src/compiler/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"),
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   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
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  20  * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
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  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:
  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
  77 ir_visitor_status
  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
  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
 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
 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 *
 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
 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 }