src/glsl/opt_if_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_if_simplification.cpp
  26  *
  27  * Moves constant branches of if statements out to the surrounding
  28  * instruction stream, and inverts if conditionals to avoid empty
  29  * "then" blocks.
  30  */
  31
  32 #include "ir.h"
  33
  34 namespace {
  35
  36 class ir_if_simplification_visitor : public ir_hierarchical_visitor {
  37 public:
  38    ir_if_simplification_visitor()
  39    {
  40       this->made_progress = false;
  41    }
  42
  43    ir_visitor_status visit_leave(ir_if *);
  44    ir_visitor_status visit_enter(ir_assignment *);
  45
  46    bool made_progress;
  47 };
  48
  49 } /* unnamed namespace */
  50
  51 /* We only care about the top level "if" instructions, so don't
  52  * descend into expressions.
  53  */
  54 ir_visitor_status
  55 ir_if_simplification_visitor::visit_enter(ir_assignment *ir)
  56 {
  57    (void) ir;
  58    return visit_continue_with_parent;
  59 }
  60
  61 bool
  62 do_if_simplification(exec_list *instructions)
  63 {
  64    ir_if_simplification_visitor v;
  65
  66    v.run(instructions);
  67    return v.made_progress;
  68 }
  69
  70
  71 ir_visitor_status
  72 ir_if_simplification_visitor::visit_leave(ir_if *ir)
  73 {
  74    /* If the if statement has nothing on either side, remove it. */
  75    if (ir->then_instructions.is_empty() &&
  76        ir->else_instructions.is_empty()) {
  77       ir->remove();
  78       this->made_progress = true;
  79       return visit_continue;
  80    }
  81
  82    /* FINISHME: Ideally there would be a way to note that the condition results
  83     * FINISHME: in a constant before processing both of the other subtrees.
  84     * FINISHME: This can probably be done with some flags, but it would take
  85     * FINISHME: some work to get right.
  86     */
  87    ir_constant *condition_constant = ir->condition->constant_expression_value();
  88    if (condition_constant) {
  89       /* Move the contents of the one branch of the conditional
  90        * that matters out.
  91        */
  92       if (condition_constant->value.b[0]) {
  93          ir->insert_before(&ir->then_instructions);
  94       } else {
  95          ir->insert_before(&ir->else_instructions);
  96       }
  97       ir->remove();
  98       this->made_progress = true;
  99       return visit_continue;
 100    }
 101
 102    /* Turn:
 103     *
 104     *     if (cond) {
 105     *     } else {
 106     *         do_work();
 107     *     }
 108     *
 109     * into :
 110     *
 111     *     if (!cond)
 112     *         do_work();
 113     *
 114     * which avoids control flow for "else" (which is usually more
 115     * expensive than normal operations), and the "not" can usually be
 116     * folded into the generation of "cond" anyway.
 117     */
 118    if (ir->then_instructions.is_empty()) {
 119       ir->condition = new(ralloc_parent(ir->condition))
 120          ir_expression(ir_unop_logic_not, ir->condition);
 121       ir->else_instructions.move_nodes_to(&ir->then_instructions);
 122       this->made_progress = true;
 123    }
 124
 125    return visit_continue;
 126 }