1 // Copyright (c) 2016 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/eliminate_dead_constant_pass.h"
16
17 #include <algorithm>
18 #include <unordered_map>
19 #include <unordered_set>
20 #include <vector>
21
22 #include "source/opt/def_use_manager.h"
23 #include "source/opt/log.h"
24 #include "source/opt/reflect.h"
25
26 namespace spvtools {
27 namespace opt {
28
Process()29 Pass::Status EliminateDeadConstantPass::Process() {
30 std::unordered_set<Instruction*> working_list;
31 // Traverse all the instructions to get the initial set of dead constants as
32 // working list and count number of real uses for constants. Uses in
33 // annotation instructions do not count.
34 std::unordered_map<Instruction*, size_t> use_counts;
35 std::vector<Instruction*> constants = context()->GetConstants();
36 for (auto* c : constants) {
37 uint32_t const_id = c->result_id();
38 size_t count = 0;
39 context()->get_def_use_mgr()->ForEachUse(
40 const_id, [&count](Instruction* user, uint32_t index) {
41 (void)index;
42 spv::Op op = user->opcode();
43 if (!(IsAnnotationInst(op) || IsDebug1Inst(op) || IsDebug2Inst(op) ||
44 IsDebug3Inst(op))) {
45 ++count;
46 }
47 });
48 use_counts[c] = count;
49 if (!count) {
50 working_list.insert(c);
51 }
52 }
53
54 // Start from the constants with 0 uses, back trace through the def-use chain
55 // to find all dead constants.
56 std::unordered_set<Instruction*> dead_consts;
57 while (!working_list.empty()) {
58 Instruction* inst = *working_list.begin();
59 // Back propagate if the instruction contains IDs in its operands.
60 switch (inst->opcode()) {
61 case spv::Op::OpConstantComposite:
62 case spv::Op::OpSpecConstantComposite:
63 case spv::Op::OpSpecConstantOp:
64 for (uint32_t i = 0; i < inst->NumInOperands(); i++) {
65 // SpecConstantOp instruction contains 'opcode' as its operand. Need
66 // to exclude such operands when decreasing uses.
67 if (inst->GetInOperand(i).type != SPV_OPERAND_TYPE_ID) {
68 continue;
69 }
70 uint32_t operand_id = inst->GetSingleWordInOperand(i);
71 Instruction* def_inst =
72 context()->get_def_use_mgr()->GetDef(operand_id);
73 // If the use_count does not have any count for the def_inst,
74 // def_inst must not be a constant, and should be ignored here.
75 if (!use_counts.count(def_inst)) {
76 continue;
77 }
78 // The number of uses should never be less then 0, so it can not be
79 // less than 1 before it decreases.
80 SPIRV_ASSERT(consumer(), use_counts[def_inst] > 0);
81 --use_counts[def_inst];
82 if (!use_counts[def_inst]) {
83 working_list.insert(def_inst);
84 }
85 }
86 break;
87 default:
88 break;
89 }
90 dead_consts.insert(inst);
91 working_list.erase(inst);
92 }
93
94 // Turn all dead instructions and uses of them to nop
95 for (auto* dc : dead_consts) {
96 context()->KillDef(dc->result_id());
97 }
98 return dead_consts.empty() ? Status::SuccessWithoutChange
99 : Status::SuccessWithChange;
100 }
101
102 } // namespace opt
103 } // namespace spvtools
104