[cfe-dev] food for optimizer developers

[Ralf W. Grosse-Kunstleve]

> Have you tried profiling the resulting program?

>
> -Alexei

ifort and g++, just enough to convince myself there isn't something silly
due to the conversion to C++.
50% of the time is spent in two lines of code.
I haven't profiled clang++, mainly because I think I couldn't do much about
the 2x speed difference compared to g++ anyway.
Ralf


On Mon, Aug 9, 2010 at 9:52 PM, Ralf W. Grosse-Kunstleve <rwgk at yahoo.com> wrote:
> I wrote a Fortran to C++ conversion program that I used to convert selected
> LAPACK sources. Comparing runtimes with different compilers I get:
>
>                          absolute  relative
> ifort 11.1.072             1.790s     1.00
> gfortran 4.4.4             2.470s     1.38
> g++ 4.4.4                  2.922s     1.63
> clang++ 2.8 (trunk 108205) 6.487s     3.62
>
> This is under Fedora 13, 64-bit, 12-core Opteron 2.2GHz
>
> All files to easily reproduce the results are here:
>
>  http://cci.lbl.gov/lapack_fem/
>
> See the README file or the example commands below.
>
> Questions:
>
> - Why is the code generated by clang++ so much slower than the g++ code?
>
> - Is there anything I could do in the C++ code generation or in the "fem"
>  Fortran EMulation library to help runtime performance?
>
> Ralf
>
>
> wget http://cci.lbl.gov/lapack_fem/lapack_fem_001.tgz
> tar zxf lapack_fem_001.tgz
> cd lapack_fem_001
> clang++ -o dsyev_test_clang++ -I. -O3 -ffast-math dsyev_test.cpp
> time dsyev_test_clang++
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