CMake: Drop support for OPT_FLAGS

[chfast]

1. Adding -march=native flag by default is bad idea, because this makes the library not portable.
2. If you want to optimize with -march=native just do

cmake -DCMAKE_CXX_FLAGS='-march=native -mtune=native'

[chfast]

@howardwu can you review this?

[howardwu]

Agreed, using -march=native -mtune=native affects portability for older CPUs.

Keeping the OPT_FLAGS is useful for preserving optimizations. How about using -g -O2 as default instead?

[chfast]

Using CMAKE_BUILD_TYPE=RelWithDebInfo will give you exactly -g -O2.

[chfast]

I usually build it with CMAKE_BUILD_TYPE=Release what is -O3 -DNDEBUG. No debug symbols.
I can set the default build type to RelWithDebInfo. We can also tweak flags for each build type. See https://cmake.org/cmake/help/v3.10/variable/CMAKE_LANG_FLAGS_RELWITHDEBINFO.html.

[howardwu]

Setting CMAKE_BUILD_TYPE to RelWithDebInfo in CMakeLists seems fine.

[chfast]

Done.

[howardwu]

LGTM

We should propagate this change up to libfqfft and libsnark, updating the respective READMEs.

[tromer]

I don't mind changing the interface to whatever is the "proper" CMake way du jour, but I suggest to keep the -march=native -mtune=native default behavior, for several reasons:

1. Compatibility with existing 3rd-party build scripts.
2. People that use libsnark for ad-hoc tests and benchmarks should get the fastest performance by default. (Experience shows that when people do naive benchmarks, and especially when they write other systems and compare it to an existing one line libsnark, they will not bother optimizing things.)
3. Conversely, people that use libsnark for release-level stuff will actually read the README and are more likely to make a conscious decision.

... unless the improvement due to -march=native -mtune=native is negligible on all known platforms.

(Sorry for not seeing this pull request earlier. Something's broken with Github notifications for new pull requests, I'm getting notifications only on discussions of existing requests)

[tromer]

Also, the README (of libff and the other libs) should helpfully offer the magical incantations, known to few mortals, like cmake CMAKE_BUILD_TYPE=RelWithDebInfo -DCMAKE_CXX_FLAGS='-march=native -mtune=native' .

[howardwu]

@tromer How about checking for march support, e.g.

include(CheckCXXCompilerFlag)
CHECK_CXX_COMPILER_FLAG("-march=native" COMPILER_SUPPORTS_MARCH_NATIVE)
if(COMPILER_SUPPORTS_MARCH_NATIVE)
    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -march=native -mtune=native")
endif()

[tromer]

@howardwu How does that help? AFAIK gcc always supports -march=native but the question is whether it should be used (which depends on whether you're going to run the executable on the local machine, as is common during development/benchmarking, or are building a version for distribution).