David Smith
One little thing I love is how Swift helps with my favorite game*: the floor^W^Wmalloc is lava
Allocation counts:
```
NSMutableArray *c = [NSMutableArray array]; //1
[c addObject: @"hello"]; //2
id d = [c mutableCopy]; //3
var a = [] //0
a.append("hello") //1
let b = a //1
NSMutableString *e = [[NSMutableString alloc] init]; //1
[e appendString: @"hello"]; //2
id f = [e copy]; //3
var g = "" //0
g.append("hello") //0
let h = g //0
```
*"malloc is lava" is a survival horror roguelike
Paul Cantrell
@Catfish_Man This one…!!
```
var g = "" //0
g.append("hello") //0
let h = g //0
```
At a guess:
- “” and “hello” are both emitted in the binary, so no alloc?
- And compiler knows that “”.append(foo) to empty can just simplify to foo?
- …because…inlining? special case magic? something else?
David Smith
@inthehands actually that one works two different ways 😂
1) the smol Strings optimization means we *never* have to allocate for <= 15 bytes of UTF8
2) appending to an empty String will indeed just return the argument (special case in the code), which is a constant in the binary
…or possibly the argument is also smol, I forget how we emit that. It works either way here.
Paul Cantrell
@Catfish_Man
Oh, did not know about the smol strings! Makes tons of sense. I take it the standard string struct is 16 bytes, and one byte is smol flag and/or len…?
(I have never actually looked directly at Swift’s memory representation of anything!)
David Smith
@inthehands yup. String being two words has other interesting consequences (like making collections of NSStrings change size as they bridge, which is awkward), but overall I'm quite happy with it.
Another fun fact: when not in smol form, the pointed-to buffer is actually an NSString subclass, which allows us to implement String -> NSString bridging by just retaining and returning it.
"But David, isn't Swift below Foundation? How can it subclass a Foundation class"
class_setSuperclass 🫠
Paul Cantrell
@Catfish_Man > ”But David, isn't Swift below Foundation? How can it subclass a Foundation class"
>class_setSuperclass 🫠
Well there goes my youthful innocence
Paul Cantrell
@Catfish_Man
“weak”?!
As in “if this class is undefined, we no longer have a superclass?”
David Smith
@inthehands I have a feature request in with the linker folks for "weak lazy subclassing", but they understandably are not thrilled with catering to my unhinged whims :D
Paul Cantrell
@Catfish_Man @steve
“This is fi ne”
David Smith
@steve @inthehands it's like an onion where every layer is made of infuriating consequences
I Can't Believe It's Not Zero!
@inthehands @Catfish_Man I’m going to be angry about the 0124567xx89 32b layout for a long time.
I Can't Believe It's Not Zero!
@inthehands @Catfish_Man well, on 64b Apple platforms. On 32b they’re 12B (holding 8 utf8 units, or 10 but non-contiguous if code was built with an older Swift version and inlined), and on Android 64b they hold 14 instead of 15 utf8 units because of memory coloring. There may be other platform-specific oddities I’m forgetting.
Paul Cantrell
@Catfish_Man Ha! This code made me angry when I finally understood it.
David Smith
@inthehands found the code I was thinking of that handles NSString * being half the size of String in collections while still avoiding a temporary buffer: https://github.com/swiftlang/swift-corelibs-foundation/blob/1b514e4242526690c19fad9f53644065dd50b69d/Darwin/Foundation-swiftoverlay/NSDictionary.swift#L102
Paul Cantrell
@Catfish_Man
This bit…!
“I’m sure those values didn’t need converting anyway”
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