Reminiscence format of worth varieties in Swift
Reminiscence is only a bunch of `1`s and `0`s, merely referred to as bits (binary digits). If we group the circulation of bits into teams of 8, we will name this new unit byte (eight bit is a byte, e.g. binary 10010110 is hex 96). We will additionally visualize these bytes in a hexadecimal kind (e.g. 96 A6 6D 74 B2 4C 4A 15 and many others). Now if we put these hexa representations into teams of 8, we’ll get a brand new unit referred to as phrase.
This 64bit reminiscence (a phrase represents 64bit) format is the essential basis of our fashionable x64 CPU structure. Every phrase is related to a digital reminiscence deal with which can also be represented by a (normally 64bit) hexadecimal quantity. Earlier than the x86-64 period the x32 ABI used 32bit lengthy addresses, with a most reminiscence limitation of 4GiB. Luckily we use x64 these days. 💪
So how will we retailer our knowledge varieties on this digital reminiscence deal with area? Properly, lengthy story brief, we allocate simply the correct quantity of area for every knowledge kind and write the hex illustration of our values into the reminiscence. It is magic, offered by the working system and it simply works.
We may additionally begin speaking about reminiscence segmentation, paging, and different low degree stuff, however truthfully talking I actually do not understand how these issues work simply but. As I am digging deeper and deeper into low degree stuff like this I am studying loads about how computer systems work underneath the hood.
One necessary factor is that I already know and I need to share with you. It’s all about reminiscence entry on numerous architectures. For instance if a CPU’s bus width is 32bit meaning the CPU can solely learn 32bit phrases from the reminiscence underneath 1 learn cycle. Now if we merely write each object to the reminiscence with out correct knowledge separation that may trigger some bother.
┌──────────────────────────┬──────┬───────────────────────────┐
│ ... │ 4b │ ... │
├──────────────────────────┴───┬──┴───────────────────────────┤
│ 32 bytes │ 32 bytes │
└──────────────────────────────┴──────────────────────────────┘
As you’ll be able to see if our reminiscence knowledge is misaligned, the primary 32bit learn cycle can solely learn the very first a part of our 4bit knowledge object. It’s going to take 2 learn cycles to get again our knowledge from the given reminiscence area. That is very inefficient and in addition harmful, that is why a lot of the programs will not permit you unaligned entry and this system will merely crash. So how does our reminiscence format seems like in Swift? Let’s take a fast have a look at our knowledge varieties utilizing the built-in MemoryLayout enum kind.
print(MemoryLayout<Bool>.measurement)
print(MemoryLayout<Bool>.stride)
print(MemoryLayout<Bool>.alignment)
print(MemoryLayout<Int>.measurement)
print(MemoryLayout<Int>.stride)
print(MemoryLayout<Int>.alignment)
As you’ll be able to see Swift shops a Bool worth utilizing 1 byte and (on 64bit programs) Int can be saved utilizing 8 bytes. So, what the heck is the distinction between measurement, stride and alignment?
The alignment will let you know how a lot reminiscence is required (a number of of the alignment worth) to save lots of issues completely aligned on a reminiscence buffer. Dimension is the variety of bytes required to truly retailer that kind. Stride will let you know concerning the distance between two components on the buffer. Don’t be concerned in the event you do not perceive a phrase about these casual definitions, it’s going to all make sense simply in a second.
struct Instance {
let foo: Int
let bar: Bool
}
print(MemoryLayout<Instance>.measurement)
print(MemoryLayout<Instance>.stride)
print(MemoryLayout<Instance>.alignment)
When developing new knowledge varieties, a struct in our case (lessons work totally different), we will calculate the reminiscence format properties, primarily based on the reminiscence format attributes of the collaborating variables.
┌─────────────────────────────────────┬─────────────────────────────────────┐
│ 16 bytes stride (8x2) │ 16 bytes stride (8x2) │
├──────────────────┬──────┬───────────┼──────────────────┬──────┬───────────┤
│ 8 bytes │ 1b │ 7 bytes │ 8 bytes │ 1b │ 7 bytes │
├──────────────────┴──────┼───────────┼──────────────────┴──────┼───────────┤
│ 9 bytes measurement (8+1) │ padding │ 9 bytes measurement (8+1) │ padding │
└─────────────────────────┴───────────┴─────────────────────────┴───────────┘
In Swift, easy varieties have the identical alignment worth measurement as their measurement. In the event you retailer commonplace Swift knowledge varieties on a contiguous reminiscence buffer there is not any padding wanted, so each stride can be equal with the alignment for these varieties.
When working with compound varieties, such because the Instance struct is, the reminiscence alignment worth for that kind can be chosen utilizing the utmost worth (8) of the properties alignments. Dimension would be the sum of the properties (8 + 1) and stride could be calculated by rounding up the dimensions to the following the following a number of of the alignment. Is that this true in each case? Properly, not precisely…
struct Instance {
let bar: Bool
let foo: Int
}
print(MemoryLayout<Instance>.measurement)
print(MemoryLayout<Instance>.stride)
print(MemoryLayout<Instance>.alignment)
What the heck occurred right here? Why did the dimensions enhance? Dimension is difficult, as a result of if the padding is available in between the saved variables, then it’s going to enhance the general measurement of our kind. You possibly can’t begin with 1 byte then put 8 extra bytes subsequent to it, since you’d misalign the integer kind, so that you want 1 byte, then 7 bytes of padding and eventually the 8 bypes to retailer the integer worth.
┌─────────────────────────────────────┬─────────────────────────────────────┐
│ 16 bytes stride (8x2) │ 16 bytes stride (8x2) │
├──────────────────┬───────────┬──────┼──────────────────┬───────────┬──────┤
│ 8 bytes │ 7 bytes │ 1b │ 8 bytes │ 7 bytes │ 1b │
└──────────────────┼───────────┼──────┴──────────────────┼───────────┼──────┘
│ padding │ │ padding │
┌──────────────────┴───────────┴──────┬──────────────────┴───────────┴──────┐
│ 16 bytes measurement (1+7+8) │ 16 bytes measurement (1+7+8) │
└─────────────────────────────────────┴─────────────────────────────────────┘
That is the principle cause why the second instance struct has a barely elevated measurement worth. Be happy to create different varieties and observe by drawing the reminiscence format for them, you’ll be able to at all times verify in the event you have been right or not by printing the reminiscence format at runtime utilizing Swift. 💡
This complete drawback is actual properly defined on the [swift unboxed] weblog. I might additionally wish to suggest this text by Steven Curtis and there’s yet another nice publish about Unsafe Swift: A street to reminiscence. These writings helped me loads to know reminiscence format in Swift. 🙏
Reference varieties and reminiscence format in Swift
I discussed earlier that lessons behave fairly totally different that is as a result of they’re reference varieties. Let me change the Instance kind to a category and see what occurs with the reminiscence format.
class Instance {
let bar: Bool = true
let foo: Int = 0
}
print(MemoryLayout<Instance>.measurement)
print(MemoryLayout<Instance>.stride)
print(MemoryLayout<Instance>.alignment)
What, why? We have been speaking about reminiscence reserved within the stack, till now. The stack reminiscence is reserved for static reminiscence allocation and there is an different factor referred to as heap for dynamic reminiscence allocation. We may merely say, that worth varieties (struct, Int, Bool, Float, and many others.) reside within the stack and reference varieties (lessons) are allotted within the heap, which isn’t 100% true. Swift is sensible sufficient to carry out extra reminiscence optimizations, however for the sake of “simplicity” let’s simply cease right here.
You may ask the query: why is there a stack and a heap? The reply is that they’re fairly totally different. The stack could be sooner, as a result of reminiscence allocation occurs utilizing push / pop operations, however you’ll be able to solely add or take away gadgets to / from it. The stack measurement can also be restricted, have you ever ever seen a stack overflow error? The heap permits random reminiscence allocations and you need to just remember to additionally deallocate what you have reserved. The opposite draw back is that the allocation course of has some overhead, however there isn’t a measurement limitation, besides the bodily quantity of RAM. The stack and the heap is sort of totally different, however they’re each extraordinarily helpful reminiscence storages. 👍
Again to the subject, how did we get 8 for each worth (measurement, stride, alignment) right here? We will calculate the true measurement (in bytes) of an object on the heap by utilizing the class_getInstanceSize methodology. A category at all times has a 16 bytes of metadata (simply print the dimensions of an empty class utilizing the get occasion measurement methodology) plus the calculated measurement for the occasion variables.
class Empty {}
print(class_getInstanceSize(Empty.self))
class Instance {
let bar: Bool = true
let foo: Int = 0
}
print(class_getInstanceSize(Instance.self))
The reminiscence format of a category is at all times 8 byte, however the precise measurement that it will take from the heap is dependent upon the occasion variable varieties. The opposite 16 byte comes from the “is a” pointer and the reference depend. If you understand concerning the Goal-C runtime a bit then this could sound acquainted, but when not, then don’t fret an excessive amount of about ISA pointers for now. We’ll speak about them subsequent time. 😅
Swift makes use of Automated Reference Counting (ARC) to trace and handle your app’s reminiscence utilization. In a lot of the circumstances you do not have to fret about guide reminiscence administration, because of ARC. You simply must just remember to do not create sturdy reference cycles between class situations. Luckily these circumstances could be resolved simply with weak or unowned references. 🔄
class Writer {
let identify: String
weak var publish: Publish?
init(identify: String) { self.identify = identify }
deinit { print("Writer deinit") }
}
class Publish {
let title: String
var writer: Writer?
init(title: String) { self.title = title }
deinit { print("Publish deinit") }
}
var writer: Writer? = Writer(identify: "John Doe")
var publish: Publish? = Publish(title: "Lorem ipsum dolor sit amet")
publish?.writer = writer
writer?.publish = publish
publish = nil
writer = nil
As you’ll be able to see within the instance above if we do not use a weak reference then objects will reference one another strongly, this creates a reference cycle they usually will not be deallocated (deinit will not be referred to as in any respect) even in the event you set particular person tips to nil. It is a very fundamental instance, however the true query is when do I’ve to make use of weak, unowned or sturdy? 🤔
I do not wish to say “it relies upon”, so as a substitute, I would wish to level you into the best route. In the event you take a more in-depth have a look at the official documentation about Closures, you may see what captures values:
- World features are closures which have a reputation and don’t seize any values.
- Nested features are closures which have a reputation and might seize values from their enclosing operate.
- Closure expressions are unnamed closures written in a light-weight syntax that may seize values from their surrounding context.
As you’ll be able to see international (static features) do not increment reference counters. Nested features alternatively will seize values, similar factor applies to closure expressions and unnamed closures, however it’s kind of extra sophisticated. I would wish to suggest the next two articles to know extra about closures and capturing values:
Lengthy story brief, retain cycles suck, however in a lot of the circumstances you’ll be able to keep away from them simply by utilizing simply the best key phrase. Below the hood, ARC does an amazing job, besides a number of edge circumstances when you need to break the cycle. Swift is a memory-safe programming language by design. The language ensures that each object can be initialized earlier than you may use them, and objects residing within the reminiscence that are not referenced anymore can be deallocated robotically. Array indices are additionally checked for out-of-bounds errors. This offers us an additional layer of security, besides in the event you write unsafe Swift code… 🤓
Anyway, in a nutshell, that is how the reminiscence format seems like within the Swift programming language.
