Re: Compiling c++ template is very slow.

[Richard Biener <richard dot guenther at gmail dot com>]

On Fri, Mar 9, 2018 at 3:33 AM, Fis Trivial <ybbs.daans@hotmail.com> wrote:
>
> I tried to use c++ template to generate code for a personal project, but
> found that the compilation time needed with g++ is much
> slower(exponentially) than with clang++.
>
> This is a code snippet for testing purpose:
>
> #include <iostream>
>
> template <int a, int b>
> struct v : v<a-1, b>, v<a, b-1>
> {
>   static int constexpr m = a;
>   static int constexpr n = b;
>   static int constexpr s = a + b;
> };
>
> template <int b>
> struct v<1, b> : v<1, b-1>
> {
>   static int constexpr m = 1;
>   static int constexpr n = b;
>   static int constexpr s = b + 1;
> };
> template <int a>
> struct v<a, 1> : v<a-1, 1>
> {
>   static int constexpr m = a;
>   static int constexpr n = 1;
>   static int constexpr s = a + 1;
> };
> template <>
> struct v<1, 1>
> {
>   static int constexpr m = 1;
>   static int constexpr n = 1;
>   static int constexpr s = 2;
> };
>
> int main()
> {
>   std::cout << v<7, 12>::s << std::endl;
>   std::cout << v<4, 3>::s << std::endl;
> };
>
>
> Here is the time information:
> ---
> $ time g++ -std=c++11 generate.cc -o bygcc
>
> real    0m39.529s
> user    0m39.418s
> sys     0m0.053s
>
> $ time clang++ -std=c++11 generate.cc -o byclang
>
> real    0m0.310s
> user    0m0.273s
> sys     0m0.024s
> ---
>
> When using greater value, gcc will require exponentially more time to
> compile while the needed time from clang grows linearly. For example,
> replacing the `main` function from above code to:
>
> int main()
> {
>   // change 7 to 8 and drop <4, 3>
>   std::cout << v<8, 12>::s << std::endl;
> };
>
> ---
> $ time g++ -std=c++11 generate.cc -o bygcc
>
> real    5m20.755s
> user    5m8.509s
> sys     0m0.260s

I can confirm this and suggest to open a bugreport.

> $ time clang++ -std=c++11 generate.cc -o byclang
>
> real    0m0.314s
> user    0m0.281s
> sys     0m0.020s
> ---
>
>
> Just for fun, making the template parameter to 128:
> ---
> $ time g++ -std=c++11 generate.cc -o bygcc
> ... not gonna happen :).
>
> $ time clang++ -std=c++11 generate.cc -o byclang
>
> real    0m18.549s
> user    0m18.410s
> sys     0m0.066s
> ---
>
>
> I am currently running Fedora 27 with following version of gcc and
> clang:
>
> $ g++ --version
> g++ (GCC) 7.3.1 20180303 (Red Hat 7.3.1-5)
> Copyright (C) 2017 Free Software Foundation, Inc.
> This is free software; see the source for copying conditions.  There is NO
> warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
>
> $ clang++ --version
> clang version 5.0.1 (tags/RELEASE_501/final)
> Target: x86_64-unknown-linux-gnu
> Thread model: posix
> InstalledDir: /usr/bin