = (ir_instruction *)n;
 108
 109       if (ir->ir_type == ir_type_return ||
 110           ir->ir_type == ir_type_loop_jump)
 111          return NULL;
 112
 113       /* So far, this code doesn't know how to look inside of flow
 114        * control to see if a discard later on at this level is
 115        * unconditional.
 116        */
 117       if (ir->ir_type == ir_type_if ||
 118           ir->ir_type == ir_type_loop)
 119          return NULL;
 120
 121       ir_discard *discard = ir->as_discard();
 122       if (discard != NULL && discard->condition == NULL)
 123          return discard;
 124    }
 125    return NULL;
 126 }
 127
 128 static bool
 129 is_only_instruction(ir_discard *discard)
 130 {
 131    return (discard->prev->is_head_sentinel() &&
 132            discard->next->is_tail_sentinel());
 133 }
 134
 135 /* We only care about the top level instructions, so don't descend
 136  * into expressions.
 137  */
 138 ir_visitor_status
 139 discard_simplifier::visit_enter(ir_assignment *ir)
 140 {
 141    (void) ir;
 142    return visit_continue_with_parent;
 143 }
 144
 145 ir_visitor_status
 146 discard_simplifier::visit_enter(ir_if *ir)
 147 {
 148    ir_discard *then_discard = find_unconditional_discard(ir->then_instructions);
 149    ir_discard *else_discard = find_unconditional_discard(ir->else_instructions);
 150
 151    if (then_discard == NULL && else_discard == NULL)
 152       return visit_continue;
 153
 154    /* If both branches result in discard, replace whole if with discard. */
 155    if (then_discard != NULL && else_discard != NULL) {
 156       this->progress = true;
 157       ir->replace_with(then_discard);
 158       return visit_continue_with_parent;
 159    }
 160
 161    /* Otherwise, one branch has a discard. */
 162    if (then_discard != NULL && !is_only_instruction(then_discard)) {
 163       this->progress = true;
 164       ir->then_instructions.make_empty();
 165       ir->then_instructions.push_tail(then_discard);
 166    } else if (else_discard != NULL && !is_only_instruction(else_discard)) {
 167       this->progress = true;
 168       ir->else_instructions.make_empty();
 169       ir->else_instructions.push_tail(else_discard);
 170    }
 171
 172    visit_list_elements(this, &ir->then_instructions);
 173    return visit_continue_with_parent;
 174 }
 175
 176 ir_visitor_status
 177 discard_simplifier::visit_enter(ir_loop *ir)
 178 {
 179    ir_discard *discard = find_unconditional_discard(ir->body_instructions);
 180
 181    if (discard) {
 182       ir->replace_with(discard);
 183       return visit_continue_with_parent;
 184    }
 185
 186    return visit_continue;
 187 }
 188
 189 bool
 190 do_discard_simplification(exec_list *instructions)
 191 {
 192    /* Look for a top-level unconditional discard */
 193    ir_discard *discard = find_unconditional_discard(*instructions);
 194    if (discard != NULL) {
 195       instructions->make_empty();
 196       instructions->push_tail(discard);
 197       return true;
 198    }
 199
 200    discard_simplifier v;
 201
 202    visit_list_elements(&v, instructions);
 203
 204    return v.progress;
 205 }