Hi, On 2014-09-04 17:27:16 -0700, Ethan L. Miller wrote: > Yes, it's possible to use CPU flags to allow the use of advanced > instruction sets automatically. runtime detection of supported instructions sets is with the current function pointer approach possible too. > The difficulty is that, if those > instructions aren't available, it's not clear which of the "basic" > approaches to use, since performance can vary based on a lot of > factors. Even with advanced instructions, there are often multiple > reasonable approaches to take, as Janne's email makes clear, so it's > impossible to say "this algorithm is always best". I agree that the current approach fits the model of implementations with different cpu/memory use better. Using ifunc would be mostly orthogonal to the issue of badly structured code. > We can certainly set up a default approach if we want, though, that > can be overridden by compile-time flags. I don't think this would be an improvement. > Incidentally, I'm starting to work on coding a version of gf-complete > (and associated erasure coding functions) in C++ using templates, > which will hopefully allow us to better separate out different > implementations. We could still have run-time dispatch for the > desired routines, but templates should allow for more compact code and > better isolation of architecture-specific code. The big drawback is > that C++ code isn't typically used in the kernel.... One possible simplification for the carry less multiplication would be relying on inlining and optimisations of compile time constants. Implement one function which does a variable number of polynomial reductions. The current functions would then just be thin wrappers which call the general function with a compile time constant for the number of reductions. Forced inlining and dead code removal will optimize branches away. The same method could be used to avoid the duplication of the inner loop for the optional xor with the destination. Janne -- To unsubscribe from this list: send the line "unsubscribe ceph-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
