Why Clang

[Fred Morcos]

I would also guess that the base system is stuck with gcc ~4.1 due to
the GPLv3-ization of later gcc version. Is that correct?

On Tue, Jun 19, 2012 at 4:43 PM, Michel Talon <talon at lpthe.jussieu.fr> wrote:
> David Brodbeck said:
>> Another way of looking at it is after 25 years of optimization GCC is
>> unable to beat a new compiler that's had almost none...
> Unfortunately this affirmation is blatantly false, recent gcc produce code
> much faster than clang. I give here an example which i like, a monte carlo computation for a spin lattice.
> Everything runs on my macbook.
>
> lilas% clang -v
> Apple clang version 2.1 (tags/Apple/clang-163.7.1) (based on LLVM 3.0svn)
> Target: x86_64-apple-darwin11.4.0
> lilas% clang -O4 test.c -lf2c
> lilas% time ./a.out
> ...
>
> real    0m2.359s
> user    0m2.341s
> sys     0m0.003s
>
> lilas% /usr/local/bin/gcc -v
> …
> gcc version 4.6.1 (GCC)
>
> lilas% /usr/local/bin/gcc -O3 test.c -lf2c
> lilas% time ./a.out
> …
>
> real    0m1.241s
> user    0m1.234s
> sys     0m0.003s
>
> So gcc gives an executable running twice faster than clang, basically, when both compilers
> are run at maximal optimization. To show the effectiveness of the optimizer, here is the running
> time without any optimization:
>
> lilas% /usr/local/bin/gcc  test.c -lf2c
> lilas% time ./a.out
> …
>
> real    0m6.895s
> user    0m6.889s
> sys     0m0.005s
>
> What this demonstrates is that for programs which do real computations, optimization is
> *very* important, and gcc is now very good (i have not shown the numbers but they match the Intel compiler)
> while clang is at the level gcc was ten years ago. So i fully agree with Wojciech Puchar, the move to clang
> is only driven by anti GPL propaganda which is frankly completely stupid, since in any events, gcc
> does not contaminate the binaries it produces (except when using contaminated accompanying libraries
> e.g. for C++). Of course, when compiling FreeBSD kernel or similar programs which do little computation
> there is no harm using clang. I suspect that the price is higher for programs like mencoder which require
> the highest efficiency.
>
> I will not comment on the better error messages coming from clang, this could be a more serious argument.
>
> --
>
> Michel Talon
> talon at lpthe.jussieu.fr
>
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