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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.
// Optimize relooper-generated label variable usage: add blocks and turn
// a label-set/break/label-check into a break into the new block.
// This assumes the very specific output the fastcomp relooper emits,
// including the name of the 'label' variable.
#include "wasm.h"
#include "pass.h"
#include "ir/utils.h"
#include "ir/manipulation.h"
namespace wasm {
static Name LABEL("label");
static Name getInnerName(int i) {
return Name(std::string("__rjti$") + std::to_string(i));
static Name getOuterName(int i) {
return Name(std::string("__rjto$") + std::to_string(i));
static If* isLabelCheckingIf(Expression* curr, Index labelIndex) {
if (!curr) return nullptr;
auto* iff = curr->dynCast<If>();
if (!iff) return nullptr;
auto* condition = iff->condition->dynCast<Binary>();
if (!(condition && condition->op == EqInt32)) return nullptr;
auto* left = condition->left->dynCast<GetLocal>();
if (!(left && left->index == labelIndex)) return nullptr;
return iff;
static Index getCheckedLabelValue(If* iff) {
return iff->condition->cast<Binary>()->right->cast<Const>()->value.geti32();
static SetLocal* isLabelSettingSetLocal(Expression* curr, Index labelIndex) {
if (!curr) return nullptr;
auto* set = curr->dynCast<SetLocal>();
if (!set) return nullptr;
if (set->index != labelIndex) return nullptr;
return set;
static Index getSetLabelValue(SetLocal* set) {
return set->value->cast<Const>()->value.geti32();
struct LabelUseFinder : public PostWalker<LabelUseFinder> {
Index labelIndex;
std::map<Index, Index>& checks; // label value => number of checks on it
std::map<Index, Index>& sets; // label value => number of sets to it
LabelUseFinder(Index labelIndex, std::map<Index, Index>& checks, std::map<Index, Index>& sets) : labelIndex(labelIndex), checks(checks), sets(sets) {}
void visitIf(If* curr) {
if (isLabelCheckingIf(curr, labelIndex)) {
void visitSetLocal(SetLocal* curr) {
if (isLabelSettingSetLocal(curr, labelIndex)) {
struct RelooperJumpThreading : public WalkerPass<ExpressionStackWalker<RelooperJumpThreading>> {
bool isFunctionParallel() override { return true; }
Pass* create() override { return new RelooperJumpThreading; }
std::map<Index, Index> labelChecks;
std::map<Index, Index> labelSets;
Index labelIndex;
Index newNameCounter = 0;
void visitBlock(Block* curr) {
// look for the if label == X pattern
auto& list = curr->list;
if (list.size() == 0) return;
for (Index i = 0; i < list.size() - 1; i++) {
// once we see something that might be irreducible, we must skip that if and the rest of the dependents
bool irreducible = false;
Index origin = i;
for (Index j = i + 1; j < list.size(); j++) {
if (auto* iff = isLabelCheckingIf(list[j], labelIndex)) {
irreducible |= hasIrreducibleControlFlow(iff, list[origin]);
if (!irreducible) {
optimizeJumpsToLabelCheck(list[origin], iff);
// if the next element is a block, it may be the holding block of label-checking ifs
if (auto* holder = list[j]->dynCast<Block>()) {
if (holder->list.size() > 0) {
if (If* iff = isLabelCheckingIf(holder->list[0], labelIndex)) {
irreducible |= hasIrreducibleControlFlow(iff, list[origin]);
if (!irreducible) {
// this is indeed a holder. we can process the ifs, and must also move
// the block to enclose the origin, so it is properly reachable
assert(holder->list.size() == 1); // must be size 1, a relooper multiple will have its own label, and is an if-else sequence and nothing more
optimizeJumpsToLabelCheck(list[origin], iff);
holder->list[0] = list[origin];
list[origin] = holder;
// reuse the if as a nop
list[j] = iff;
break; // we didn't see something we like, so stop here
void doWalkFunction(Function* func) {
// if there isn't a label variable, nothing for us to do
if (func->localIndices.count(LABEL)) {
labelIndex = func->getLocalIndex(LABEL);
LabelUseFinder finder(labelIndex, labelChecks, labelSets);
void visitFunction(Function* curr) {
// we may alter types
ReFinalize().walkFunctionInModule(curr, getModule());
bool hasIrreducibleControlFlow(If* iff, Expression* origin) {
// Gather the checks in this if chain. If all the label values checked are only set in origin,
// then since origin is right before us, this is not irreducible - we can replace all sets
// in origin with jumps forward to us, and since there is nothing else, this is safe and complete.
// We must also have the property that there is just one check for the label value, as otherwise
// node splitting has complicated things.
std::map<Index, Index> labelChecksInOrigin;
std::map<Index, Index> labelSetsInOrigin;
LabelUseFinder finder(labelIndex, labelChecksInOrigin, labelSetsInOrigin);
while (iff) {
auto num = getCheckedLabelValue(iff);
assert(labelChecks[num] > 0);
if (labelChecks[num] > 1) return true; // checked more than once, somewhere in function
assert(labelChecksInOrigin[num] == 0);
if (labelSetsInOrigin[num] != labelSets[num]) {
assert(labelSetsInOrigin[num] < labelSets[num]);
// the label is set outside of the origin
// if the only other location is inside the if body, then it is ok - it must be in a loop
// and returning to the top of the loop body, so we don't need to do anything for that
// label setting anyhow
std::map<Index, Index> labelChecksInIfTrue;
std::map<Index, Index> labelSetsInIfTrue;
LabelUseFinder finder(labelIndex, labelChecksInIfTrue, labelSetsInIfTrue);
if (labelSetsInOrigin[num] + labelSetsInIfTrue[num] < labelSets[num]) {
// label set somewhere we can't see now, could be irreducible control flow
// TODO: one case where this happens is instead of an if-chain, we have
// ifs and a switch on label|0, in separate elements. perhaps not
// emitting switches on label|0 in the relooper would avoid that.
return true;
iff = isLabelCheckingIf(iff->ifFalse, labelIndex);
return false;
// optimizes jumps to a label check
// * origin is where the jumps originate, and also where we should write our output
// * iff is the if
void optimizeJumpsToLabelCheck(Expression*& origin, If* iff) {
Index nameCounter = newNameCounter++;
Index num = getCheckedLabelValue(iff);
// create a new block for this jump target
Builder builder(*getModule());
// origin is where all jumps to this target must come from - the element right before this if
// we break out of inner to reach the target. instead of flowing out of normally, we break out of the outer, so we skip the target.
auto innerName = getInnerName(nameCounter);
auto outerName = getOuterName(nameCounter);
auto* ifFalse = iff->ifFalse;
// all assignments of label to the target can be replaced with breaks to the target, via innerName
struct JumpUpdater : public PostWalker<JumpUpdater> {
Index labelIndex;
Index targetNum;
Name targetName;
void visitSetLocal(SetLocal* curr) {
if (curr->index == labelIndex) {
if (Index(curr->value->cast<Const>()->value.geti32()) == targetNum) {
JumpUpdater updater;
updater.labelIndex = labelIndex;
updater.targetNum = num;
updater.targetName = innerName;
// restructure code
auto* inner = builder.blockifyWithName(origin, innerName, builder.makeBreak(outerName));
auto* outer = builder.makeSequence(inner, iff->ifTrue);
outer->name = outerName;
origin = outer;
// if another label value is checked here, handle that too
if (ifFalse) {
optimizeJumpsToLabelCheck(origin, ifFalse->cast<If>());
// declare pass
Pass *createRelooperJumpThreadingPass() {
return new RelooperJumpThreading();
} // namespace wasm