src/glsl/lower_discard_flow.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
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   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 "glsl_types.h"
  48 #include "ir.h"
  49 #include "program/hash_table.h"
  50
  51 namespace {
  52
  53 class lower_discard_flow_visitor : public ir_hierarchical_visitor {
  54 public:
  55    lower_discard_flow_visitor(ir_variable *discarded)
  56    : discarded(discarded)
  57    {
  58       mem_ctx = ralloc_parent(discarded);
  59    }
  60
  61    ~lower_discard_flow_visitor()
  62    {
  63    }
  64
  65    ir_visitor_status visit_enter(ir_discard *ir);
  66    ir_visitor_status visit_enter(ir_loop_jump *ir);
  67    ir_visitor_status visit_enter(ir_loop *ir);
  68    ir_visitor_status visit_enter(ir_function_signature *ir);
  69
  70    ir_if *generate_discard_break();
  71
  72    ir_variable *discarded;
  73    void *mem_ctx;
  74 };
  75
  76 } /* anonymous namespace */
  77
  78 ir_visitor_status
  79 lower_discard_flow_visitor::visit_enter(ir_loop_jump *ir)
  80 {
  81    if (ir->mode != ir_loop_jump::jump_continue)
  82       return visit_continue;
  83
  84    ir->insert_before(generate_discard_break());
  85
  86    return visit_continue;
  87 }
  88
  89 ir_visitor_status
  90 lower_discard_flow_visitor::visit_enter(ir_discard *ir)
  91 {
  92    ir_dereference *lhs = new(mem_ctx) ir_dereference_variable(discarded);
  93    ir_rvalue *rhs;
  94    if (ir->condition) {
  95       /* discarded <- condition, use (var_ref discarded) as the condition */
  96       rhs = ir->condition;
  97       ir->condition = new(mem_ctx) ir_dereference_variable(discarded);
  98    } else {
  99       rhs = new(mem_ctx) ir_constant(true);
 100    }
 101    ir_assignment *assign = new(mem_ctx) ir_assignment(lhs, rhs);
 102    ir->insert_before(assign);
 103
 104    return visit_continue;
 105 }
 106
 107 ir_visitor_status
 108 lower_discard_flow_visitor::visit_enter(ir_loop *ir)
 109 {
 110    ir->body_instructions.push_tail(generate_discard_break());
 111
 112    return visit_continue;
 113 }
 114
 115 ir_visitor_status
 116 lower_discard_flow_visitor::visit_enter(ir_function_signature *ir)
 117 {
 118    if (strcmp(ir->function_name(), "main") != 0)
 119       return visit_continue;
 120
 121    ir_dereference *lhs = new(mem_ctx) ir_dereference_variable(discarded);
 122    ir_rvalue *rhs = new(mem_ctx) ir_constant(false);
 123    ir_assignment *assign = new(mem_ctx) ir_assignment(lhs, rhs);
 124    ir->body.push_head(assign);
 125
 126    return visit_continue;
 127 }
 128
 129 ir_if *
 130 lower_discard_flow_visitor::generate_discard_break()
 131 {
 132    ir_rvalue *if_condition = new(mem_ctx) ir_dereference_variable(discarded);
 133    ir_if *if_inst = new(mem_ctx) ir_if(if_condition);
 134
 135    ir_instruction *br = new(mem_ctx) ir_loop_jump(ir_loop_jump::jump_break);
 136    if_inst->then_instructions.push_tail(br);
 137
 138    return if_inst;
 139 }
 140
 141 void
 142 lower_discard_flow(exec_list *ir)
 143 {
 144    void *mem_ctx = ir;
 145
 146    ir_variable *var = new(mem_ctx) ir_variable(glsl_type::bool_type,
 147                                                "discarded",
 148                                                ir_var_temporary);
 149
 150    ir->push_head(var);
 151
 152    lower_discard_flow_visitor v(var);
 153
 154    visit_list_elements(&v, ir);
 155 }