在观看Titus Winters的"Live at Head"演讲后,他提到StrCat()是人们最喜欢的功能之一,我决定尝试实现类似的东西,看看是否能在运行时性能方面击败std::string::append(或operator+,我认为它在内部使用append)。我的想法是,作为可变参数模板实现的strcat()函数将能够确定其所有类似字符串的参数的组合大小,并进行单个分配以存储最终结果,而不是在operator+的情况下不断重新分配,因为它没有关于调用上下文的整体知识。
然而,当我在quick-bench上将自定义实现与operator+进行比较时,我发现我的strcat()实现在最近版本的clang和gcc上比operator+慢约4倍,编译选项为
有人知道这里可能导致减速的原因吗?
然而,当我在quick-bench上将自定义实现与operator+进行比较时,我发现我的strcat()实现在最近版本的clang和gcc上比operator+慢约4倍,编译选项为
-std=c++17 -O3
。我已经包含了以下快速测试代码以供参考。有人知道这里可能导致减速的原因吗?
#include <cstring>
#include <iostream>
#include <string>
// Get the size of string-like args
int getsize(const std::string& s) { return s.size(); }
int getsize(const char* s) { return strlen(s); }
template <typename S>
int strcat_size(const S& s) {
return getsize(s);
}
template <typename S, typename... Strings>
int strcat_size(const S& first, Strings... rest) {
if (sizeof...(Strings) == 0) {
return 0;
} else {
return getsize(first) + strcat_size(rest...);
}
}
// Populate a pre-allocated string with content from another string-like object
template <typename S>
void strcat_fill(std::string& res, const S& first) {
res += first;
}
template <typename S, typename... Strings>
void strcat_fill(std::string& res, const S& first, Strings... rest) {
res += first;
strcat_fill(res, rest...);
}
template <typename S, typename... Strings>
std::string strcat(const S& first, Strings... rest) {
int totalsize = strcat_size(first, rest...);
std::string res;
res.reserve(totalsize);
strcat_fill(res, first, rest...);
return res;
}
const char* s1 = "Hello World! ";
std::string s2 = "Here is a string to concatenate. ";
std::string s3 = "Here is a longer string to concatenate that avoids small string optimization";
const char* s4 = "How about some more strings? ";
std::string s5 = "And more strings? ";
std::string s6 = "And even more strings to use!";
static void strcat_bench(benchmark::State& state) {
// Code inside this loop is measured repeatedly
for (auto _ : state) {
std::string s = strcat(s1, s2, s3, s4, s5, s6);
benchmark::DoNotOptimize(s);
}
}
BENCHMARK(strcat_bench);
static void append_bench(benchmark::State& state) {
for (auto _ : state) {
std::string s = s1 + s2 + s3 + s4 + s5 + s6;
benchmark::DoNotOptimize(s);
}
}
BENCHMARK(append_bench);
Strings&&... rest
接受并使用std::forward
,就像这个答案中所示。 - kmdreko