@register_passable
You can add the @register_passable
decorator on a struct to tell Mojo that
the type should be passed in machine registers (such as a CPU register; subject
to the details of the underlying architecture). For tiny data types like an
integer or floating-point number, this is much more efficient than storing
values in stack memory. This means the type is always passed by value and
cannot be passed by reference.
The basic @register_passable
decorator does not change the fundamental
behavior of a type: it still needs an __init__()
and __copyinit__()
method
to be copyable (and it may have a __del__()
method, if necessary). For example:
@register_passable
struct Pair:
var a: Int
var b: Int
fn __init__(inout self, one: Int, two: Int):
self.a = one
self.b = two
fn __copyinit__(inout self, existing: Self):
self.a = existing.a
self.b = existing.b
fn test_pair():
var x = Pair(5, 10)
var y = x
print(y.a, y.b)
y.a = 10
y.b = 20
print(y.a, y.b)
@register_passable
struct Pair:
var a: Int
var b: Int
fn __init__(inout self, one: Int, two: Int):
self.a = one
self.b = two
fn __copyinit__(inout self, existing: Self):
self.a = existing.a
self.b = existing.b
fn test_pair():
var x = Pair(5, 10)
var y = x
print(y.a, y.b)
y.a = 10
y.b = 20
print(y.a, y.b)
test_pair()
test_pair()
This behavior is what we expect from Pair
, with or without the decorator.
You should be aware of a few other observable effects:
-
@register_passable
types cannot hold instances of types that are not also@register_passable
. -
@register_passable
types do not have a predictable identity, and so theself
pointer is not stable/predictable (e.g. in hash tables). -
@register_passable
arguments and result are exposed to C and C++ directly, instead of being passed by-pointer. -
@register_passable
types cannot have a__moveinit__()
constructor, because values passed in a register cannot be passed by reference.
@register_passable("trivial")
Most types that use @register_passable
are just "bags of bits," which we call
"trivial" types. These trivial types are simple and should be copied, moved,
and destroyed without any custom constructors or a destructor. For these types,
you can add the "trivial"
argument, and Mojo synthesizes all the lifecycle
methods as appropriate for a trivial register-passable type:
@register_passable("trivial")
struct Pair:
var a: Int
var b: Int
@register_passable("trivial")
struct Pair:
var a: Int
var b: Int
This is similar to the @value
decorator,
except when using @register_passable("trivial")
the only lifecycle method
you're allowed to define is the __init__()
constructor (but you don't have
to)—you cannot define any copy or move constructors or a destructor.
Examples of trivial types include:
- Arithmetic types such as
Int
,Bool
,Float64
etc. - Pointers (the address value is trivial, not the data being pointed to).
- Arrays of other trivial types, including SIMD.
For more information about lifecycle methods (constructors and destructors) see the section about Value lifecycle.
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