C++ 属性： likely, unlikely (C++20 起)
来自cppreference.com
< cpp | language | attributes
允许编译器为包含该语句的执行路径，比任何其他不包含该语句的执行路径，更可能或更不可能的情况进行优化。
语法
[[likely]]
|
(1) | ||||||||
[[unlikely]]
|
(2) | ||||||||
解释
这些属性可应用于标号或语句（除了声明语句）。它们不可同时应用到同一标号或语句。
1) 应用到语句，允许编译器为包含该语句的执行路径，比任何其他不包含该语句的执行路径更可能的情况进行优化。
2) 应用到语句，允许编译器为包含该语句的执行路径，比任何其他不包含该语句的执行路径更不可能的情况进行优化。
当且仅当执行路径中含有到某个标号的跳转时，才认为该执行路径包含该标号：
int f(int i) { switch (i) { case 1: [[fallthrough]]; [[likely]] case 2: return 1; } return 2; }
i == 2 被认为比 i 的任何其他值更可能，但 [[likely]] 在 i == 1 的情况无效果，尽管它直落到 case 2: 标号。
示例
运行此代码
#include <chrono> #include <cmath> #include <iomanip> #include <iostream> #include <random> namespace with_attributes { constexpr double pow(double x, long long n) noexcept { if (n > 0) [[likely]] return x * pow(x, n - 1); else [[unlikely]] return 1; } constexpr long long fact(long long n) noexcept { if (n > 1) [[likely]] return n * fact(n - 1); else [[unlikely]] return 1; } constexpr double cos(double x) noexcept { constexpr long long precision{16LL}; double y{}; for (auto n{0LL}; n < precision; n += 2LL) [[likely]] y += pow(x, n) / (n & 2LL ? -fact(n) : fact(n)); return y; } } // namespace with_attributes namespace no_attributes { constexpr double pow(double x, long long n) noexcept { if (n > 0) return x * pow(x, n - 1); else return 1; } constexpr long long fact(long long n) noexcept { if (n > 1) return n * fact(n - 1); else return 1; } constexpr double cos(double x) noexcept { constexpr long long precision{16LL}; double y{}; for (auto n{0LL}; n < precision; n += 2LL) y += pow(x, n) / (n & 2LL ? -fact(n) : fact(n)); return y; } } // namespace no_attributes double gen_random() noexcept { static std::random_device rd; static std::mt19937 gen(rd()); static std::uniform_real_distribution<double> dis(-1.0, 1.0); return dis(gen); } volatile double sink{}; // 确保有副作用 int main() { for (const auto x : {0.125, 0.25, 0.5, 1. / (1 << 26)}) std::cout << std::setprecision(53) << "x = " << x << '\n' << std::cos(x) << '\n' << with_attributes::cos(x) << '\n' << (std::cos(x) == with_attributes::cos(x) ? "equal" : "differ") << '\n'; auto benchmark = [](auto fun, auto rem) { const auto start = std::chrono::high_resolution_clock::now(); for (auto size{1ULL}; size != 10'000'000ULL; ++size) sink = fun(gen_random()); const std::chrono::duration<double> diff = std::chrono::high_resolution_clock::now() - start; std::cout << "Time: " << std::fixed << std::setprecision(6) << diff.count() << " sec " << rem << std::endl; }; benchmark(with_attributes::cos, "(with attributes)"); benchmark(no_attributes::cos, "(without attributes)"); benchmark([](double t) { return std::cos(t); }, "(std::cos)"); }
可能的输出：
x = 0.125 0.99219766722932900560039115589461289346218109130859375 0.99219766722932900560039115589461289346218109130859375 equal x = 0.25 0.96891242171064473343022882545483298599720001220703125 0.96891242171064473343022882545483298599720001220703125 equal x = 0.5 0.8775825618903727587394314468838274478912353515625 0.8775825618903727587394314468838274478912353515625 equal x = 1.490116119384765625e-08 0.99999999999999988897769753748434595763683319091796875 0.99999999999999988897769753748434595763683319091796875 equal Time: 0.579122 sec (with attributes) Time: 0.722553 sec (without attributes) Time: 0.425963 sec (std::cos)
引用
- C++23 标准（ISO/IEC 14882:2024）：
- 9.12.7 Likelihood attributes [dcl.attr.likelihood]
- C++20 标准（ISO/IEC 14882:2020）：
- 9.12.6 Likelihood attributes [dcl.attr.likelihood]