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// Copyright (c) 2018 Google LLC
// 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 <cstdint>
#include <unordered_map>
#include <vector>
#include "source/opt/constants.h"
namespace spvtools {
namespace opt {
// Folding Rules:
// The folding mechanism is built around the concept of a |FoldingRule|. A
// folding rule is a function that implements a method of simplifying an
// instruction.
// The inputs to a folding rule are:
// |inst| - the instruction to be simplified.
// |constants| - if an in-operands is an id of a constant, then the
// corresponding value in |constants| contains that
// constant value. Otherwise, the corresponding entry in
// |constants| is |nullptr|.
// A folding rule returns true if |inst| can be simplified using this rule. If
// the instruction can be simplified, then |inst| is changed to the simplified
// instruction. Otherwise, |inst| remains the same.
// See folding_rules.cpp for examples on how to write a folding rule. It is
// important to note that if |inst| can be folded to the result of an
// instruction that feed it, then |inst| should be changed to an OpCopyObject
// that copies that id.
// Be sure to add new folding rules to the table of folding rules in the
// constructor for FoldingRules. The new rule should be added to the list for
// every opcode that it applies to. Note that earlier rules in the list are
// given priority. That is, if an earlier rule is able to fold an instruction,
// the later rules will not be attempted.
using FoldingRule = std::function<bool(
IRContext* context, Instruction* inst,
const std::vector<const analysis::Constant*>& constants)>;
class FoldingRules {
using FoldingRuleSet = std::vector<FoldingRule>;
explicit FoldingRules(IRContext* ctx) : context_(ctx) {}
virtual ~FoldingRules() = default;
const FoldingRuleSet& GetRulesForInstruction(Instruction* inst) const {
if (inst->opcode() != SpvOpExtInst) {
auto it = rules_.find(inst->opcode());
if (it != rules_.end()) {
return it->second;
} else {
uint32_t ext_inst_id = inst->GetSingleWordInOperand(0);
uint32_t ext_opcode = inst->GetSingleWordInOperand(1);
auto it = ext_rules_.find({ext_inst_id, ext_opcode});
if (it != ext_rules_.end()) {
return it->second;
return empty_vector_;
IRContext* context() { return context_; }
// Adds the folding rules for the object.
virtual void AddFoldingRules();
// The folding rules for core instructions.
std::unordered_map<uint32_t, FoldingRuleSet> rules_;
// The folding rules for extended instructions.
struct Key {
uint32_t instruction_set;
uint32_t opcode;
friend bool operator<(const Key& a, const Key& b) {
if (a.instruction_set < b.instruction_set) {
return true;
if (a.instruction_set > b.instruction_set) {
return false;
return a.opcode < b.opcode;
std::map<Key, FoldingRuleSet> ext_rules_;
IRContext* context_;
FoldingRuleSet empty_vector_;
} // namespace opt
} // namespace spvtools