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:
ir_if_simplification_visitor()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
visit_enter(ir_assignment * ir)55 ir_if_simplification_visitor::visit_enter(ir_assignment *ir)
56 {
57 (void) ir;
58 return visit_continue_with_parent;
59 }
60
61 bool
do_if_simplification(exec_list * instructions)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
visit_leave(ir_if * ir)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 }
127