Use of Base.@propagate_inbounds in OffsetArrays.jl

General Usage
1

I am trying to understand how Base.@propagate_inbounds works by studying its use in OffsetArrays.jl. Currently setindex! is implemented as

@propagate_inbounds function Base.setindex!(A::OffsetArray{T,N}, val, I::Vararg{Int,N}) where {T,N}
    @boundscheck checkbounds(A, I...)
    J = @inbounds map(parentindex, axes(A), I)
    @inbounds parent(A)[J...] = val
    val
end

However the use of @propagate_inbounds here seems unnecessary to me, as the underlying parent array is indexed with @inbounds anyway. Am I correct in this, and would it be the same if the @propagate_inbounds macro is removed from the function definition?

2

It disables the @boundscheck line if the caller used @inbounds:

julia> using OffsetArrays

julia> o = OffsetVector([1, 2, 3], -1);

julia> function f(o)
           @inbounds o[-1] = 1
           return o
       end
f (generic function with 1 method)

julia> function g(o)
           o[-1] = 1
           return o
       end
g (generic function with 1 method)

julia> f(o)
3-element OffsetArray(::Array{Int64,1}, 0:2) with eltype Int64 with indices 0:2:
 1
 2
 3

julia> g(o)
ERROR: BoundsError: attempt to access 3-element OffsetArray(::Array{Int64,1}, 0:2) with eltype Int64 with indices 0:2 at index [-1]
Stacktrace:
 [1] throw_boundserror(::OffsetArray{Int64,1,Array{Int64,1}}, ::Tuple{Int64}) at ./abstractarray.jl:537
 [2] checkbounds at ./abstractarray.jl:502 [inlined]
 [3] setindex! at /home/fredrik/.julia/packages/OffsetArrays/JNUm0/src/OffsetArrays.jl:164 [inlined]
 [4] g(::OffsetArray{Int64,1,Array{Int64,1}}) at ./REPL[4]:2
 [5] top-level scope at REPL[6]:1
1 Like
3

Isn’t this what @inbounds is supposed to do anyway if the function gets inlined? I understood @propagate_inbounds to push the @inbounds down one layer, but in this case the inner function is called with bounds checking disabled anyway, so propagating the @inbounds should have no effect.

In other words, I understand if there’s an @inline here, but the @propagate_inbounds seems unnecessary. Is there something that I’m missing?