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/*
* Copyright 2017 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.
*/
//
// Hoists repeated constants to a local. A local.get takes 2 bytes
// in most cases, and if a const is larger than that, it may be
// better to store it to a local, then get it from that local.
//
// WARNING: this often shrinks code size, but can *increase* gzip
// size. apparently having the constants in their proper
// places lets them be compressed better, across
// functions, etc. TODO investigate
// TODO: hoisting a zero does not even require an initial set!
// TODO: hoisting a float or double zero is especially beneficial as there
// is no LEB compression for them, and no need for the set, so
// each f32.const is 5 bytes and f64.const is 9 bytes, while it is
// <= 1 byte to declare the local and 2-3 to use it!
//
#include <map>
#include <wasm.h>
#include <pass.h>
#include <wasm-binary.h>
#include <wasm-builder.h>
namespace wasm {
// with fewer uses than this, it is never beneficial to hoist
static const Index MIN_USES = 2;
struct ConstHoisting : public WalkerPass<PostWalker<ConstHoisting>> {
bool isFunctionParallel() override { return true; }
Pass* create() override { return new ConstHoisting; }
std::map<Literal, std::vector<Expression**>> uses;
void visitConst(Const* curr) {
uses[curr->value].push_back(getCurrentPointer());
}
void visitFunction(Function* curr) {
std::vector<Expression*> prelude;
for (auto& pair : uses) {
auto value = pair.first;
auto& vec = pair.second;
auto num = vec.size();
if (worthHoisting(value, num)) {
prelude.push_back(hoist(vec));
}
}
if (!prelude.empty()) {
Builder builder(*getModule());
// merge-blocks can optimize this into a single block later in most cases
curr->body = builder.makeSequence(
builder.makeBlock(prelude),
curr->body
);
}
}
private:
bool worthHoisting(Literal value, Index num) {
if (num < MIN_USES) return false;
// measure the size of the constant
Index size = 0;
switch (value.type) {
case i32: {
size = getWrittenSize(S32LEB(value.geti32()));
break;
}
case i64: {
size = getWrittenSize(S64LEB(value.geti64()));
break;
}
case f32:
case f64: {
size = getTypeSize(value.type);
break;
}
case v128: {
// v128 not implemented yet
return false;
}
case none:
case unreachable: {
WASM_UNREACHABLE();
}
}
// compute the benefit, of replacing the uses with
// one use + a set and then a get for each use
// doing the algebra, the criterion here is when
// size > 2(1+num)/(num-1)
// or
// num > (size+2)/(size-2)
auto before = num * size;
auto after = size + 2 /* local.set */ + (2 /* local.get */ * num);
return after < before;
}
template<typename T>
Index getWrittenSize(const T& thing) {
BufferWithRandomAccess buffer;
buffer << thing;
return buffer.size();
}
// replace all the uses with gets, for a local set at the top. returns
// the set.
Expression* hoist(std::vector<Expression**>& vec) {
auto type = (*(vec[0]))->type;
Builder builder(*getModule());
auto temp = builder.addVar(getFunction(), type);
auto* ret = builder.makeSetLocal(
temp,
*(vec[0])
);
for (auto item : vec) {
*item = builder.makeGetLocal(temp, type);
}
return ret;
}
};
Pass *createConstHoistingPass() {
return new ConstHoisting();
}
} // namespace wasm
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