(cppcon)用表达式模板实现一个一个简单安全的log
14 Jun 2018
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传统写法
#define LOG(msg) \
if (s_bLoggingEnabled) \
std::cout<<__FILE__<<"("<<__LINE__<<")"<<msg<<std::endl;
汇编长这样
0x0000000000400c9d <+53>: mov %rax,%rdi
0x0000000000400ca0 <+56>: callq 0x400af0 <_ZNSaIcED1Ev@plt>
0x0000000000400ca5 <+61>: movzbl 0x201528(%rip),%eax # 0x6021d4 <s_bLoggingEnabled>
0x0000000000400cac <+68>: test %al,%al
0x0000000000400cae <+70>: je 0x400d2c <main()+196>
0x0000000000400cb0 <+72>: mov $0x400e5a,%esi
0x0000000000400cb5 <+77>: mov $0x6020c0,%edi
0x0000000000400cba <+82>: callq 0x400ad0 <_ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc@plt>
0x0000000000400cbf <+87>: mov $0x400e65,%esi
0x0000000000400cc4 <+92>: mov %rax,%rdi
0x0000000000400cc7 <+95>: callq 0x400ad0 <_ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc@plt>
0x0000000000400ccc <+100>: mov $0xe,%esi
0x0000000000400cd1 <+105>: mov %rax,%rdi
0x0000000000400cd4 <+108>: callq 0x400a70 <_ZNSolsEi@plt>
0x0000000000400cd9 <+113>: mov $0x400e67,%esi
0x0000000000400cde <+118>: mov %rax,%rdi
0x0000000000400ce1 <+121>: callq 0x400ad0 <_ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc@plt>
0x0000000000400ce6 <+126>: mov $0x400e69,%esi
0x0000000000400ceb <+131>: mov %rax,%rdi
0x0000000000400cee <+134>: callq 0x400ad0 <_ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc@plt>
0x0000000000400cf3 <+139>: mov %rax,%rdx
0x0000000000400cf6 <+142>: lea -0x40(%rbp),%rax
0x0000000000400cfa <+146>: mov %rax,%rsi
0x0000000000400cfd <+149>: mov %rdx,%rdi
0x0000000000400d00 <+152>: callq 0x400b50 <_ZStlsIcSt11char_traitsIcESaIcEERSt13basic_ostreamIT_T0_ES7_RKNSt7__cxx1112basic_stringIS4_S5_T1_EE@plt>
0x0000000000400d05 <+157>: mov $0x37,%esi
0x0000000000400d0a <+162>: mov %rax,%rdi
0x0000000000400d0d <+165>: callq 0x400a70 <_ZNSolsEi@plt>
0x0000000000400d12 <+170>: mov $0x400e6d,%esi
0x0000000000400d17 <+175>: mov %rax,%rdi
0x0000000000400d1a <+178>: callq 0x400ad0 <_ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc@plt>
0x0000000000400d1f <+183>: mov $0x400b10,%esi
0x0000000000400d24 <+188>: mov %rax,%rdi
作者说了log相关的指令问题，可能会阻止编译器优化，icache也不友好
针对此，要达到减少指令，保留速度，类型安全还方便，所以就用到表达式模板了，把工作放到编译期
怎么做？把所有打印的参数用表达式墨宝封装一下，typelist登场
拆成两部分，log 和logdata
logdata就是个表达式模板typelist，把所有的参数串起来
using namespace std;
#define LOG(msg) \
if (s_bLoggingEnabled) \
(log(__FILE__,__LINE__,LogData<None>()<<msg));
template <typename List>
struct LogData{
typedef List type;
List list;
};
struct None{};
template <typename Begin, typename Value>
LogData<std::pair<Begin&&, Value&&>> operator<<(LogData<Begin>&& begin,
Value&& v) noexcept {
return ;
}
template <typename Begin, size_t n >
LogData<std::pair<Begin&&, const char* >> operator<<(LogData<Begin>&& begin,
const char(&sz)[n]) noexcept{
return ;
}
template<typename TLogData>
void log(const char* file, int line, TLogData&& data)
noexcept {
std::cout<< file<<" "<<line<<": ";
LogRecursive(std::cout, std::forward<typename TLogData::type>(data.list));
std::cout<<std::endl;
}
template<typename TLogDataPair>
void LogRecursive(std::ostream& os, TLogDataPair&& data) noexcept{
LogRecursive(os,std::forward<typename TLogDataPair::first_type>(data.first));
os<<std::forward<typename TLogDataPair::second_type>(data.second);
}
inline void LogRecursive(std::ostream& os, None) noexcept{}
int main()
{
s_bLoggingEnabled = true;
string s{"blaa"};
LOG("sth"<<s<<55<<"!!");
}
0x0000000000400cab <+67>: movzbl 0x201522(%rip),%eax # 0x6021d4 <s_bLoggingEnabled>
0x0000000000400cb2 <+74>: test %al,%al
0x0000000000400cb4 <+76>: je 0x400d42 <main()+218>
0x0000000000400cba <+82>: movl $0x37,-0x44(%rbp)
0x0000000000400cc1 <+89>: lea -0x11(%rbp),%rax
0x0000000000400cc5 <+93>: mov $0x40130a,%esi
0x0000000000400cca <+98>: mov %rax,%rdi
0x0000000000400ccd <+101>: callq 0x400e24 <operator<< <None, 4ul>(LogData<None>&&, char const (&) [4ul])>
0x0000000000400cd2 <+106>: mov %rax,-0x30(%rbp)
0x0000000000400cd6 <+110>: mov %rdx,-0x28(%rbp)
0x0000000000400cda <+114>: lea -0xa0(%rbp),%rdx
0x0000000000400ce1 <+121>: lea -0x30(%rbp),%rax
0x0000000000400ce5 <+125>: mov %rdx,%rsi
0x0000000000400ce8 <+128>: mov %rax,%rdi
0x0000000000400ceb <+131>: callq 0x400ebd <operator<< <std::pair<None&&, char const*>, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&>(LogData<std::pair<None&&, char const*> >&&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&)>
0x0000000000400cf0 <+136>: mov %rax,-0x40(%rbp)
0x0000000000400cf4 <+140>: mov %rdx,-0x38(%rbp)
---Type <return> to continue, or q <return> to quit---
0x0000000000400cf8 <+144>: lea -0x44(%rbp),%rdx
0x0000000000400cfc <+148>: lea -0x40(%rbp),%rax
0x0000000000400d00 <+152>: mov %rdx,%rsi
0x0000000000400d03 <+155>: mov %rax,%rdi
0x0000000000400d06 <+158>: callq 0x400f6a <operator<< <std::pair<std::pair<None&&, char const*>&&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&>, int>(LogData<std::pair<std::pair<None&&, char const*>&&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&> >&&, int&&)>
0x0000000000400d0b <+163>: mov %rax,-0x60(%rbp)
0x0000000000400d0f <+167>: mov %rdx,-0x58(%rbp)
0x0000000000400d13 <+171>: lea -0x60(%rbp),%rax
0x0000000000400d17 <+175>: mov $0x40130e,%esi
0x0000000000400d1c <+180>: mov %rax,%rdi
0x0000000000400d1f <+183>: callq 0x401002 <operator<< <std::pair<std::pair<std::pair<None&&, char const*>&&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&>&&, int&&>, 3ul>(LogData<std::pair<std::pair<std::pair<None&&, char const*>&&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&>&&, int&&> >&&, char const (&) [3ul])>
0x0000000000400d24 <+188>: mov %rax,-0x70(%rbp)
0x0000000000400d28 <+192>: mov %rdx,-0x68(%rbp)
0x0000000000400d2c <+196>: lea -0x70(%rbp),%rax
0x0000000000400d30 <+200>: mov %rax,%rdx
0x0000000000400d33 <+203>: mov $0x31,%esi
0x0000000000400d38 <+208>: mov $0x401311,%edi
0x0000000000400d3d <+213>: callq 0x401054 <log<LogData<std::pair<std::pair<std::pair<std::pair<None&&, char const*>&&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&>&&, int&&>&&, char const*> > >(char const*, int, LogData<std::pair<std::pair<std::pair<std::pair<None&&, char const*>&&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&>&&, int&&>&&, char const*> >&&)>
0x0000000000400d42 <+218>: lea -0xa0(%rbp),%rax
我个人测验，感觉并没有好到哪里去。。gcc开O3的话两个长得基本一致了。。就当学一下typelist转发技术好了
