inlined sum, sumabs2, mean, and similar?

[stevengj]

It seems like it would be worthwhile to inline common reduction operations for small vectors rather than calling the generic Base reduce code?

julia> @which sum(SVector(3,4,5))
sum(a) at reduce.jl:229

[c42f]

In this case, the optimizer actually seems to be doing very well at turning the generic Base code into something efficient:

julia> using StaticArrays

julia> @code_native sum(SVector(1,2,3))
        .text
Filename: reduce.jl
        pushq   %rbp
        movq    %rsp, %rbp
Source line: 108
        movq    8(%rdi), %rax
        addq    (%rdi), %rax
        addq    16(%rdi), %rax
Source line: 229
        popq    %rbp
        retq
        nopw    %cs:(%rax,%rax)

I get similar results for sumabs2 and mean.

With the current compiler and standard library, it's hard to know what to do about similar: it inherently needs to give a mutable container, but SVector is immutable. As soon as we create a mutable container (MVector) we end up with heap allocations. If you have any good ideas about how to avoid this, we're all ears! Some of the problems are discussed here: #32

[c42f]

Ah wait. @andyferris has implemented mapreduce, and base calls that which is why this works at all.

Fair enough to have these @inline, if annoying to have to replicate even more of Base.

[andyferris]

Yes, I've been aiming for some compatibility for Base functions. It seems sad to have to specialize sum simply because Base doesn't include an @inline but is otherwise perfect. 😢 Fortunately, as @c42f points out, sum(::SVector{3}) does happen to inline, but this doesn't work for longer vectors.

Also note that just because @which goes to Base it doesn't mean that's not intentional, or that generically written code in Base isn't performant with StaticArrays. I would actually prefer more of that generic code to work fast.

Steven, do you think adding @inline to a bunch of obvious places like this into Base would be welcome (e.g. here where sum is more-or-less an alias for another function)? Or better to do that here?

[andyferris]

I've done all the ones I can think of... I'll close this but feel free to reopen with further requests.