isbits Union Optimizations
In Julia, the Array type holds both "bits" values as well as heap-allocated "boxed" values. The distinction is whether the value itself is stored inline (in the direct allocated memory of the array), or if the memory of the array is simply a collection of pointers to objects allocated elsewhere. In terms of performance, accessing values inline is clearly an advantage over having to follow a pointer to the actual value. The definition of "isbits" generally means any Julia type with a fixed, determinate size, meaning no "pointer" fields, see ?isbitstype.
Julia also supports Union types, quite literally the union of a set of types. Custom Union type definitions can be extremely handy for applications wishing to "cut across" the nominal type system (i.e. explicit subtype relationships) and define methods or functionality on these, otherwise unrelated, set of types. A compiler challenge, however, is in determining how to treat these Union types. The naive approach (and indeed, what Julia itself did pre-0.7), is to simply make a "box" and then a pointer in the box to the actual value, similar to the previously mentioned "boxed" values. This is unfortunate, however, because of the number of small, primitive "bits" types (think UInt8, Int32, Float64, etc.) that would easily fit themselves inline in this "box" without needing any indirection for value access. There are two main ways Julia can take advantage of this optimization as of 0.7: isbits Union fields in types, and isbits Union Arrays.
isbits Union Structs
Julia now includes an optimization wherein "isbits Union" fields in types (mutable struct, struct, etc.) will be stored inline. This is accomplished by determining the "inline size" of the Union type (e.g. Union{UInt8, Int16} will have a size of two bytes, which represents the size needed of the largest Union type Int16), and in addition, allocating an extra "type tag byte" (UInt8), whose value signals the type of the actual value stored inline of the "Union bytes". The type tag byte value is the index of the actual value’s type in the Union type’s order of types. For example, a type tag value of 0x02 for a field with type Union{Nothing, UInt8, Int16} would indicate that an Int16 value is stored in the 16 bits of the field in the structure’s memory; a 0x01 value would indicate that a UInt8 value was stored in the first 8 bits of the 16 bits of the field’s memory. Lastly, a value of 0x00 signals that the nothing value will be returned for this field, even though, as a singleton type with a single type instance, it technically has a size of 0. The type tag byte for a type’s Union field is stored directly after the field’s computed Union memory.
isbits Union Arrays
Julia can now also store "isbits Union" values inline in an Array, as opposed to requiring an indirection box. The optimization is accomplished by storing an extra "type tag array" of bytes, one byte per array element, alongside the bytes of the actual array data. This type tag array serves the same function as the type field case: its value signals the type of the actual stored Union value in the array. In terms of layout, a Julia Array can include extra "buffer" space before and after its actual data values, which are tracked in the a->offset and a->maxsize fields of the jl_array_t* type. The "type tag array" is treated exactly as another jl_array_t*, but which shares the same a->offset, a->maxsize, and a->len fields. So the formula to access an isbits Union Array’s type tag bytes is a->data + (a->maxsize - a->offset) * a->elsize + a->offset; i.e. the Array’s a->data pointer is already shifted by a->offset, so correcting for that, we follow the data all the way to the max of what it can hold a->maxsize, then adjust by a->offset more bytes to account for any present "front buffering" the array might be doing. This layout in particular allows for very efficient resizing operations as the type tag data only ever has to move when the actual array’s data has to move.