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Make map() and broadcast() faster for Union{T, Missing/Nothing} element types #25828

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Feb 6, 2018
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Make map() and broadcast() faster for Union{T, Missing/Nothing} element types #25828

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5 changes: 2 additions & 3 deletions base/array.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -573,12 +573,11 @@ function collect_to!(dest::AbstractArray{T}, itr, offs, st) where T
i = offs
while !done(itr, st)
el, st = next(itr, st)
S = typeof(el)
if S === T || S <: T
if el isa T || typeof(el) === T
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The need for typeof(el) === T is a bit of a mystery, but it was weird to need it in the existing code already. See #25799.

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I assume that was some attempt at providing a fast-path for uninferred code. But S === T is often much harder to infer than S <: T. And since isconcretetype(S), it won't be faster at runtime either.

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Yet if I remove that check I get a worse performance than before with map(identity, y) (see what I noted at #25799). That's really weird, but...

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Keno recently changed the result of el isa T from Const(true) to Conditional(:el, T, Union{}) – maybe there's was a mistake in the implementation of that?

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Where's the relevant commit? Can I try reverting it?

@inbounds dest[i] = el::T
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if you remove the typeof(el) === T condition above, inference will be able to infer the el::T result and you can remove this redundant type-check

i += 1
else
R = promote_typejoin(T, S)
R = promote_typejoin(T, typeof(el))
new = similar(dest, R)
copyto!(new,1, dest,1, i-1)
@inbounds new[i] = el
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5 changes: 2 additions & 3 deletions base/broadcast.jl
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Expand Up @@ -500,14 +500,13 @@ end
@nexprs $nargs i->(@inbounds val_i = _broadcast_getindex(A_i, I_i))
# call the function
V = @ncall $nargs f val
S = typeof(V)
# store the result
if S <: eltype(B)
if V isa eltype(B)
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+1

This syntax allows the compiler to see / "prove" that S <: T if this branch is taken, and thus likely eliminate dispatch in the assignment below

@inbounds B[I] = V
else
# This element type doesn't fit in B. Allocate a new B with wider eltype,
# copy over old values, and continue
newB = Base.similar(B, promote_typejoin(eltype(B), S))
newB = Base.similar(B, promote_typejoin(eltype(B), typeof(V)))
for II in Iterators.take(iter, count)
newB[II] = B[II]
end
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