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Implementation.rst

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Implementation notes

Code use C++14 extensively.

For now code is made to be compiled with:

  • >=clang 3.4
  • >=g++-4.9.1
  • icc 15.0.2 and greater

Due to the limitations in g++ and icc, these features cannot be used:

  • Relaxed constexpr functions
  • Variable templates

Due to a bug in CLang, what I call "universal enable_if" cannot be used and the dummy value must be used instead.

Once we migrate to g++-5.0 and if we drop support for icc, the following changes could be made:

  • Don't use cpp14_constexpr
  • Use variable templates for several of the traits
  • Use variable templates for vectorizable inside traits

Compile-Time

The time to compile expressions it currently not great. It is starting to take quite long.

There are several possible solutions

  • Type erasure of some sort. For instance, some algorithms only need some sizes and data pointer, this would save some instantiations, but could mean major changes
  • Simplify the interface template by removing some enable_if and making some things not template.

However, some things are harder than expected. For instance, it is not possible to remove SFINAE on the operator+ funtion because otherwise it would match etl::complex or iterators from dyn matrix. This because forwarding is "too perfect" and because ADL is used.

Coverage

Coverage is quite low (less than 50%). The biggest problem now is that it is not possible to merge the coverage statistics of several runs correctly. I do believe that the real coverage is in fact higher than this. The merge process only takes the maximum coverage from each profile, but does not merge individual functions meaning that a lot of information is lost. One other problems is that some files are polluting the results since they are not meant to be covered, for instance SFINAE selection is never meant to be "executed" and no_vectorization as well.

How to improve coverage:
  • Improve the merge of multiple coverage profile
  • Remove some files from the coverage analysis
  • Remove some lines from the coverage analysis (asserts) (not possible in sonar unfortunately)
  • Test more things, obviously

Notes

There are too many corner cases in evaluation of expressions:
  • direct_evaluate
  • compound expressions
  • forced expressions
  • reductions

This should be improved