[eigen] MSVC bad inlining behavior

Hi Eigen developers,

I’m using Eigen (personally and professionally) for quite a few years, mainly to solve many small fixed-size problems. While the template _expression_ engine is a wonderful tool to optimize the generated code, it only works reasonable well using clang/gcc. Using MSVC we’re constantly running into the issue that a lot of code is not properly inlined, which in turn hurts performance massively.

The issue can usually be resolved by patching Eigen and replacing some ‘inline’ with ‘EIGEN_STRONG_INLINE’. I’ve collected some very simple examples which demonstrate the problem:

https://godbolt.org/g/Q8Bvxv

The example is compiled using MSVC 2017 and Clang 4. In each of them the former does not properly inline the calls, while Clang does.

There is a lot of other cases where this happens. The most frustrating ones are some where calls to ‘derived()’ are found in the code as they should essentially be no-ops. While I don’t see Eigen as a culprit, I would like to propose adapting Eigen to accommodate for the bad inlining engine of MSVC. Some time ago, I’ve created a PR which at least fixes the most annoying inlining problems for us:

https://bitbucket.org/eigen/eigen/pull-requests/345

I am open to different solutions for this problem. If adding ‘EIGEN_STRONG_INLINE’ to the core structures of Eigen, I will give it another more complete try.

Best regards,

Basil