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 /** @file lower_discard_flow.cpp
25 *
26 * Implements the GLSL 1.30 revision 9 rule for fragment shader
27 * discard handling:
28 *
29 * "Control flow exits the shader, and subsequent implicit or
30 * explicit derivatives are undefined when this control flow is
31 * non-uniform (meaning different fragments within the primitive
32 * take different control paths)."
33 *
34 * There seem to be two conflicting things here. "Control flow exits
35 * the shader" sounds like the discarded fragments should effectively
36 * jump to the end of the shader, but that breaks derivatives in the
37 * case of uniform control flow and causes rendering failure in the
38 * bushes in Unigine Tropics.
39 *
40 * The question, then, is whether the intent was "loops stop at the
41 * point that the only active channels left are discarded pixels" or
42 * "discarded pixels become inactive at the point that control flow
43 * returns to the top of a loop". This implements the second
44 * interpretation.
45 */
46
47 #include "compiler/glsl_types.h"
48 #include "ir.h"
49
50 namespace {
51
52 class lower_discard_flow_visitor : public ir_hierarchical_visitor {
53 public:
lower_discard_flow_visitor(ir_variable * discarded)54 lower_discard_flow_visitor(ir_variable *discarded)
55 : discarded(discarded)
56 {
57 mem_ctx = ralloc_parent(discarded);
58 }
59
~lower_discard_flow_visitor()60 ~lower_discard_flow_visitor()
61 {
62 }
63
64 ir_visitor_status visit(ir_loop_jump *ir);
65 ir_visitor_status visit_enter(ir_discard *ir);
66 ir_visitor_status visit_enter(ir_loop *ir);
67 ir_visitor_status visit_enter(ir_function_signature *ir);
68
69 ir_if *generate_discard_break();
70
71 ir_variable *discarded;
72 void *mem_ctx;
73 };
74
75 } /* anonymous namespace */
76
77 ir_visitor_status
visit(ir_loop_jump * ir)78 lower_discard_flow_visitor::visit(ir_loop_jump *ir)
79 {
80 if (ir->mode != ir_loop_jump::jump_continue)
81 return visit_continue;
82
83 ir->insert_before(generate_discard_break());
84
85 return visit_continue;
86 }
87
88 ir_visitor_status
visit_enter(ir_discard * ir)89 lower_discard_flow_visitor::visit_enter(ir_discard *ir)
90 {
91 ir_dereference *lhs = new(mem_ctx) ir_dereference_variable(discarded);
92 ir_rvalue *rhs;
93 if (ir->condition) {
94 /* discarded <- condition, use (var_ref discarded) as the condition */
95 rhs = ir->condition;
96 ir->condition = new(mem_ctx) ir_dereference_variable(discarded);
97 } else {
98 rhs = new(mem_ctx) ir_constant(true);
99 }
100 ir_assignment *assign = new(mem_ctx) ir_assignment(lhs, rhs);
101 ir->insert_before(assign);
102
103 return visit_continue;
104 }
105
106 ir_visitor_status
visit_enter(ir_loop * ir)107 lower_discard_flow_visitor::visit_enter(ir_loop *ir)
108 {
109 ir->body_instructions.push_tail(generate_discard_break());
110
111 return visit_continue;
112 }
113
114 ir_visitor_status
visit_enter(ir_function_signature * ir)115 lower_discard_flow_visitor::visit_enter(ir_function_signature *ir)
116 {
117 if (strcmp(ir->function_name(), "main") != 0)
118 return visit_continue;
119
120 ir_dereference *lhs = new(mem_ctx) ir_dereference_variable(discarded);
121 ir_rvalue *rhs = new(mem_ctx) ir_constant(false);
122 ir_assignment *assign = new(mem_ctx) ir_assignment(lhs, rhs);
123 ir->body.push_head(assign);
124
125 return visit_continue;
126 }
127
128 ir_if *
generate_discard_break()129 lower_discard_flow_visitor::generate_discard_break()
130 {
131 ir_rvalue *if_condition = new(mem_ctx) ir_dereference_variable(discarded);
132 ir_if *if_inst = new(mem_ctx) ir_if(if_condition);
133
134 ir_instruction *br = new(mem_ctx) ir_loop_jump(ir_loop_jump::jump_break);
135 if_inst->then_instructions.push_tail(br);
136
137 return if_inst;
138 }
139
140 void
lower_discard_flow(exec_list * ir)141 lower_discard_flow(exec_list *ir)
142 {
143 void *mem_ctx = ir;
144
145 ir_variable *var = new(mem_ctx) ir_variable(glsl_type::bool_type,
146 "discarded",
147 ir_var_temporary);
148
149 ir->push_head(var);
150
151 lower_discard_flow_visitor v(var);
152
153 visit_list_elements(&v, ir);
154 }
155