Docs on enabling LTO (-flto) with clang++

[springmeyer]

Getting Link Time optimization working takes certain combinations of modern tools and build settings. It would be great to have some general docs on this so that LTO is demystified and more commonly used.

Doc should cover:

Linux details

• upgraded binutils gold (can be installed via mason)
• at least clang++ 3.9.1 via mason
• llvm-ar and llvm-ranlib on path (and set in environment AR=/path/to/llvm-ar RANLIB=/path/to/llvm-ranlib)
• -flto in the CXXFLAGS

OS X details

• Apple built in compiler or clang++ via mason
• -flto in the CXXFLAGS and LDFLAGS

Troubleshooting:

This error indicates that the linker does not understand the LTO format. This will happen on linux if you've not installed binutils from mason.

file.o: file not recognized: File format not recognized
clang-5.0: error: linker command failed with exit code 1 (use -v to see invocation)

Refs https://github.com/mapbox/spatial-algorithms/pull/10

[daniel-j-h]

Flagging: CMake's interprocedural optimization property does not enable lto on gcc and clang - it only does so for the Intel compiler. I ran into this some time ago since it looks fitting. Something to look out for.

Related: maybe a note or two about how to check cxxflags and ldflags in CMake's generated files would be great for debugging - I think the files are called CMakeFiles/TARGET.dir/{flags.make,link.txt}.

[springmeyer]

per https://github.com/mapbox/carmen-cache/issues/117#issuecomment-369051742 also we need to ensure the same clang++ major version is used to compile all binaries and libraries" otherwise you'll see an error like:

LLVM gold plugin has failed to create LTO module:
  Invalid value (Producer: 'LLVM5.0.0git-30c4647' Reader: 'LLVM 3.9.1')

[springmeyer]

node-cpp-skel now enables LTO by default: https://github.com/mapbox/node-cpp-skel/issues/110

[jfirebaugh]

It seems that -flto and -Os cannot currently be used simultaneously when using the Android NDK: https://github.com/android-ndk/ndk/issues/721.