Howto enable stronger optimization builds

Question

I am trying to build PETSc and have problems to enable optimization. Without specifying, PETSc always creates a debugging build, but I can turn that off with passing --with-debugging=0 to cmake. However, this only enables -O1 by default, but as my application is extreme time consuming and very time critical, I want to have at least -O2. I can't find an option except --CFLAGS, which works, but always appends options to the end, so -O1 would override my -O2.

I greped for "-O" to set the flag manually, this gave me a million lines, mostly from the configure.log file and doesn't help.

Does anybody know the file where to set the flag, or a workaround like ...another option that disables the usage of the last specified -O#, but enables the strongest or first?

Answer 1

Citing PETSc' install instructions:

Configure defaults to building PETSc in debug mode. One can switch to using optimzed mode with the toggle option --with-debugging [defaults to debug enabled]. Additionally one can specify more suitable optimization flags with the options COPTFLAGS, FOPTFLAGS, CXXOPTFLAGS. ./configure --with-cc=gcc --with-fc=gfortran --with-debugging=0 COPTFLAGS='-O3 -march=p4 -mtune=p4' FOPTFLAGS='-O3 -qarch=p4 -qtune=p4'