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Description
If a user writes code such as:
function resting_sum((_, x, y...),)
return x, +(x, y...)
end
The y... in the argument destructuring is lowered via a Base.rest(); this is inferred with terrible effects, as shown in the following example:
using Cthulhu
@noinline function resting_sum((_, x, y...),)
return x, +(x, y...)
end
function foo(x, ys)
return resting_sum((nothing, x, ys...),)
end
N = 12
x = 1
ys = ntuple(idx -> 1, N-1)
@assert foo(x, ys) == (1, N)
@descend foo(x, ys)Which results in:
%13 = invoke resting_sum(::Tuple{Nothing, Vararg{Int64, 12}})::Tuple{Int64, Int64} (!c,!e,!n,!t,!s,!m,+i)
Note the difference if we apply a simple change to avoid the destructuring and instead use Base.tail() to pull out the last elements:
@noinline function tailing_sum(xs)
return xs[2], +(Base.tail(xs)...)
end
function bar(x, ys)
return tailing_sum((nothing, x, ys...),)
end
@assert bar(x, ys) == (1, N)
@descend bar(x, ys)
Which results in:
%13 = invoke tailing_sum(::Tuple{Nothing, Vararg{Int64, 12}})::Tuple{Int64, Int64} (+c,+e,+n,+t,+s,+m,+i)
We should improve the inferrability/effects of Base.rest() to allow users to use this convenient syntax.
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