Julia Language Tuples Dispatching on tuple types
Example
Because Julia function parameter lists are themselves tuples, dispatching on various kinds of tuples is often easier done through the method parameters themselves, often with liberal usage for the "splatting" ... operator. For instance, consider the implementation of reverse for tuples, from Base:
revargs() = ()
revargs(x, r...) = (revargs(r...)..., x)
reverse(t::Tuple) = revargs(t...)
Implementing methods on tuples this way preserves type stability, which is crucial for performance. We can see that there is no overhead to this approach using the @code_warntype macro:
julia> @code_warntype reverse((1, 2, 3))
Variables:
#self#::Base.#reverse
t::Tuple{Int64,Int64,Int64}
Body:
begin
SSAValue(1) = (Core.getfield)(t::Tuple{Int64,Int64,Int64},2)::Int64
SSAValue(2) = (Core.getfield)(t::Tuple{Int64,Int64,Int64},3)::Int64
return (Core.tuple)(SSAValue(2),SSAValue(1),(Core.getfield)(t::Tuple{Int64,Int64,Int64},1)::Int64)::Tuple{Int64,Int64,Int64}
end::Tuple{Int64,Int64,Int64}
Although somewhat hard to read, the code here is simply getting creating a new tuple with values 3rd, 2nd, and 1st elements of the original tuple, respectively. On many machines, this compiles down to extremely efficient LLVM code, which consists of loads and stores.
julia> @code_llvm reverse((1, 2, 3))
define void @julia_reverse_71456([3 x i64]* noalias sret, [3 x i64]*) #0 {
top:
%2 = getelementptr inbounds [3 x i64], [3 x i64]* %1, i64 0, i64 1
%3 = getelementptr inbounds [3 x i64], [3 x i64]* %1, i64 0, i64 2
%4 = load i64, i64* %3, align 1
%5 = load i64, i64* %2, align 1
%6 = getelementptr inbounds [3 x i64], [3 x i64]* %1, i64 0, i64 0
%7 = load i64, i64* %6, align 1
%.sroa.0.0..sroa_idx = getelementptr inbounds [3 x i64], [3 x i64]* %0, i64 0, i64 0
store i64 %4, i64* %.sroa.0.0..sroa_idx, align 8
%.sroa.2.0..sroa_idx1 = getelementptr inbounds [3 x i64], [3 x i64]* %0, i64 0, i64 1
store i64 %5, i64* %.sroa.2.0..sroa_idx1, align 8
%.sroa.3.0..sroa_idx2 = getelementptr inbounds [3 x i64], [3 x i64]* %0, i64 0, i64 2
store i64 %7, i64* %.sroa.3.0..sroa_idx2, align 8
ret void
}
