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/*
* Copyright 2019 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
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
//
// Simplify and optimize globals and their use.
//
// * Turns never-written and unwritable (not imported or exported)
// globals immutable.
// * If an immutable global is a copy of another, use the earlier one,
// to allow removal of the copies later.
// * Apply the constant values of immutable globals.
//
// Some globals may not have uses after these changes, which we leave
// to other passes to optimize.
//
#include <atomic>
#include "ir/utils.h"
#include "pass.h"
#include "wasm-builder.h"
#include "wasm.h"
namespace wasm {
namespace {
struct GlobalInfo {
bool imported = false;
bool exported = false;
std::atomic<bool> written;
};
using GlobalInfoMap = std::map<Name, GlobalInfo>;
struct GlobalUseScanner : public WalkerPass<PostWalker<GlobalUseScanner>> {
bool isFunctionParallel() override { return true; }
GlobalUseScanner(GlobalInfoMap* infos) : infos(infos) {}
GlobalUseScanner* create() override { return new GlobalUseScanner(infos); }
void visitGlobalSet(GlobalSet* curr) { (*infos)[curr->name].written = true; }
private:
GlobalInfoMap* infos;
};
using NameNameMap = std::map<Name, Name>;
using NameSet = std::set<Name>;
struct GlobalUseModifier : public WalkerPass<PostWalker<GlobalUseModifier>> {
bool isFunctionParallel() override { return true; }
GlobalUseModifier(NameNameMap* copiedParentMap)
: copiedParentMap(copiedParentMap) {}
GlobalUseModifier* create() override {
return new GlobalUseModifier(copiedParentMap);
}
void visitGlobalGet(GlobalGet* curr) {
auto iter = copiedParentMap->find(curr->name);
if (iter != copiedParentMap->end()) {
curr->name = iter->second;
}
}
private:
NameNameMap* copiedParentMap;
};
struct ConstantGlobalApplier
: public WalkerPass<PostWalker<ConstantGlobalApplier>> {
bool isFunctionParallel() override { return true; }
ConstantGlobalApplier(NameSet* constantGlobals)
: constantGlobals(constantGlobals) {}
ConstantGlobalApplier* create() override {
return new ConstantGlobalApplier(constantGlobals);
}
void visitGlobalGet(GlobalGet* curr) {
if (constantGlobals->count(curr->name)) {
auto* global = getModule()->getGlobal(curr->name);
assert(global->init->is<Const>());
replaceCurrent(ExpressionManipulator::copy(global->init, *getModule()));
}
}
private:
NameSet* constantGlobals;
};
} // anonymous namespace
struct SimplifyGlobals : public Pass {
PassRunner* runner;
Module* module;
GlobalInfoMap map;
void run(PassRunner* runner_, Module* module_) override {
runner = runner_;
module = module_;
analyze();
preferEarlierImports();
propagateConstantsToGlobals();
propagateConstantsToCode();
}
void analyze() {
// First, find out all the relevant info.
for (auto& global : module->globals) {
auto& info = map[global->name];
if (global->imported()) {
info.imported = true;
}
}
for (auto& ex : module->exports) {
if (ex->kind == ExternalKind::Global) {
map[ex->value].exported = true;
}
}
GlobalUseScanner(&map).run(runner, module);
// We now know which are immutable in practice.
for (auto& global : module->globals) {
auto& info = map[global->name];
if (global->mutable_ && !info.imported && !info.exported &&
!info.written) {
global->mutable_ = false;
}
}
}
void preferEarlierImports() {
// Optimize uses of immutable globals, prefer the earlier import when
// there is a copy.
NameNameMap copiedParentMap;
for (auto& global : module->globals) {
auto child = global->name;
if (!global->mutable_ && !global->imported()) {
if (auto* get = global->init->dynCast<GlobalGet>()) {
auto parent = get->name;
if (!module->getGlobal(get->name)->mutable_) {
copiedParentMap[child] = parent;
}
}
}
}
if (!copiedParentMap.empty()) {
// Go all the way back.
for (auto& global : module->globals) {
auto child = global->name;
if (copiedParentMap.count(child)) {
while (copiedParentMap.count(copiedParentMap[child])) {
copiedParentMap[child] = copiedParentMap[copiedParentMap[child]];
}
}
}
// Apply to the gets.
GlobalUseModifier(&copiedParentMap).run(runner, module);
}
}
// Constant propagation part 1: even an mutable global with a constant
// value can have that value propagated to another global that reads it,
// since we do know the value during startup, it can't be modified until
// code runs.
void propagateConstantsToGlobals() {
// Go over the list of globals in order, which is the order of
// initialization as well, tracking their constant values.
std::map<Name, Literal> constantGlobals;
for (auto& global : module->globals) {
if (!global->imported()) {
if (auto* c = global->init->dynCast<Const>()) {
constantGlobals[global->name] = c->value;
} else if (auto* get = global->init->dynCast<GlobalGet>()) {
auto iter = constantGlobals.find(get->name);
if (iter != constantGlobals.end()) {
Builder builder(*module);
global->init = builder.makeConst(iter->second);
}
}
}
}
}
// Constant propagation part 2: apply the values of immutable globals
// with constant values to to global.gets in the code.
void propagateConstantsToCode() {
NameSet constantGlobals;
for (auto& global : module->globals) {
if (!global->mutable_ && !global->imported() &&
global->init->is<Const>()) {
constantGlobals.insert(global->name);
}
}
if (!constantGlobals.empty()) {
ConstantGlobalApplier(&constantGlobals).run(runner, module);
}
}
};
Pass* createSimplifyGlobalsPass() { return new SimplifyGlobals(); }
} // namespace wasm
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