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1 // Copyright (c) 2017 Google Inc.
2 //
3 // Licensed under the Apache License, Version 2.0 (the "License");
4 // you may not use this file except in compliance with the License.
5 // You may obtain a copy of the License at
6 //
7 //     http://www.apache.org/licenses/LICENSE-2.0
8 //
9 // Unless required by applicable law or agreed to in writing, software
10 // distributed under the License is distributed on an "AS IS" BASIS,
11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12 // See the License for the specific language governing permissions and
13 // limitations under the License.
14 
15 #include "source/opt/dead_variable_elimination.h"
16 
17 #include <vector>
18 
19 #include "source/opt/ir_context.h"
20 #include "source/opt/reflect.h"
21 
22 namespace spvtools {
23 namespace opt {
24 
25 // This optimization removes global variables that are not needed because they
26 // are definitely not accessed.
Process()27 Pass::Status DeadVariableElimination::Process() {
28   // The algorithm will compute the reference count for every global variable.
29   // Anything with a reference count of 0 will then be deleted.  For variables
30   // that might have references that are not explicit in this context, we use
31   // the value kMustKeep as the reference count.
32   std::vector<uint32_t> ids_to_remove;
33 
34   // Get the reference count for all of the global OpVariable instructions.
35   for (auto& inst : context()->types_values()) {
36     if (inst.opcode() != spv::Op::OpVariable) {
37       continue;
38     }
39 
40     size_t count = 0;
41     uint32_t result_id = inst.result_id();
42 
43     // Check the linkage.  If it is exported, it could be reference somewhere
44     // else, so we must keep the variable around.
45     get_decoration_mgr()->ForEachDecoration(
46         result_id, uint32_t(spv::Decoration::LinkageAttributes),
47         [&count](const Instruction& linkage_instruction) {
48           uint32_t last_operand = linkage_instruction.NumOperands() - 1;
49           if (spv::LinkageType(linkage_instruction.GetSingleWordOperand(
50                   last_operand)) == spv::LinkageType::Export) {
51             count = kMustKeep;
52           }
53         });
54 
55     if (count != kMustKeep) {
56       // If we don't have to keep the instruction for other reasons, then look
57       // at the uses and count the number of real references.
58       count = 0;
59       get_def_use_mgr()->ForEachUser(result_id, [&count](Instruction* user) {
60         if (!IsAnnotationInst(user->opcode()) &&
61             user->opcode() != spv::Op::OpName) {
62           ++count;
63         }
64       });
65     }
66     reference_count_[result_id] = count;
67     if (count == 0) {
68       ids_to_remove.push_back(result_id);
69     }
70   }
71 
72   // Remove all of the variables that have a reference count of 0.
73   bool modified = false;
74   if (!ids_to_remove.empty()) {
75     modified = true;
76     for (auto result_id : ids_to_remove) {
77       DeleteVariable(result_id);
78     }
79   }
80   return (modified ? Status::SuccessWithChange : Status::SuccessWithoutChange);
81 }
82 
DeleteVariable(uint32_t result_id)83 void DeadVariableElimination::DeleteVariable(uint32_t result_id) {
84   Instruction* inst = get_def_use_mgr()->GetDef(result_id);
85   assert(inst->opcode() == spv::Op::OpVariable &&
86          "Should not be trying to delete anything other than an OpVariable.");
87 
88   // Look for an initializer that references another variable.  We need to know
89   // if that variable can be deleted after the reference is removed.
90   if (inst->NumOperands() == 4) {
91     Instruction* initializer =
92         get_def_use_mgr()->GetDef(inst->GetSingleWordOperand(3));
93 
94     // TODO: Handle OpSpecConstantOP which might be defined in terms of other
95     // variables.  Will probably require a unified dead code pass that does all
96     // instruction types.  (Issue 906)
97     if (initializer->opcode() == spv::Op::OpVariable) {
98       uint32_t initializer_id = initializer->result_id();
99       size_t& count = reference_count_[initializer_id];
100       if (count != kMustKeep) {
101         --count;
102       }
103 
104       if (count == 0) {
105         DeleteVariable(initializer_id);
106       }
107     }
108   }
109   context()->KillDef(result_id);
110 }
111 }  // namespace opt
112 }  // namespace spvtools
113