XL (xlc, xlC, xlf)
Get the latest releases:
GNU Compiler Collection (GCC) [1]
Get the latest release:
Clang
Get the latest release: http://releases.llvm.org/
Architecture
Generate instructions that run on POWER8 -mcpu=power8 (XLC default)
-qarch=pwr8 (XLF default)
-mcpu=power8 -target powerpcle-unknown-linux-gnu
-mcpu=pwr8
Generate instructions that run on POWER9 -mcpu=power9
-qarch=pwr9
-mcpu=power9 -target powerpcle-unknown-linux-gnu
-mcpu=pwr9
Optimization levels
Disable all optimizations -O0 -qnoopt (default) -O0 (default) -O0 (default)
Optimization levels
-O
-O2
-O3
-O4
-O5[2]
-O or -O1
-O2
-O3
-Ofast
-O0
-O2
-O3
-Os
Recommended optimization
(A good balance between run-time performance and compilation time)
Commercial code:
-O3 or -O3 -qipa
Technical computing/analytic:
-O3 or -Ofast
Commercial code:
-O3
Technical computing/analytic:
-Ofast -funroll-loops
-O2
Additional optimizations
Feedback directed optimization -qpdf1
-qpdf2
-fprofile-generate
-fprofile-use
-fprofile-instr-generate
-fprofile-instr-use
Interprocedural optimizations -qipa -flto  
OpenMP -qsmp=omp -fopenmp -fopenmp
Loop optimizations -qhot -fpeel-loops
-funroll-loops
-funroll-loops

[1] For the latest version of GCC for Linux on Power, plus optimized libraries that can be used with GCC or XL, see the Advance toolchain for Linux on Power pages.

[2] XL also accepts most GNU options