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
34namespace {
35
36class ir_if_simplification_visitor : public ir_hierarchical_visitor {
37public:
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 */
54ir_visitor_status
55ir_if_simplification_visitor::visit_enter(ir_assignment *ir)
56{
57   (void) ir;
58   return visit_continue_with_parent;
59}
60
61bool
62do_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
71ir_visitor_status
72ir_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 =
88      ir->condition->constant_expression_value(ralloc_parent(ir));
89   if (condition_constant) {
90      /* Move the contents of the one branch of the conditional
91       * that matters out.
92       */
93      if (condition_constant->value.b[0]) {
94         ir->insert_before(&ir->then_instructions);
95      } else {
96         ir->insert_before(&ir->else_instructions);
97      }
98      ir->remove();
99      this->made_progress = true;
100      return visit_continue;
101   }
102
103   /* Turn:
104    *
105    *     if (cond) {
106    *     } else {
107    *         do_work();
108    *     }
109    *
110    * into :
111    *
112    *     if (!cond)
113    *         do_work();
114    *
115    * which avoids control flow for "else" (which is usually more
116    * expensive than normal operations), and the "not" can usually be
117    * folded into the generation of "cond" anyway.
118    */
119   if (ir->then_instructions.is_empty()) {
120      ir->condition = new(ralloc_parent(ir->condition))
121	 ir_expression(ir_unop_logic_not, ir->condition);
122      ir->else_instructions.move_nodes_to(&ir->then_instructions);
123      this->made_progress = true;
124   }
125
126   return visit_continue;
127}
128