@aclements, do you want to improve these docs for Go 1.9?

The first word in a variable or in an allocated struct, array or slice can be relied upon to be 64-bit aligned.

I think this is fair.

And "the first word in an allocated slice" is not an accurate description, it should be the first element in an allocated slice which elements are 64-bit words.

Can you expand on why you think this isn't an accurate description? It seems accurate to me.

(Really, this whole statement is somewhat inaccurate, as it's only true if the allocation itself is at least 8 bytes. But if that's not the case, then there's no way to use the 64-bit atomic ops anyway.)

CL https://golang.org/cl/48112 mentions this issue.

A slice is composed a header and an underlying array.
When we say "an allocated slice", we generally mean the header of the slice is allocated.
But we would never do 64-bit atomic operations on slice headers.
And for an allocated slice, its underlying array may be not allocated.
For example, its underlying array may be a field of a struct value.

And "the first word in an allocated slice" is not an accurate description, it should be the first element in an allocated slice which elements are 64-bit words.

The above texts may be also not good. I prefer to use the following one:

The first word in a variable or in an allocated struct and array can be relied upon to be 64-bit aligned. The elements of a slice are stored in an underlying array of the slice.