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/*
* Copyright 2022 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.
*/
//
// Sorts globals by their static use count. This helps reduce the size of wasm
// binaries because fewer bytes are needed to encode references to frequently
// used globals.
//
#include "memory"
#include "ir/find_all.h"
#include "pass.h"
#include "support/topological_sort.h"
#include "wasm.h"
namespace wasm {
using NameCountMap = std::unordered_map<Name, std::atomic<Index>>;
struct UseCountScanner : public WalkerPass<PostWalker<UseCountScanner>> {
bool isFunctionParallel() override { return true; }
bool modifiesBinaryenIR() override { return false; }
UseCountScanner(NameCountMap& counts) : counts(counts) {}
std::unique_ptr<Pass> create() override {
return std::make_unique<UseCountScanner>(counts);
}
void visitGlobalGet(GlobalGet* curr) {
// We can't add a new element to the map in parallel.
assert(counts.count(curr->name) > 0);
counts[curr->name]++;
}
void visitGlobalSet(GlobalSet* curr) {
assert(counts.count(curr->name) > 0);
counts[curr->name]++;
}
private:
NameCountMap& counts;
};
struct ReorderGlobals : public Pass {
// Whether to always reorder globals, even if there are very few and the
// benefit is minor. That is useful for testing.
bool always;
ReorderGlobals(bool always) : always(always) {}
void run(Module* module) override {
if (module->globals.size() < 128 && !always) {
// The module has so few globals that they all fit in a single-byte U32LEB
// value, so no reordering we can do can actually decrease code size. Note
// that this is the common case with wasm MVP modules where the only
// globals are typically the stack pointer and perhaps a handful of others
// (however, features like wasm GC there may be a great many globals).
return;
}
NameCountMap counts;
// Fill in info, as we'll operate on it in parallel.
for (auto& global : module->globals) {
counts[global->name];
}
// Count uses.
UseCountScanner scanner(counts);
scanner.run(getPassRunner(), module);
scanner.runOnModuleCode(getPassRunner(), module);
// Do a toplogical sort to ensure we keep dependencies before the things
// that need them. For example, if $b's definition depends on $a then $b
// must appear later:
//
// (global $a i32 (i32.const 10))
// (global $b i32 (global.get $a)) ;; $b depends on $a
//
struct DependencySort : TopologicalSort<Name, DependencySort> {
Module& wasm;
const NameCountMap& counts;
std::unordered_map<Name, std::vector<Name>> deps;
DependencySort(Module& wasm, const NameCountMap& counts)
: wasm(wasm), counts(counts) {
// Sort a list of global names by their counts.
auto sort = [&](std::vector<Name>& globals) {
std::stable_sort(
globals.begin(), globals.end(), [&](const Name& a, const Name& b) {
return counts.at(a) < counts.at(b);
});
};
// Sort the globals.
std::vector<Name> sortedNames;
for (auto& global : wasm.globals) {
sortedNames.push_back(global->name);
}
sort(sortedNames);
// Everything is a root (we need to emit all globals).
for (auto global : sortedNames) {
push(global);
}
// The dependencies are the globals referred to.
for (auto& global : wasm.globals) {
if (global->imported()) {
continue;
}
std::vector<Name> vec;
for (auto* get : FindAll<GlobalGet>(global->init).list) {
vec.push_back(get->name);
}
sort(vec);
deps[global->name] = std::move(vec);
}
}
void pushPredecessors(Name global) {
for (auto pred : deps[global]) {
push(pred);
}
}
};
std::unordered_map<Name, Index> sortedIndexes;
for (auto global : DependencySort(*module, counts)) {
auto index = sortedIndexes.size();
sortedIndexes[global] = index;
}
std::sort(
module->globals.begin(),
module->globals.end(),
[&](const std::unique_ptr<Global>& a, const std::unique_ptr<Global>& b) {
return sortedIndexes[a->name] < sortedIndexes[b->name];
});
module->updateMaps();
}
};
Pass* createReorderGlobalsPass() { return new ReorderGlobals(false); }
Pass* createReorderGlobalsAlwaysPass() { return new ReorderGlobals(true); }
} // namespace wasm
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