性能C ++ 0x异常

问题描述

在C ++ 0x中使用异常的性能影响是什么？这个编译器有多少依赖？

What are the performance implications of using exceptions in C++0x? How much is this compiler dependent? Should we expect to use exceptions more for general logic handling like in Java?

推荐答案

#include <iostream>
#include <stdexcept>

struct SpaceWaster {
    SpaceWaster(int l, SpaceWaster *p) : level(l), prev(p) {}
    // we want the destructor to do something
    ~SpaceWaster() { prev = 0; }
    bool checkLevel() { return level == 0; }
    int level;
    SpaceWaster *prev;
};

void thrower(SpaceWaster *current) {
    if (current->checkLevel()) throw std::logic_error("some error message goes here\n");
    SpaceWaster next(current->level - 1, current);
    // typical exception-using code doesn't need error return values
    thrower(&next);
    return;
}

int returner(SpaceWaster *current) {
    if (current->checkLevel()) return -1;
    SpaceWaster next(current->level - 1, current);
    // typical exception-free code requires that return values be handled
    if (returner(&next) == -1) return -1;
    return 0;
}

int main() {
    const int repeats = 1001;
    int returns = 0;
    SpaceWaster first(1000, 0);

    for (int i = 0; i < repeats; ++i) {
        #ifdef THROW
            try {
                thrower(&first);
            } catch (std::exception &e) {
                ++returns;
            }
        #else
            returner(&first);
            ++returns;
        #endif
    }
    #ifdef THROW
        std::cout << returns << " exceptions\n";
    #else
        std::cout << returns << " returns\n";
    #endif
}

Mickey Mouse基准测试结果：

Mickey Mouse benchmarking results:

$ make throw -B && time ./throw
g++     throw.cpp   -o throw
1001 returns

real    0m0.547s
user    0m0.421s
sys     0m0.046s

$ make throw CPPFLAGS=-DTHROW -B && time ./throw
g++  -DTHROW   throw.cpp   -o throw
1001 exceptions

real    0m2.047s
user    0m1.905s
sys     0m0.030s

所以在这种情况下，抛出一个异常1000堆栈级别，而不是正常返回， 1.5ms。这包括进入try块，我相信在一些系统是在执行时是免费的，在其他人每次进入尝试时都会产生成本，而在其他每次输入包含尝试的函数时，只会产生成本。对于更可能的100个堆栈级别，我将重复上升到10k，因为一切都快了10倍。因此，例外花费0.1ms。

So in this case, throwing an exception up 1000 stack levels, rather than returning normally, takes about 1.5ms. That includes entering the try block, which I believe on some systems is free at execution time, on others incurs a cost each time you enter try, and on others only incurs a cost each time you enter the function which contains the try. For a more likely 100 stack levels, I upped the repeats to 10k because everything was 10 times faster. So the exception cost 0.1ms.

对于10000个堆栈级别，它是18.7s vs 4.1s，因此大约14ms的额外成本的异常。因此，对于这个例子，我们看到一个非常一致的开销，每个级别的堆栈1.5us（每个级别都破坏一个对象）。

For 10 000 stack levels, it was 18.7s vs 4.1s, so about 14ms additional cost for the exception. So for this example we're looking at a pretty consistent overhead of 1.5us per level of stack (where each level is destructing one object).

显然C ++ 0x不指定异常（或任何其他，除了算法和数据结构的大O复杂性）的性能。我不认为它改变了异常，会严重影响许多实现，无论是积极还是消极。

Obviously C++0x doesn't specify performance for exceptions (or anything else, other than big-O complexity for algorithms and data structures). I don't think it changes exceptions in a way which will seriously impact many implementations, either positively or negatively.

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