mirror of
https://github.com/Light-City/CPlusPlusThings.git
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dfe1c933b3
- const README: rewrite the space-saving claim per issue #5 follow-ups (constant-expression const vs #define immediates; which const qualify as constant expressions) - const README: fix the class-section diagnosis per issue #338 (overload resolution picks the const overload; the restriction only surfaces once that overload is removed) - sizeof README: virtual-inheritance result is compiler-dependent (GCC merges vbptr+vfptr -> 16, MSVC keeps them separate -> 32) #246 - static README: new section on internal linkage / access restriction with anonymous-namespace comparison #142 - 10-day practice: full problem list table + fixed day links #136 - new design_pattern/singleton/singleton_default.hpp: reusable thread-safe CRTP singleton template, compile+concurrency tested #303
96 lines
2.8 KiB
Markdown
96 lines
2.8 KiB
Markdown
> 来自 [C++那些事网站](https://light-city.github.io/stories_things/cpp-things/cpp-restrict-optimization/) 的文章，由站点同步脚本导出。
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# C++ restrict 与编译优化
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## restrict性能优化
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```cpp
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#include <vector>
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#include <memory>
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#include <benchmark/benchmark.h>
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#define LEN 1<<20
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using AggregateDataPtr = char *;
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struct AvgState {
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uint64_t numerator{0};
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uint64_t denominator{0};
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double divide() { return numerator / denominator; }
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};
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class Column {
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public:
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std::vector<uint64_t> & data() {return data_; }
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private:
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std::vector<uint64_t> data_;
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};
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template <class Derived>
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class IAggregate {
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public:
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void addBatch(size_t batch_size, AggregateDataPtr state, Column *args) __attribute__((noinline)) {
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for (size_t i = 0; i < batch_size; ++i) {
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static_cast<const Derived *>(this)->add(state, args, i);
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}
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}
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void addBatchWithoutOpt(size_t batch_size, AggregateDataPtr state, Column *args) __attribute__((noinline)) {
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for (size_t i = 0; i < batch_size; ++i) {
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static_cast<const Derived *>(this)->addWithoutOpt(state, args, i);
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}
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}
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};
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class AvgAggregator final : public IAggregate<AvgAggregator> {
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public:
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static AvgState& data(AggregateDataPtr __restrict place) { return *reinterpret_cast<AvgState *>(place); }
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static AvgState& data_without_opt(AggregateDataPtr place) { return *reinterpret_cast<AvgState *>(place); }
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void add(AggregateDataPtr __restrict state, Column * args, uint64_t row_num) const {
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data(state).numerator += args[0].data()[row_num];
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++(data(state).denominator);
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}
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void addWithoutOpt(AggregateDataPtr state, Column * args, uint64_t row_num) const {
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data_without_opt(state).numerator += args[0].data()[row_num];
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++(data_without_opt(state).denominator);
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}
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};
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static void BM_loop_with_optimize(benchmark::State& state) {
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AvgAggregator avgAggregator;
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Column column;
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for (size_t i = 0 ; i < LEN; i++) {
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column.data().push_back(i);
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}
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Column arg0[] = {column};
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// Code inside this loop is measured repeatedly
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for(auto _ : state) {
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auto avgState = std::make_unique<AvgState>();
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avgAggregator.addBatch(LEN, reinterpret_cast<AggregateDataPtr>(avgState.get()), arg0);
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}
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}
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static void BM_loop_without_optimize(benchmark::State& state) {
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AvgAggregator avgAggregator;
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Column column;
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for (size_t i = 0 ; i < LEN; i++) {
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column.data().push_back(i);
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}
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Column arg0[] = {column};
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// Code inside this loop is measured repeatedly
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for(auto _ : state) {
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auto avgState = std::make_unique<AvgState>();
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avgAggregator.addBatchWithoutOpt(LEN, reinterpret_cast<AggregateDataPtr>(avgState.get()), arg0);
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}
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}
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BENCHMARK(BM_loop_without_optimize);
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BENCHMARK(BM_loop_with_optimize);
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```
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> https://quick-bench.com/q/ccjuWr_JcLozAfy3Gxf3uVu76Yk
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https://github.com/ClickHouse/ClickHouse/pull/19946 |