topic I usually recommend -fast to in Intel® Fortran Compiler

best ifort equivalent flag to gfortran -O3 -ffast-math?

Edgardo_Doerner — Thu, 07 Aug 2014 17:57:00 GMT

Hi to everyone,

I am parallelizing a MC code written in Fortran using OpenMP. During the development process I used the Parallel Studio Suite with great success (really, I would not be able to do it without the help of the Advisor, Amplifier and Inspector programs... I am not an expert (actually, I am physicist who loves programming, but I have had to do self-training during my career)

Now I am testing the performance of my program under various OS, compiler and CPU's. Originally, this code was expected to be compiled with gfortran and "-O3 -ffast-math" flags, is there a close equivalent to this configuration for ifort?

As a first approach I am using ifort with "-O3" and I obtain a ~20% performance gain compared with gfortran, so I would like to check if there are other flags that could improve the performance of my code. Thanks for your help!

I usually recommend -fast to

Steven_L_Intel1 — Thu, 07 Aug 2014 18:35:23 GMT

I usually recommend -fast to start out - this is shorthand for a set of options that usually improve performance. It includes -O3 as well as -xHost, which lets the compiler take advantage of advanced instructions on the processor you're using.

gfortran -O -ffast-math

TimP — Thu, 07 Aug 2014 19:06:34 GMT

gfortran -O -ffast-math implies the equivalent of -complex-limited-range which affects only complex data types and is risky. It also implies the equivalent of -assume protect_parens,byterecl,buffered_io,minus0 which are all good options.

You probably do want some unrolling, which is implied by the ifort option you mentioned, but not the gfortran.

The nearest gfortran equivalent to -xHost, which Steve recommended, is -march=native. gfortran 64-bit defaults to -msse2, as does ifort.

Thanks for your answers, I

Edgardo_Doerner — Thu, 07 Aug 2014 19:48:14 GMT

Thanks for your answers, I used "-O3 -fast" with ifort and comparing with gfortran -O3 -ffast-math I obtained a ~35% performance gain, but I have a small problem. When I use "-fast" I have some warnings during the compilation:

ipo: warning #11021: unresolved omp_get_num_threads_
Referenced in /tmp/ipo_ifortAfxRhT.o
(a lot of this kind of warnings, all related to omp_ functions)

The program executes normally and the results are correct, but I am worried about this messages, should I worry about them?

@Tim: I will try your suggestions with gfortran, thanks!

Interesting that you would

Steven_L_Intel1 — Thu, 07 Aug 2014 21:34:59 GMT

Interesting that you would get that warning - it's during the link phase. Do you have any OpenMP code in this program? Do you have a test case we can look at? If you don't see actual errors, I wouldn't worry, but something isn't right.

Yes, I use OpenMP to

Edgardo_Doerner — Thu, 07 Aug 2014 21:54:41 GMT

Yes, I use OpenMP to parallelize it, and I call that functions to output the number of OpenMP threads, to initialize the RNG, etc...

I have a test code (attached, it was an example to another problem that I had with OpenMP - from Jim Dempsey), there I have
the following message when compiling with ifort -openmp -O3 -fast

ipo: warning #11021: unresolved omp_get_thread_num
Referenced in /tmp/ipo_iforts9U7NX.o
ipo: warning #11021: unresolved omp_set_num_threads
Referenced in /tmp/ipo_iforts9U7NX.o
ipo: remark #11001: performing single-file optimizations
ipo: remark #11006: generating object file /tmp/ipo_iforts9U7NX.o

thanks for your help.

If you failed to set -qopenmp

TimP — Thu, 07 Aug 2014 22:36:35 GMT

If you failed to set -qopenmp or -openmp according to ifort version (equivalent to gfortran -fopenmp), and didn't follow the method of protecting omp function calls in #ifdef _OPENMP ... #endif you might expect such unresolved references at link time.

The legacy method of

integer omp_num_threads ! not using OpenMP header or USE

...

n = omp_get_num_threads()

should work if you don't change default integer type, and link against OpenMP library, but certain Intel tools will warn about it, as it's contrary to OpenMP standard.

thanks for the help, Tim, I

Edgardo_Doerner — Sun, 10 Aug 2014 02:00:08 GMT

thanks for the help, Tim, I had no idea about the correct method to use the omp functions... and by the way, I had the doubt of how to use them and being able to compile the program without the use of the omp library... thanks!