Clang at NSC

Clang is a C/ObjC/C++ compiler based on the LLVM infrastructure. Its main advantages over other compilers are very fast compile times and “expressive” error messages and diagnostics. Clang also implements all of the ISO C++ 2011 standard.

Currently, you can find Clang and LLVM installed on Triolith under /software/apps/llvm+clang. To put the clang and llvm- commands in your PATH, load the clang module:

module load clang/3.3

How to use

To compile normally:

clang -o myprogram.exe myprogram.c

To compile with Intel’s MKL for BLAS, LAPACK, and FFTW3 functionality:

clang -o myprogram.exe -I$MKLROOT/include -L$MKLROOT/lib/intel64 -lmkl_gf_lp64 -lmkl_core -lmkl_sequential -lpthread -lm  myprogram.c

To run it, you need to set the LD_LIBRARY_PATH, because NSC’s linker wrapper does not work for clang yet.

LD_LIBRARY_PATH=$MKLROOT/lib/intel64 ./myprogram.exe

Compiler flag recommendations

Clang has four optimization levels (-O1,-O2,-O3,-Ofast,-Os). They roughly correspond to the ones for gcc, but are not identical. There is rudimentary auto-vectorization in Clang, but it is not as efficient as the one in gcc or Intel’s compiler, so your mileage may very. For high optimzation, try:

clang -O3 -march=native -mtune=native ...

Currently, the link-time optimization (-O4 level) does not seem to work on Triolith.

Static analysis

Clang is useful just for the extensive set of warnings and static checks of C code. The simplest way is just to compile your code with all warnings enabled:

clang -Weverything ...

Please note that this is different from the -Wall option, which does not actually enable all warnings. If you want to run Clang only as a static checker and not compile the files, add -fsyntax-only

clang -fsyntax-only -Weverything ...

If you want to go beyond running clang on a single file, there is utility called scan-buildthat can run static analysis over your whole project while building it. If you have a makefile for your project, the following should work:

scan-build make