4 ms·

Can anyone explain why C++ beats out C so clearly?

mwattsun 5y ago
I was curious too and found this: https://stackoverflow.com/questions/2513741/c-vs-c-for-performance-in-memory-allocation https://stackoverflow.com/questions/2513741/c-vs-c-for-perfo... from which I gather that if you're using an allocator function that zero's out memory then it will be slower of course.
khoobid_shoma 5y ago
I think the C code in the benchmarks are not really efficient. An experienced C programmer does not simulate C++ style memory management if speed matters.
dorinlazar 5y ago
C++ is just a stricter C - if you write proper C, it will be a correct C++ program, therefore it would run at the same speed. There's no way C can be other than marginally faster than C++ - if you write C and compile it with a C++ compiler. Then you have various optimizations that come from things like templates, constexpr. Look at the qsort method - to have a modicum of genericity you need to pass a function that will be invoked. Instead, a C++ version would be able to be optimized to compare the elements directly, without calling a function, avoiding a big number of function calls. There's no way to make C faster than well written C++. But C++ is complex and a lot of people write bad C++, that's why it has developed a bad reputation. It's really undeserved.
- ReleaseCandidat 5y ago
  > There's no way C can be other than marginally faster than C++ In theory `restrict` (to disallow pointer aliasing like in Fortran) could lead to optimization gains, same as not having exceptions (the possibility of exceptions prohibits certain optimizations of the C++ compiler).
  - dorinlazar 5y ago
    I agree; of course, both clang and gcc extend C++ with __restrict__ if I remember correctly; from what I know, there are more things that can be done in C++ to improve over C than the other way around.
    - khoobid_shoma 5y ago
      IIRC, __restrict__ in clang and g++ does nothing when you write C++ code. It is there just for compatibility with C.
ReleaseCandidat 5y ago
There is no reason, the fasted C++ code is actually C - I really don't know why somebody wrote that as C++. https://github.com/hanabi1224/Programming-Language-Benchmarks/blob/main/bench/algorithm/spectral-norm/8.cpp https://github.com/hanabi1224/Programming-Language-Benchmark... https://github.com/hanabi1224/Programming-Language-Benchmarks/blob/main/bench/algorithm/spectral-norm/7.cpp https://github.com/hanabi1224/Programming-Language-Benchmark...
sdfdf4434r34r 5y ago
Because it's not C++ code: https://github.com/hanabi1224/Programming-Language-Benchmarks/blob/main/bench/algorithm/spectral-norm/8.cpp https://github.com/hanabi1224/Programming-Language-Benchmark...

I was curious too and found this: https://stackoverflow.com/questions/2513741/c-vs-c-for-performance-in-memory-allocation https://stackoverflow.com/questions/2513741/c-vs-c-for-perfo... from which I gather that if you're using an allocator function that zero's out memory then it will be slower of course.

khoobid_shoma 5y ago

I think the C code in the benchmarks are not really efficient. An experienced C programmer does not simulate C++ style memory management if speed matters.

dorinlazar 5y ago

C++ is just a stricter C - if you write proper C, it will be a correct C++ program, therefore it would run at the same speed. There's no way C can be other than marginally faster than C++ - if you write C and compile it with a C++ compiler. Then you have various optimizations that come from things like templates, constexpr. Look at the qsort method - to have a modicum of genericity you need to pass a function that will be invoked. Instead, a C++ version would be able to be optimized to compare the elements directly, without calling a function, avoiding a big number of function calls. There's no way to make C faster than well written C++. But C++ is complex and a lot of people write bad C++, that's why it has developed a bad reputation. It's really undeserved.

ReleaseCandidat 5y ago
> There's no way C can be other than marginally faster than C++ In theory `restrict` (to disallow pointer aliasing like in Fortran) could lead to optimization gains, same as not having exceptions (the possibility of exceptions prohibits certain optimizations of the C++ compiler).
- dorinlazar 5y ago
  I agree; of course, both clang and gcc extend C++ with __restrict__ if I remember correctly; from what I know, there are more things that can be done in C++ to improve over C than the other way around.
  - khoobid_shoma 5y ago
    IIRC, __restrict__ in clang and g++ does nothing when you write C++ code. It is there just for compatibility with C.

ReleaseCandidat 5y ago

There is no reason, the fasted C++ code is actually C - I really don't know why somebody wrote that as C++. https://github.com/hanabi1224/Programming-Language-Benchmarks/blob/main/bench/algorithm/spectral-norm/8.cpp https://github.com/hanabi1224/Programming-Language-Benchmark... https://github.com/hanabi1224/Programming-Language-Benchmarks/blob/main/bench/algorithm/spectral-norm/7.cpp https://github.com/hanabi1224/Programming-Language-Benchmark...

sdfdf4434r34r 5y ago

Because it's not C++ code: https://github.com/hanabi1224/Programming-Language-Benchmarks/blob/main/bench/algorithm/spectral-norm/8.cpp https://github.com/hanabi1224/Programming-Language-Benchmark...