Performance penalty using anonymous function in Julia

Question

I have noticed that there is a performance penalty associated with using anonymous functions in Julia. To illustrate I have two implementations of quicksort (taken from the micro performance benchmarks in the Julia distribution). The first sorts in ascending order

function qsort!(a,lo,hi)
    i, j = lo, hi
    while i < hi
        pivot = a[(lo+hi)>>>1]
        while i <= j
            while a[i] < pivot; i += 1; end
            while pivot < a[j]; j -= 1; end
            if i <= j
                a[i], a[j] = a[j], a[i]
                i, j = i+1, j-1
            end
        end
        if lo < j; qsort!(a,lo,j); end
        lo, j = i, hi
    end
    return a
end

The second takes an additional parameter: an anonymous function that can be used to specify ascending or descending sort, or comparison for more exotic types

function qsort_generic!(a,lo,hi,op=(x,y)->x<y)
    i, j = lo, hi
    while i < hi
        pivot = a[(lo+hi)>>>1]
        while i <= j
            while op(a[i], pivot); i += 1; end
            while op(pivot, a[j]); j -= 1; end
            if i <= j
                a[i], a[j] = a[j], a[i]
                i, j = i+1, j-1
            end
        end
        if lo < j; qsort_generic!(a,lo,j,op); end
        lo, j = i, hi
    end
    return a
end

There is a significant performance penalty when sorting Arrays of Int64, with the default version an order of magnitude faster. Here are times for sorting arrays of length N in seconds:

N       qsort_generic   qsort
2048    0.00125         0.00018
4096    0.00278         0.00029
8192    0.00615         0.00061
16384   0.01184         0.00119
32768   0.04482         0.00247
65536   0.07773         0.00490

The question is: Is this due to limitations in the compiler that will be ironed out in time, or is there an idiomatic way to pass functors/anonymous functions that should be used in cases like this?

update From the answers it looks like this is something that will be fixed up in the compiler.

In the mean time, there were two suggested work arounds. Both approaches are fairly straightforward, though they do start to feel like the sort of jiggery-pokery that you have to use in C++ (though not on the same scale of awkward).

The first is the FastAnon package suggested by @Toivo Henningsson. I didn't try this approach, but it looks good.

I tried out the second method suggested by @simonstar, which gave me equivalent performance to the non-generic qsort! implementation:

abstract OrderingOp
immutable AscendingOp<:OrderingOp end
immutable DescendingOp<:OrderingOp end
evaluate(::AscendingOp, x, y) = x<y
evaluate(::DescendingOp, x, y) = x>y

function qsort_generic!(a,lo,hi,op=AscendingOp())
    i, j = lo, hi
    while i < hi
        pivot = a[(lo+hi)>>>1]
        while i <= j
            while evaluate(op, a[i], pivot); i += 1; end
            while evaluate(op, pivot, a[j]); j -= 1; end
            if i <= j
                a[i], a[j] = a[j], a[i]
                i, j = i+1, j-1
            end
        end
        if lo < j; qsort_generic!(a,lo,j,op); end
        lo, j = i, hi
    end
    return a
end

Thanks everyone for the help.

Answer 1

It's a problem and will be fixed with an upcoming type system overhaul.

Update: This has now been fixed in the 0.5 version of Julia.

Answer 2

As others have noted, the code you've written is idiomatic Julia and will someday be fast, but the compiler isn't quite there yet. Besides using FastAnonymous, another option is to pass types instead of anonymous functions. For this pattern, you define an immutable with no fields and a method (let's call it evaluate) that accepts an instance of the type and some arguments. Your sorting function would then accept an op object instead of a function and call evaluate(op, x, y) instead of op(x, y). Because functions are specialized on their input types, there is no runtime overhead to the abstraction. This is the basis for reductions and specification of sort order in the standard library, as well as NumericExtensions.

For example:

immutable AscendingSort; end
evaluate(::AscendingSort, x, y) = x < y

function qsort_generic!(a,lo,hi,op=AscendingSort())
    i, j = lo, hi
    while i < hi
        pivot = a[(lo+hi)>>>1]
        while i <= j
            while evaluate(op, a[i], pivot); i += 1; end
            while evaluate(op, pivot, a[j]); j -= 1; end
            if i <= j
                a[i], a[j] = a[j], a[i]
                i, j = i+1, j-1
            end
        end
        if lo < j; qsort_generic!(a,lo,j,op); end
        lo, j = i, hi
    end
    return a
end

Answer 3

Yes, it's due to limitations in the compiler, and there are plans to fix it, see e.g. this issue. In the meantime, the FastAnonymous package might provide a workaround.

The way that you have done it looks pretty idiomatic, there's unfortunately no magic trick that you are missing (except for possibly the FastAnonymous package).