Reminiscence format of worth sorts in Swift
Reminiscence is only a bunch of 1s and 0s, merely referred to as bits (binary digits). If we group the circulate of bits into teams of 8, we are able to name this new unit byte (eight bit is a byte, e.g. binary 10010110 is hex 96). We are able to additionally visualize these bytes in a hexadecimal type (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 fundamental basis of our fashionable x64 CPU structure. Every phrase is related to a digital reminiscence deal with which can 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. Happily we use x64 these days. 💪
So how can we retailer our information sorts on this digital reminiscence deal with area? Effectively, lengthy story brief, we allocate simply the correct quantity of area for every information sort and write the hex illustration of our values into the reminiscence. It is magic, offered by the working system and it simply works.
We might additionally begin speaking about reminiscence segmentation, paging, and different low stage stuff, however actually talking I actually do not know the way these issues work simply but. As I am digging deeper and deeper into low stage stuff like this I am studying so much about how computer systems work below the hood.
One vital factor is that I already know and I need to share with you. It’s all about reminiscence entry on varied architectures. For instance if a CPU’s bus width is 32bit meaning the CPU can solely learn 32bit phrases from the reminiscence below 1 learn cycle. Now if we merely write each object to the reminiscence with out correct information separation that may trigger some bother.
┌──────────────────────────┬──────┬───────────────────────────┐
│ ... │ 4b │ ... │
├──────────────────────────┴───┬──┴───────────────────────────┤
│ 32 bytes │ 32 bytes │
└──────────────────────────────┴──────────────────────────────┘
As you possibly can see if our reminiscence information is misaligned, the primary 32bit learn cycle can solely learn the very first a part of our 4bit information object. It’s going to take 2 learn cycles to get again our information from the given reminiscence area. That is very inefficient and in addition harmful, that is why a lot of the programs will not enable you unaligned entry and this system will merely crash. So how does our reminiscence format seems like in Swift? Let’s take a fast take a look at our information sorts utilizing the built-in MemoryLayout enum sort.
print(MemoryLayout<Bool>.dimension)
print(MemoryLayout<Bool>.stride)
print(MemoryLayout<Bool>.alignment)
print(MemoryLayout<Int>.dimension)
print(MemoryLayout<Int>.stride)
print(MemoryLayout<Int>.alignment) As you possibly can see Swift shops a Bool worth utilizing 1 byte and (on 64bit programs) Int will likely be saved utilizing 8 bytes. So, what the heck is the distinction between dimension, stride and alignment?
The alignment will let you know how a lot reminiscence is required (a number of of the alignment worth) to avoid wasting issues completely aligned on a reminiscence buffer. Measurement is the variety of bytes required to truly retailer that sort. Stride will let you know in regards to the distance between two components on the buffer. Don’t fret if you happen to 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>.dimension)
print(MemoryLayout<Instance>.stride)
print(MemoryLayout<Instance>.alignment) When developing new information sorts, a struct in our case (courses work completely different), we are able to calculate the reminiscence format properties, based mostly on the reminiscence format attributes of the taking part variables.
┌─────────────────────────────────────┬─────────────────────────────────────┐
│ 16 bytes stride (8x2) │ 16 bytes stride (8x2) │
├──────────────────┬──────┬───────────┼──────────────────┬──────┬───────────┤
│ 8 bytes │ 1b │ 7 bytes │ 8 bytes │ 1b │ 7 bytes │
├──────────────────┴──────┼───────────┼──────────────────┴──────┼───────────┤
│ 9 bytes dimension (8+1) │ padding │ 9 bytes dimension (8+1) │ padding │
└─────────────────────────┴───────────┴─────────────────────────┴───────────┘
In Swift, easy sorts have the identical alignment worth dimension as their dimension. In the event you retailer customary Swift information sorts on a contiguous reminiscence buffer there is not any padding wanted, so each stride will likely be equal with the alignment for these sorts.
When working with compound sorts, such because the Instance struct is, the reminiscence alignment worth for that sort will likely be chosen utilizing the utmost worth (8) of the properties alignments. Measurement would be the sum of the properties (8 + 1) and stride may 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? Effectively, not precisely…
struct Instance {
let bar: Bool
let foo: Int
}
print(MemoryLayout<Instance>.dimension)
print(MemoryLayout<Instance>.stride)
print(MemoryLayout<Instance>.alignment) What the heck occurred right here? Why did the dimensions improve? Measurement is hard, as a result of if the padding is available in between the saved variables, then it’s going to improve the general dimension of our sort. You’ll be able to’t begin with 1 byte then put 8 extra bytes subsequent to it, since you’d misalign the integer sort, 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 dimension (1+7+8) │ 16 bytes dimension (1+7+8) │
└─────────────────────────────────────┴─────────────────────────────────────┘
That is the principle purpose why the second instance struct has a barely elevated dimension worth. Be happy to create different sorts and observe by drawing the reminiscence format for them, you possibly can all the time examine if you happen to have been appropriate 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 may be yet another nice put up about Unsafe Swift: A street to reminiscence. These writings helped me so much to know reminiscence format in Swift. 🙏
Reference sorts and reminiscence format in Swift
I discussed earlier that courses behave fairly completely different that is as a result of they’re reference sorts. Let me change the Instance sort to a category and see what occurs with the reminiscence format.
class Instance {
let bar: Bool = true
let foo: Int = 0
}
print(MemoryLayout<Instance>.dimension)
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 might merely say, that worth sorts (struct, Int, Bool, Float, and many others.) reside within the stack and reference sorts (courses) are allotted within the heap, which isn’t 100% true. Swift is wise sufficient to carry out further reminiscence optimizations, however for the sake of “simplicity” let’s simply cease right here.
You would possibly ask the query: why is there a stack and a heap? The reply is that they’re fairly completely different. The stack may be quicker, as a result of reminiscence allocation occurs utilizing push / pop operations, however you possibly can solely add or take away objects to / from it. The stack dimension can 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 is no such thing as a dimension limitation, besides the bodily quantity of RAM. The stack and the heap is kind of completely different, however they’re each extraordinarily helpful reminiscence storage. 👍
Again to the subject, how did we get 8 for each worth (dimension, stride, alignment) right here? We are able to calculate the actual dimension (in bytes) of an object on the heap through the use of the class_getInstanceSize methodology. A category all the time has a 16 bytes of metadata (simply print the dimensions of an empty class utilizing the get occasion dimension methodology) plus the calculated dimension 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 all the time 8 byte, however the precise dimension that it will take from the heap will depend on the occasion variable sorts. The opposite 16 byte comes from the “is a” pointer and the reference depend. If in regards to the Goal-C runtime a bit then this will sound acquainted, but when not, then don’t be concerned an excessive amount of about ISA pointers for now. We’ll speak about them subsequent time. 😅
Swift makes use of Computerized Reference Counting (ARC) to trace and handle your app’s reminiscence utilization. In a lot of the instances you do not have to fret about handbook reminiscence administration, due to ARC. You simply must just remember to do not create robust reference cycles between class situations. Happily these instances may be resolved simply with weak or unowned references. 🔄
class Writer {
let title: String
weak var put up: Submit?
init(title: String) { self.title = title }
deinit { print("Writer deinit") }
}
class Submit {
let title: String
var writer: Writer?
init(title: String) { self.title = title }
deinit { print("Submit deinit") }
}
var writer: Writer? = Writer(title: "John Doe")
var put up: Submit? = Submit(title: "Lorem ipsum dolor sit amet")
put up?.writer = writer
writer?.put up = put up
put up = nil
writer = nil
As you possibly can 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 and so they will not be deallocated (deinit will not be referred to as in any respect) even if you happen to set particular person tips to nil. It is a very primary instance, however the actual query is when do I’ve to make use of weak, unowned or robust? 🤔
I do not wish to say “it relies upon”, so as a substitute, I might wish to level you into the precise path. In the event you take a better take 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 possibly can see world (static features) do not increment reference counters. Nested features then again will seize values, similar factor applies to closure expressions and unnamed closures, however it is a bit extra sophisticated. I might 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 instances you possibly can keep away from them simply through the use of simply the precise key phrase. Beneath the hood, ARC does an incredible job, besides a number of edge instances when you need to break the cycle. Swift is a memory-safe programming language by design. The language ensures that each object will likely be initialized earlier than you might use them, and objects dwelling within the reminiscence that are not referenced anymore will likely be deallocated mechanically. Array indices are additionally checked for out-of-bounds errors. This provides us an additional layer of security, besides if you happen to write unsafe Swift code… 🤓
Anyway, in a nutshell, that is how the reminiscence format seems like within the Swift programming language.