Bug 6657 – dotProduct overload for small fixed size arrays

When the arrays given to std.numeric.dotProduct are very small, its performance compared to an inlined foreach loop becomes quite bad (a simple benchmarks on request). This is a hard problem to solve in general (you probably need profile-driven optimizations, that maybe future LLVM versions will perform with -O5 or -O6 level optimization or more), but there is an important special case where I think there is a way to improve the situation: int[3] a, b; auto r = dotProduct(a, b); In this situation dotProduct may just recognize (with template constraints) that both a and b are fixed-sized arrays, that their length is the same (so no need to test it again at runtime), and that this length is small (less than 6? 8?). In this case a function template overload for dotProduct performs a "static foreach" on the items of a and b (in theory you are also allowed to use inline asm with few instructions that use SIMD registers, but currently with DMD this kills inlining, so it's not good enough). I think in this case DMD will *inline* this very simple dotProduct function overload, allowing good enough performance. This low performance problem has caused problems in my code. See also bug 4393

@n8sh created dlang/phobos pull request #6990 "Fix Issue 6657 - dotProduct overload for small fixed size arrays" fixing this issue: - Fix Issue 6657 - dotProduct overload for small fixed size arrays https://github.com/dlang/phobos/pull/6990

dlang/phobos pull request #6990 "Fix Issue 6657 - dotProduct overload for small fixed size arrays" was merged into master: - c334d376011a04356dc150f765a54e7ad1c05810 by Nathan Sashihara: Fix Issue 6657 - dotProduct overload for small fixed size arrays https://github.com/dlang/phobos/pull/6990