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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
*
* 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.
*/
#include "pass.h"
#include "wasm.h"
#include "wasm-binary.h"
#include "wasm-builder.h"
namespace wasm {
// Adding segments adds overhead, this is a rough estimate
const Index OVERHEAD = 8;
struct MemoryPacking : public Pass {
bool modifiesBinaryenIR() override { return false; }
void run(PassRunner* runner, Module* module) override {
// Conservatively refuse to change segments if bulk memory is enabled to
// avoid invalidating segment indices or segment contents referenced from
// memory.init instructions.
// TODO: optimize in the presence of memory.init instructions
if (!module->memory.exists || runner->options.features.hasBulkMemory()) {
return;
}
std::vector<Memory::Segment> packed;
// we can only handle a constant offset for splitting
auto isSplittable = [&](const Memory::Segment& segment) {
return segment.offset->is<Const>();
};
for (auto& segment : module->memory.segments) {
if (!isSplittable(segment)) {
packed.push_back(segment);
}
}
size_t numRemaining = module->memory.segments.size() - packed.size();
// Split only if we have room for more segments
auto shouldSplit = [&]() {
return WebLimitations::MaxDataSegments > packed.size() + numRemaining;
};
for (auto& segment : module->memory.segments) {
if (!isSplittable(segment)) continue;
// skip final zeros
while (segment.data.size() > 0 && segment.data.back() == 0) {
segment.data.pop_back();
}
if (!shouldSplit()) {
packed.push_back(segment);
continue;
}
auto* offset = segment.offset->cast<Const>();
// Find runs of zeros, and split
auto& data = segment.data;
auto base = offset->value.geti32();
Index start = 0;
// create new segments
while (start < data.size()) {
// skip initial zeros
while (start < data.size() && data[start] == 0) {
start++;
}
Index end = start; // end of data-containing part
Index next = end; // after zeros we can skip. preserves next >= end
if (!shouldSplit()) {
next = end = data.size();
}
while (next < data.size() && (next - end < OVERHEAD)) {
if (data[end] != 0) {
end++;
next = end; // we can try to skip zeros from here
} else {
// end is on a zero, we are looking to skip
if (data[next] != 0) {
end = next; // we must extend the segment, including some zeros
} else {
next++;
}
}
}
if (end != start) {
packed.emplace_back(Builder(*module).makeConst(Literal(int32_t(base + start))), &data[start], end - start);
}
start = next;
}
numRemaining--;
}
module->memory.segments.swap(packed);
}
};
Pass *createMemoryPackingPass() {
return new MemoryPacking();
}
} // namespace wasm
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