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// Copyright (c) 2016 Google Inc.
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
#include "source/opt/unify_const_pass.h"
#include <memory>
#include <unordered_map>
#include <utility>
#include <vector>
#include "source/opt/def_use_manager.h"
#include "source/opt/ir_context.h"
#include "source/util/make_unique.h"
namespace spvtools {
namespace opt {
namespace {
// The trie that stores a bunch of result ids and, for a given instruction,
// searches the result id that has been defined with the same opcode, type and
// operands.
class ResultIdTrie {
ResultIdTrie() : root_(new Node) {}
// For a given instruction, extracts its opcode, type id and operand words
// as an array of keys, looks up the trie to find a result id which is stored
// with the same opcode, type id and operand words. If none of such result id
// is found, creates a trie node with those keys, stores the instruction's
// result id and returns that result id. If an existing result id is found,
// returns the existing result id.
uint32_t LookupEquivalentResultFor(const Instruction& inst) {
auto keys = GetLookUpKeys(inst);
auto* node = root_.get();
for (uint32_t key : keys) {
node = node->GetOrCreateTrieNodeFor(key);
if (node->result_id() == 0) {
return node->result_id();
// The trie node to store result ids.
class Node {
using TrieNodeMap = std::unordered_map<uint32_t, std::unique_ptr<Node>>;
Node() : result_id_(0), next_() {}
uint32_t result_id() const { return result_id_; }
// Sets the result id stored in this node.
void SetResultId(uint32_t id) { result_id_ = id; }
// Searches for the child trie node with the given key. If the node is
// found, returns that node. Otherwise creates an empty child node with
// that key and returns that newly created node.
Node* GetOrCreateTrieNodeFor(uint32_t key) {
auto iter = next_.find(key);
if (iter == next_.end()) {
// insert a new node and return the node.
return next_.insert(std::make_pair(key, MakeUnique<Node>()))
return iter->second.get();
// The result id stored in this node. 0 means this node is empty.
uint32_t result_id_;
// The mapping from the keys to the child nodes of this node.
TrieNodeMap next_;
// Returns a vector of the opcode followed by the words in the raw SPIR-V
// instruction encoding but without the result id.
std::vector<uint32_t> GetLookUpKeys(const Instruction& inst) {
std::vector<uint32_t> keys;
// Need to use the opcode, otherwise there might be a conflict with the
// following case when <op>'s binary value equals xx's id:
// OpSpecConstantOp tt <op> yy zz
// OpSpecConstantComposite tt xx yy zz;
for (const auto& operand : inst) {
if (operand.type == SPV_OPERAND_TYPE_RESULT_ID) continue;
keys.insert(keys.end(), operand.words.cbegin(), operand.words.cend());
return keys;
std::unique_ptr<Node> root_; // The root node of the trie.
} // anonymous namespace
Pass::Status UnifyConstantPass::Process() {
bool modified = false;
ResultIdTrie defined_constants;
for (Instruction *next_instruction,
*inst = &*(context()->types_values_begin());
inst; inst = next_instruction) {
next_instruction = inst->NextNode();
// Do not handle the instruction when there are decorations upon the result
// id.
if (get_def_use_mgr()->GetAnnotations(inst->result_id()).size() != 0) {
// The overall algorithm is to store the result ids of all the eligible
// constants encountered so far in a trie. For a constant defining
// instruction under consideration, use its opcode, result type id and
// words in operands as an array of keys to lookup the trie. If a result id
// can be found for that array of keys, a constant with exactly the same
// value must has been defined before, the constant under processing
// should be replaced by the constant previously defined. If no such result
// id can be found for that array of keys, this must be the first time a
// constant with its value be defined, we then create a new trie node to
// store the result id with the keys. When replacing a duplicated constant
// with a previously defined constant, all the uses of the duplicated
// constant, which must be placed after the duplicated constant defining
// instruction, will be updated. This way, the descendants of the
// previously defined constant and the duplicated constant will both refer
// to the previously defined constant. So that the operand ids which are
// used in key arrays will be the ids of the unified constants, when
// processing is up to a descendant. This makes comparing the key array
// always valid for judging duplication.
switch (inst->opcode()) {
case SpvOp::SpvOpConstantTrue:
case SpvOp::SpvOpConstantFalse:
case SpvOp::SpvOpConstant:
case SpvOp::SpvOpConstantNull:
case SpvOp::SpvOpConstantSampler:
case SpvOp::SpvOpConstantComposite:
// Only spec constants defined with OpSpecConstantOp and
// OpSpecConstantComposite should be processed in this pass. Spec
// constants defined with OpSpecConstant{|True|False} are decorated with
// 'SpecId' decoration and all of them should be treated as unique.
// 'SpecId' is not applicable to SpecConstants defined with
// OpSpecConstant{Op|Composite}, their values are not necessary to be
// unique. When all the operands/compoents are the same between two
// OpSpecConstant{Op|Composite} results, their result values must be the
// same so are unifiable.
case SpvOp::SpvOpSpecConstantOp:
case SpvOp::SpvOpSpecConstantComposite: {
uint32_t id = defined_constants.LookupEquivalentResultFor(*inst);
if (id != inst->result_id()) {
// The constant is a duplicated one, use the cached constant to
// replace the uses of this duplicated one, then turn it to nop.
context()->ReplaceAllUsesWith(inst->result_id(), id);
modified = true;
return modified ? Status::SuccessWithChange : Status::SuccessWithoutChange;
} // namespace opt
} // namespace spvtools