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
50namespace {
51
52class lower_discard_flow_visitor : public ir_hierarchical_visitor {
53public:
54   lower_discard_flow_visitor(ir_variable *discarded)
55   : discarded(discarded)
56   {
57      mem_ctx = ralloc_parent(discarded);
58   }
59
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
77ir_visitor_status
78lower_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
88ir_visitor_status
89lower_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
106ir_visitor_status
107lower_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
114ir_visitor_status
115lower_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
128ir_if *
129lower_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
140void
141lower_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