performance of cat of scalars of mixed types

[rwgardner]

The performance of

ar = [a, b, c, d, e]

is much worse than the performance of

ar = Array(Float64, 5)
ar[1]=a
ar[2]=b
ar[3]=c
ar[4]=d
ar[5]=e

where a-e are Float64's because the former syntax uses the cat operation (for a good reason, as explained by @JeffBezanson - a-e could be many things including arrays etc., which makes the "bracket" syntax require an extremely dynamic operation (i.e., cat) in the general case).

This is a request to analyze constructs in the "bracket" notation (i.e., the first example above) and optimize the creation of the array in cases where the types of the elements can be determined statically. For example, translate the first block of code to the second internally, or do something more optimal.

(Changing the [ ] code to manually declare and assign arrays as above in our code increases speed by about 25% currently. However, the syntax is undesirable in our context from a readability standpoint.)

[kmsquire]

This does not address the request per se, but an alternate syntax is

ar = Float64[a, b, c, d, e]

How does the performance of this compare with your second code block?

[rwgardner]

Ah, thanks. ( @JeffBezanson had actually mentioned something like that too, which I forgot.)

The performance is nearly the same as the second block of code (maybe 1-2% slower, although my tests were limited so this is probably like 2% plus or minus 3%.)

That syntax is much nicer than my second block of code, but I'd still request optimization of the other case as well.

[JeffBezanson]

I started looking at this. If a,b,c,d,e are all the same type, then the two versions ([] vs. T[]) are the same, but if they're not then T[] is drastically faster. I can close much of this particular gap.

[rwgardner]

Ah that makes sense. One of the variables was an integer.

[KristofferC]

These now both take the same time (30 ns) using the function below

function f()
	a, b, c, d, e = 1.0, 2.0, 3.0, 4.0, 5.0
	println(@benchmark ar = [$a, $b, $c, $d, $e])
	println(@benchmark begin ar = Array{Float64}(5)
	ar[1]=$a
	ar[2]=$b
	ar[3]=$c
	ar[4]=$d
	ar[5]=$e
	end)
end