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* Copyright 2016 WebAssembly Community Group participants
* 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.
This is an optimized C++ implemention of the Relooper algorithm originally
developed as part of Emscripten. This implementation includes optimizations
added since the original academic paper [1] was published about it.
[1] Alon Zakai. 2011. Emscripten: an LLVM-to-JavaScript compiler. In Proceedings of the ACM international conference companion on Object oriented programming systems languages and applications companion (SPLASH '11). ACM, New York, NY, USA, 301-312. DOI=10.1145/2048147.2048224
#include <assert.h>
#include <stdio.h>
#include <stdarg.h>
#include <stdlib.h>
#include <deque>
#include <list>
#include <map>
#include <memory>
#include <set>
#include "wasm.h"
#include "wasm-builder.h"
namespace CFG {
class RelooperBuilder : public wasm::Builder {
wasm::Index labelHelper;
RelooperBuilder(wasm::Module& wasm, wasm::Index labelHelper) : wasm::Builder(wasm), labelHelper(labelHelper) {}
wasm::GetLocal* makeGetLabel() {
return makeGetLocal(labelHelper, wasm::i32);
wasm::SetLocal* makeSetLabel(wasm::Index value) {
return makeSetLocal(labelHelper, makeConst(wasm::Literal(int32_t(value))));
wasm::Binary* makeCheckLabel(wasm::Index value) {
return makeBinary(wasm::EqInt32, makeGetLabel(), makeConst(wasm::Literal(int32_t(value))));
// breaks are on blocks, as they can be specific, we make one wasm block per basic block
wasm::Break* makeBlockBreak(int id) {
return wasm::Builder::makeBreak(getBlockBreakName(id));
// continues are on shapes, as there is one per loop, and if we have more than one
// going there, it is irreducible control flow anyhow
wasm::Break* makeShapeContinue(int id) {
return wasm::Builder::makeBreak(getShapeContinueName(id));
wasm::Name getBlockBreakName(int id) {
return wasm::Name(std::string("block$") + std::to_string(id) + "$break");
wasm::Name getShapeContinueName(int id) {
return wasm::Name(std::string("shape$") + std::to_string(id) + "$continue");
struct Block;
struct Shape;
// Info about a branching from one block to another
struct Branch {
enum FlowType {
Direct = 0, // We will directly reach the right location through other means, no need for continue or break
Break = 1,
Continue = 2
Shape *Ancestor; // If not NULL, this shape is the relevant one for purposes of getting to the target block. We break or continue on it
Branch::FlowType Type; // If Ancestor is not NULL, this says whether to break or continue
// A branch either has a condition expression if the block ends in ifs, or if the block ends in a switch, then a list of indexes, which
// becomes the indexes in the table of the switch. If not a switch, the condition can be any expression.
wasm::Expression* Condition;
std::unique_ptr<std::vector<wasm::Index>> SwitchValues; // switches are rare, so have just a pointer here
wasm::Expression* Code; // If provided, code that is run right before the branch is taken. This is useful for phis
Branch(wasm::Expression* ConditionInit, wasm::Expression* CodeInit = nullptr);
Branch(std::vector<wasm::Index>&& ValuesInit, wasm::Expression* CodeInit = nullptr);
// Emits code for branch
wasm::Expression* Render(RelooperBuilder& Builder, Block *Target, bool SetLabel);
// like std::set, except that begin() -> end() iterates in the
// order that elements were added to the set (not in the order
// of operator<(T, T))
template<typename T>
struct InsertOrderedSet
std::map<T, typename std::list<T>::iterator> Map;
std::list<T> List;
typedef typename std::list<T>::iterator iterator;
iterator begin() { return List.begin(); }
iterator end() { return List.end(); }
void erase(const T& val) {
auto it = Map.find(val);
if (it != Map.end()) {
void erase(iterator position) {
// cheating a bit, not returning the iterator
void insert(const T& val) {
auto it = Map.find(val);
if (it == Map.end()) {
Map.insert(std::make_pair(val, --List.end()));
size_t size() const { return Map.size(); }
bool empty() const { return Map.empty(); }
void clear() {
size_t count(const T& val) const { return Map.count(val); }
InsertOrderedSet() {}
InsertOrderedSet(const InsertOrderedSet& other) {
*this = other;
InsertOrderedSet& operator=(const InsertOrderedSet& other) {
for (auto i : other.List) {
insert(i); // inserting manually creates proper iterators
return *this;
// like std::map, except that begin() -> end() iterates in the
// order that elements were added to the map (not in the order
// of operator<(Key, Key))
template<typename Key, typename T>
struct InsertOrderedMap
std::map<Key, typename std::list<std::pair<Key,T>>::iterator> Map;
std::list<std::pair<Key,T>> List;
T& operator[](const Key& k) {
auto it = Map.find(k);
if (it == Map.end()) {
List.push_back(std::make_pair(k, T()));
auto e = --List.end();
Map.insert(std::make_pair(k, e));
return e->second;
return it->second->second;
typedef typename std::list<std::pair<Key,T>>::iterator iterator;
iterator begin() { return List.begin(); }
iterator end() { return List.end(); }
void erase(const Key& k) {
auto it = Map.find(k);
if (it != Map.end()) {
void erase(iterator position) {
size_t size() const { return Map.size(); }
bool empty() const { return Map.empty(); }
size_t count(const Key& k) const { return Map.count(k); }
InsertOrderedMap() {}
InsertOrderedMap(InsertOrderedMap& other) {
abort(); // TODO, watch out for iterators
InsertOrderedMap& operator=(const InsertOrderedMap& other) {
abort(); // TODO, watch out for iterators
typedef InsertOrderedSet<Block*> BlockSet;
typedef InsertOrderedMap<Block*, Branch*> BlockBranchMap;
// Represents a basic block of code - some instructions that end with a
// control flow modifier (a branch, return or throw).
struct Block {
// Branches become processed after we finish the shape relevant to them. For example,
// when we recreate a loop, branches to the loop start become continues and are now
// processed. When we calculate what shape to generate from a set of blocks, we ignore
// processed branches.
// Blocks own the Branch objects they use, and destroy them when done.
BlockBranchMap BranchesOut;
BlockSet BranchesIn;
BlockBranchMap ProcessedBranchesOut;
BlockSet ProcessedBranchesIn;
Shape *Parent; // The shape we are directly inside
int Id; // A unique identifier, defined when added to relooper
wasm::Expression* Code; // The code in this block. This can be arbitrary wasm code, including internal control flow, it should just not branch to the outside
wasm::Expression* SwitchCondition; // If nullptr, then this block ends in ifs (or nothing). otherwise, this block ends in a switch, done on this condition
bool IsCheckedMultipleEntry; // If true, we are a multiple entry, so reaching us requires setting the label variable
Block(wasm::Expression* CodeInit, wasm::Expression* SwitchConditionInit = nullptr);
// Add a branch: if the condition holds we branch (or if null, we branch if all others failed)
// Note that there can be only one branch from A to B (if you need multiple conditions for the branch,
// create a more interesting expression in the Condition).
// If a Block has no outgoing branches, the contents in Code must contain a terminating
// instruction, as the relooper doesn't know whether you want control flow to stop with
// an `unreachable` or a `return` or something else (if you forget to do this, control
// flow may continue into the block that happens to be emitted right after it).
void AddBranchTo(Block *Target, wasm::Expression* Condition, wasm::Expression* Code = nullptr);
// Add a switch branch: if the switch condition is one of these values, we branch (or if the list is empty, we are the default)
// Note that there can be only one branch from A to B (if you need multiple values for the branch, that's what the array and default are for).
void AddSwitchBranchTo(Block *Target, std::vector<wasm::Index>&& Values, wasm::Expression* Code = nullptr);
// Emit code for the block, including its contents and branchings out
wasm::Expression* Render(RelooperBuilder& Builder, bool InLoop);
// Represents a structured control flow shape, one of
// Simple: No control flow at all, just instructions in a single
// basic block.
// Multiple: A shape with at least one entry. We may visit one of
// the entries, or none, before continuing to the next
// shape after this.
// Loop: An infinite loop. We assume the property that a loop
// will always visit one of its entries, and so for example
// we cannot have a loop containing a multiple and nothing
// else (since we might not visit any of the multiple's
// blocks). Multiple entries are possible for the block,
// however, which is necessary for irreducible control
// flow, of course.
struct SimpleShape;
struct MultipleShape;
struct LoopShape;
struct Shape {
int Id; // A unique identifier. Used to identify loops, labels are Lx where x is the Id. Defined when added to relooper
Shape *Next; // The shape that will appear in the code right after this one
Shape *Natural; // The shape that control flow gets to naturally (if there is Next, then this is Next)
enum ShapeType {
ShapeType Type;
Shape(ShapeType TypeInit) : Id(-1), Next(NULL), Type(TypeInit) {}
virtual ~Shape() {}
virtual wasm::Expression* Render(RelooperBuilder& Builder, bool InLoop) = 0;
static SimpleShape *IsSimple(Shape *It) { return It && It->Type == Simple ? (SimpleShape*)It : NULL; }
static MultipleShape *IsMultiple(Shape *It) { return It && It->Type == Multiple ? (MultipleShape*)It : NULL; }
static LoopShape *IsLoop(Shape *It) { return It && It->Type == Loop ? (LoopShape*)It : NULL; }
struct SimpleShape : public Shape {
Block *Inner;
SimpleShape() : Shape(Simple), Inner(NULL) {}
wasm::Expression* Render(RelooperBuilder& Builder, bool InLoop) override;
typedef std::map<int, Shape*> IdShapeMap;
struct MultipleShape : public Shape {
IdShapeMap InnerMap; // entry block ID -> shape
MultipleShape() : Shape(Multiple) {}
wasm::Expression* Render(RelooperBuilder& Builder, bool InLoop) override;
struct LoopShape : public Shape {
Shape *Inner;
BlockSet Entries; // we must visit at least one of these
LoopShape() : Shape(Loop), Inner(NULL) {}
wasm::Expression* Render(RelooperBuilder& Builder, bool InLoop) override;
// Implements the relooper algorithm for a function's blocks.
// Usage:
// 1. Instantiate this struct.
// 2. Call AddBlock with the blocks you have. Each should already
// have its branchings in specified (the branchings out will
// be calculated by the relooper).
// 3. Call Render().
// Implementation details: The Relooper instance has
// ownership of the blocks and shapes, and frees them when done.
struct Relooper {
std::deque<Block*> Blocks;
std::deque<Shape*> Shapes;
Shape *Root;
bool MinSize;
int BlockIdCounter;
int ShapeIdCounter;
void AddBlock(Block *New, int Id=-1);
// Calculates the shapes
void Calculate(Block *Entry);
// Renders the result.
wasm::Expression* Render(RelooperBuilder& Builder);
// Sets us to try to minimize size
void SetMinSize(bool MinSize_) { MinSize = MinSize_; }
typedef InsertOrderedMap<Block*, BlockSet> BlockBlockSetMap;
struct Debugging {
static void Dump(BlockSet &Blocks, const char *prefix=NULL);
static void Dump(Shape *S, const char *prefix=NULL);
} // namespace CFG