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/unify_const_pass.h"
16
17#include <memory>
18#include <unordered_map>
19#include <utility>
20#include <vector>
21
22#include "source/opt/def_use_manager.h"
23#include "source/opt/ir_context.h"
24#include "source/util/make_unique.h"
25
26namespace spvtools {
27namespace opt {
28
29namespace {
30
31// The trie that stores a bunch of result ids and, for a given instruction,
32// searches the result id that has been defined with the same opcode, type and
33// operands.
34class ResultIdTrie {
35 public:
36 ResultIdTrie() : root_(new Node) {}
37
38 // For a given instruction, extracts its opcode, type id and operand words
39 // as an array of keys, looks up the trie to find a result id which is stored
40 // with the same opcode, type id and operand words. If none of such result id
41 // is found, creates a trie node with those keys, stores the instruction's
42 // result id and returns that result id. If an existing result id is found,
43 // returns the existing result id.
44 uint32_t LookupEquivalentResultFor(const Instruction& inst) {
45 auto keys = GetLookUpKeys(inst);
46 auto* node = root_.get();
47 for (uint32_t key : keys) {
48 node = node->GetOrCreateTrieNodeFor(key);
49 }
50 if (node->result_id() == 0) {
51 node->SetResultId(inst.result_id());
52 }
53 return node->result_id();
54 }
55
56 private:
57 // The trie node to store result ids.
58 class Node {
59 public:
60 using TrieNodeMap = std::unordered_map<uint32_t, std::unique_ptr<Node>>;
61
62 Node() : result_id_(0), next_() {}
63 uint32_t result_id() const { return result_id_; }
64
65 // Sets the result id stored in this node.
66 void SetResultId(uint32_t id) { result_id_ = id; }
67
68 // Searches for the child trie node with the given key. If the node is
69 // found, returns that node. Otherwise creates an empty child node with
70 // that key and returns that newly created node.
71 Node* GetOrCreateTrieNodeFor(uint32_t key) {
72 auto iter = next_.find(key);
73 if (iter == next_.end()) {
74 // insert a new node and return the node.
75 return next_.insert(std::make_pair(key, MakeUnique<Node>()))
76 .first->second.get();
77 }
78 return iter->second.get();
79 }
80
81 private:
82 // The result id stored in this node. 0 means this node is empty.
83 uint32_t result_id_;
84 // The mapping from the keys to the child nodes of this node.
85 TrieNodeMap next_;
86 };
87
88 // Returns a vector of the opcode followed by the words in the raw SPIR-V
89 // instruction encoding but without the result id.
90 std::vector<uint32_t> GetLookUpKeys(const Instruction& inst) {
91 std::vector<uint32_t> keys;
92 // Need to use the opcode, otherwise there might be a conflict with the
93 // following case when <op>'s binary value equals xx's id:
94 // OpSpecConstantOp tt <op> yy zz
95 // OpSpecConstantComposite tt xx yy zz;
96 keys.push_back(static_cast<uint32_t>(inst.opcode()));
97 for (const auto& operand : inst) {
98 if (operand.type == SPV_OPERAND_TYPE_RESULT_ID) continue;
99 keys.insert(keys.end(), operand.words.cbegin(), operand.words.cend());
100 }
101 return keys;
102 }
103
104 std::unique_ptr<Node> root_; // The root node of the trie.
105};
106} // anonymous namespace
107
108Pass::Status UnifyConstantPass::Process() {
109 bool modified = false;
110 ResultIdTrie defined_constants;
111
112 for (Instruction *next_instruction,
113 *inst = &*(context()->types_values_begin());
114 inst; inst = next_instruction) {
115 next_instruction = inst->NextNode();
116
117 // Do not handle the instruction when there are decorations upon the result
118 // id.
119 if (get_def_use_mgr()->GetAnnotations(inst->result_id()).size() != 0) {
120 continue;
121 }
122
123 // The overall algorithm is to store the result ids of all the eligible
124 // constants encountered so far in a trie. For a constant defining
125 // instruction under consideration, use its opcode, result type id and
126 // words in operands as an array of keys to lookup the trie. If a result id
127 // can be found for that array of keys, a constant with exactly the same
128 // value must has been defined before, the constant under processing
129 // should be replaced by the constant previously defined. If no such result
130 // id can be found for that array of keys, this must be the first time a
131 // constant with its value be defined, we then create a new trie node to
132 // store the result id with the keys. When replacing a duplicated constant
133 // with a previously defined constant, all the uses of the duplicated
134 // constant, which must be placed after the duplicated constant defining
135 // instruction, will be updated. This way, the descendants of the
136 // previously defined constant and the duplicated constant will both refer
137 // to the previously defined constant. So that the operand ids which are
138 // used in key arrays will be the ids of the unified constants, when
139 // processing is up to a descendant. This makes comparing the key array
140 // always valid for judging duplication.
141 switch (inst->opcode()) {
142 case SpvOp::SpvOpConstantTrue:
143 case SpvOp::SpvOpConstantFalse:
144 case SpvOp::SpvOpConstant:
145 case SpvOp::SpvOpConstantNull:
146 case SpvOp::SpvOpConstantSampler:
147 case SpvOp::SpvOpConstantComposite:
148 // Only spec constants defined with OpSpecConstantOp and
149 // OpSpecConstantComposite should be processed in this pass. Spec
150 // constants defined with OpSpecConstant{|True|False} are decorated with
151 // 'SpecId' decoration and all of them should be treated as unique.
152 // 'SpecId' is not applicable to SpecConstants defined with
153 // OpSpecConstant{Op|Composite}, their values are not necessary to be
154 // unique. When all the operands/compoents are the same between two
155 // OpSpecConstant{Op|Composite} results, their result values must be the
156 // same so are unifiable.
157 case SpvOp::SpvOpSpecConstantOp:
158 case SpvOp::SpvOpSpecConstantComposite: {
159 uint32_t id = defined_constants.LookupEquivalentResultFor(*inst);
160 if (id != inst->result_id()) {
161 // The constant is a duplicated one, use the cached constant to
162 // replace the uses of this duplicated one, then turn it to nop.
163 context()->ReplaceAllUsesWith(inst->result_id(), id);
164 context()->KillInst(inst);
165 modified = true;
166 }
167 break;
168 }
169 default:
170 break;
171 }
172 }
173 return modified ? Status::SuccessWithChange : Status::SuccessWithoutChange;
174}
175
176} // namespace opt
177} // namespace spvtools
178