On 19.06.2012 16:43, Michel Talon 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.
Really - just to throw in another opinion:
As an average user I don't see any performance impact on my clang-built
desktop-every-day-workstation. The only thing that is getting on my
nerves are some ports I frequently have to rebuild with gcc.
It would be nice if the porting team could set up some automagic for
that. There seems to be no harm in running a mixed clang/gcc built
userland. Some members of this list seem to fear some kind of communist
infiltration by gcc - I hope this is no serious issue, is it?
I will not comment on the better error messages coming from clang, this could
be a more serious argument.
I don't know what they mean, but they really do look good :-)
Greetings
Peter
--
Michel Talon
ta...@lpthe.jussieu.fr
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