What is the purpose of @inbounds in Julia?

General Usage
1 
function sum(A::AbstractArray)
       r = zero(el type(A))
       for i = 1: length(A)
            @inbounds r += A[i]
       end
       return r
end

@inbounds(blk)

Eliminates array bounds checking within expressions.
In the example above the in-range check for referencing element i of array A is skipped to improve performance. My question is, I do not quite understand why is the @inbounds ever needed in the above example?

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2

Since it has a getindex. Logically the setup constrains it to be a valid index, but the compiler may or may not figure this out, so an @inbounds can help.

(Incidentally, the above example from the docstring is outdated, now eachindex(A) is the recommended index iterator.)

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3

Wait, does the compiler sometimes figure it out? I just always use @inbounds (where appropriate) because I figured the compiler wasn’t going to figure out that stuff.

4

For simple for loops, yes, as a general rule the compiler will figure it out.

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5

I use @inbounds super-rarely these days, since 1.2-pre is getting so good about figuring out the simple stuff, and for the complicated access patterns and index calculations I usually prefer to error if I make mistakes.

I think that peppering code with @inbounds by default is becoming less and less advisable.

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