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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 <unordered_map>
#include <vector>
#include "source/opt/constants.h"
namespace spvtools {
namespace opt {
// Constant Folding Rules:
// The folding mechanism is built around the concept of a |ConstantFoldingRule|.
// A constant folding rule is a function that implements a method of simplifying
// an instruction to a constant.
// 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 constant folding rule returns a pointer to an Constant if |inst| can be
// simplified using this rule. Otherwise, it returns |nullptr|.
// See const_folding_rules.cpp for examples on how to write a constant folding
// rule.
// Be sure to add new constant folding rules to the table of constant folding
// rules in the constructor for ConstantFoldingRules. 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 ConstantFoldingRule = std::function<const analysis::Constant*(
IRContext* ctx, Instruction* inst,
const std::vector<const analysis::Constant*>& constants)>;
class ConstantFoldingRules {
// The |Key| and |Value| structs are used to by-pass a "decorated name length
// exceeded, name was truncated" warning on VS2013 and VS2015.
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;
struct Value {
std::vector<ConstantFoldingRule> value;
void push_back(ConstantFoldingRule rule) { value.push_back(rule); }
ConstantFoldingRules(IRContext* ctx) : context_(ctx) {}
virtual ~ConstantFoldingRules() = default;
// Returns true if there is at least 1 folding rule for |opcode|.
bool HasFoldingRule(const Instruction* inst) const {
return !GetRulesForInstruction(inst).empty();
// Returns true if there is at least 1 folding rule for |inst|.
const std::vector<ConstantFoldingRule>& GetRulesForInstruction(
const Instruction* inst) const {
if (inst->opcode() != SpvOpExtInst) {
auto it = rules_.find(inst->opcode());
if (it != rules_.end()) {
return it->second.value;
} 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.value;
return empty_vector_;
// Add the folding rules.
virtual void AddFoldingRules();
// |rules[opcode]| is the set of rules that can be applied to instructions
// with |opcode| as the opcode.
std::unordered_map<uint32_t, Value> rules_;
// The folding rules for extended instructions.
std::map<Key, Value> ext_rules_;
// The context that the instruction to be folded will be a part of.
IRContext* context_;
// The empty set of rules to be used as the default return value in
// |GetRulesForInstruction|.
std::vector<ConstantFoldingRule> empty_vector_;
} // namespace opt
} // namespace spvtools