AttributedContainer.init Performance Issues

[k.benua (Christian)]

Current state of AttributedContainer.init Performance

My team and I have faced unexpected degradation when performed migration to UIButton.Configuration with a AttributedString. At first, I've logged the duration locally and reproduced it in Release builds on iOS simulator and on real device.

Here is screenshot from Xcode Instruments Time Profiler:

Then we've measured this code in production (on DispatchQueue.global(qos: .default)):

func configure() {
    var warmup = UIButton.Configuration.plain()
    var container = AttributeContainer()
    container.font = UIFont.systemFont(ofSize: 12)
    warmup.attributedTitle = AttributedString("w", attributes: container)
}

Percentile	Duration
p10	44 ms
p25	50 ms
p50	66 ms
p75	100 ms
p90	200 ms

So even on top-notch hardware it still takes significant amount of time - about 50 ms.

Why it is so slow

Main reasons are checks that type conforms to AttributedStringKey (here),MarkdownDecodableAttributedStringKey (here) and to AttributeScope (here).

The first conformance check for given type and protocol are mostly slow for any given (type, protocol), because to create existential container we need protocol witness table. To find protocol witness table, we need to find matching protocol conformance description, to find it the runtime must scan all protocol conformance type descriptors. Then it gets cached into ConcurrentReadableHashMap, that's why only first conformance check is expensive. That's why existing benchmark doesn't really cover this performance bottleneck.

So as an application grows and accumulates more protocol conformance descriptors, the cost of the first conformance checks increases proportionally. For instance, our app binary has more than 205k protocol conformance descriptors.

Optimization idea #1

So if protocol cast is expensive, we can introduce fast-circuit checks for all types that conform these types inside FoundationEssentials module. Because AttributeStringKey, MarkdownDecodableAttributedStringKey and AttributedScope are public, we'll be forced to fallback to original casts if our fast-circuit checks failed. Unfortunately, we're unable to make the same optimisation for types from UIKitCore, SwiftUICore, Accessibility frameworks, that are loaded dynamically in _loadDefaultAttributes, but still, we can significantly reduce overhead.

Draft changes for idea #1

First: build a dictionary with all known types for fast-circuit:

fileprivate enum _KnownFieldKind : Sendable {
    case attribute(any AttributedStringKey.Type)
    case markdownAttribute(any MarkdownDecodableAttributedStringKey.Type)
    case scope(any AttributeScope.Type)
}

fileprivate let _knownFieldKinds: [ObjectIdentifier : _KnownFieldKind] = {
    typealias F = AttributeScopes.FoundationAttributes
    let attributes: [any AttributedStringKey.Type] = [
        F.LinkAttribute.self,
        F.PersonNameComponentAttribute.self,
        F.DateFieldAttribute.self,
        F.AlternateDescriptionAttribute.self,
        F.ImageURLAttribute.self,
        F.ReplacementIndexAttribute.self,
        F.MeasurementAttribute.self,
        F.ByteCountAttribute.self,
        F.DurationFieldAttribute.self,
        F.WritingDirectionAttribute.self,
        F.InlinePresentationIntentAttribute.self,
        F.PresentationIntentAttribute.self,
        F.MarkdownSourcePositionAttribute.self,
        F.ListItemDelimiterAttribute.self,
        F.NumberFormatAttributes.SymbolAttribute.self,
        F.NumberFormatAttributes.NumberPartAttribute.self
    ]
    let markdownAttributes: [any MarkdownDecodableAttributedStringKey.Type] = [
        F.LanguageIdentifierAttribute.self,
        F.AgreementConceptAttribute.self,
        F.AgreementArgumentAttribute.self,
        F.ReferentConceptAttribute.self,
        F.LocalizedNumberFormatAttribute.self,
        F.InflectionAlternativeAttribute.self,
        F.MorphologyAttribute.self,
        F.InflectionRuleAttribute.self,
        F.AssumedFallbackInflectionAttribute.self
    ]
    let scopes: [any AttributeScope.Type] = [
        F.self,
        F.NumberFormatAttributes.self
    ]

    var table = [ObjectIdentifier : _KnownFieldKind]()
    for type in attributes { table[ObjectIdentifier(type)] = .attribute(type) }
    for type in markdownAttributes { table[ObjectIdentifier(type)] = .markdownAttribute(type) }
    for type in scopes { table[ObjectIdentifier(type)] = .scope(type) }
    return table
}()

Second: use it

for field in Type(Self.self).fields {
    let fieldType = field.type.swiftType
    // fast-circuit
    if let known = _knownFieldKinds[ObjectIdentifier(fieldType)] {
        switch known {
        case .attribute(let attribute):
            desc.attributes[attribute.name] = attribute
        case .markdownAttribute(let attribute):
            desc.attributes[attribute.name] = attribute
            desc.markdownAttributes[attribute.markdownName] = attribute
        case .scope(let scope):
            desc.merge(scope.scopeDescription)
        }
        continue
    }
    switch fieldType {
    case let attribute as any AttributedStringKey.Type:
        desc.attributes[attribute.name] = attribute
        if let markdownAttribute = attribute as? any MarkdownDecodableAttributedStringKey.Type {
            desc.markdownAttributes[markdownAttribute.markdownName] = markdownAttribute
        }
    case let scope as any AttributeScope.Type:
        desc.merge(scope.scopeDescription)
    default: break
    }
}

Optimization idea #2

internal static func markdownKeyTypes() is actually never used in this repo. But we compute data for it sacrificing performance while building markdownAttributes here:

if let markdownAttribute = attribute as? any MarkdownDecodableAttributedStringKey.Type {
    desc.markdownAttributes[markdownAttribute.markdownName] = markdownAttribute
}

If markdownKeyTypes() isn't used at all, we can just delete it. If it is used somehow internally but not actually needed along with attributeKeyTypes (that is used in _loadDefaultAttributes), we can calculate it when we really need it. If all these assumptions are wrong - we can leave it as it is.

Draft changes for idea #2

internal static func markdownKeyTypes() -> [String : any MarkdownDecodableAttributedStringKey.Type] {
    let desc = Self.scopeDescription
    if let markdownAttributes = desc.markdownAttributes {
        return markdownAttributes
    }
    
    var markdownAttributes = [String : any MarkdownDecodableAttributedStringKey.Type]()
    for attribute in desc.orderedAttributes {
        if let markdownAttribute = _markdownAttribute(attribute) {
            markdownAttributes[markdownAttribute.markdownName] = markdownAttribute
        }
    }
    let _markdownAttributes = markdownAttributes
    _loadedScopeCache.withLock {
        $0[Self.self]?.markdownAttributes = _markdownAttributes
    }
    return markdownAttributes
}

Conclusion

Sadly, optimizations here are strongly limited by dynamic loading AttributeScopes from UIKitCore, SwiftUICore and Accessibility frameworks, because described short-circuit logic can't be applied to types from these libraries.

But by using ideas from this topic it is possible to reduce overhead that almost each app is facing on main thread by approx 20-40%.

[jmschonfeld (Jeremy Schonfeld)]

Thanks for bringing this up! I can't speak to the UIKit usage of this API (and would recommend you file a feedback to Apple about that usage if you find it problematic since that's outside the scope of the Swift forums), but focusing on the Foundation implementations you mention in the general usage sense:

@k.benua

we can introduce fast-circuit checks for all types that conform these types inside FoundationEssentials module

We could do this, but I'd be curious to better understand how much of a performance benefit this gives us (in contrast to the increased maintenance burden of listing all of the Foundation attributes in multiple places). The Foundation attributes are certainly very commonly used, but non-Foundation attributes from AppKit, UIKit, SwiftUI, or even third party apps are also certainly very common and I don't know offhand the distribution of their usage. We can look into whether this is worthwhile, but I have a suspicion that this may not have a large effect since it would only affect a single scope and this information is already cached and only calculated on first use.

@k.benua

If markdownKeyTypes() isn't used at all, we can just delete it. If it is used somehow internally but not actually needed along with attributeKeyTypes (that is used in _loadDefaultAttributes), we can calculate it when we really need it. If all these assumptions are wrong - we can leave it as it is.

It's not used in swift-foundation because swift-foundation does not yet have the AttributedString(markdown:) API. Once we port that API to swift-foundation, it would use the markdown keys. It's currently used by the Foundation.framework build of the project and cannot be deleted. We could choose to calculate it when first calling AttributedString(markdown:), but given that this is only calculated once on first use, I wonder if iterating the scope a second time to perform those casts lazily would actually be less performant, especially for common scopes like Foundation/SwiftUI that will almost certainly be used in both APIs.

@k.benua

But by using ideas from this topic it is possible to reduce overhead that almost each app is facing on main thread by approx 20-40%.

Can you elaborate a bit on how you measured this amount and what benchmarks you used?

[k.benua (Christian)]

Hi. Jeremy! Thanks for quick response!

Can you elaborate a bit on how you measured this amount and what benchmarks you used?

This is a bit tricky part. I haven't conduct direct measurements, I've got this approximation by setting symbolic breakpoint to `swift_conformsToProtocolMaybeInstantiateSuperclasses` and printed out its arguments like pairs of (type, protocol).

Here they are, as a md table:

Casting history

#	Type	Protocol	Result	Optimized by type check	depth	Asked by
1	AttributeScopes.UIKitAttributes.FontAttribute	MarkdownDecodableAttributedStringKey	no	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
2	AttributeScopes.UIKitAttributes.ParagraphStyleAttribute	AttributedStringKey	YES	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
3	AttributeScopes.UIKitAttributes.ParagraphStyleAttribute	MarkdownDecodableAttributedStringKey	no	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
4	AttributeScopes.UIKitAttributes.ForegroundColorAttribute	AttributedStringKey	YES	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
5	AttributeScopes.UIKitAttributes.ForegroundColorAttribute	MarkdownDecodableAttributedStringKey	no	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
6	AttributeScopes.UIKitAttributes.BackgroundColorAttribute	AttributedStringKey	YES	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
7	AttributeScopes.UIKitAttributes.BackgroundColorAttribute	MarkdownDecodableAttributedStringKey	no	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
8	AttributeScopes.UIKitAttributes.LigatureAttribute	AttributedStringKey	YES	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
9	AttributeScopes.UIKitAttributes.LigatureAttribute	MarkdownDecodableAttributedStringKey	no	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
10	AttributeScopes.UIKitAttributes.KernAttribute	AttributedStringKey	YES	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
11	AttributeScopes.UIKitAttributes.KernAttribute	MarkdownDecodableAttributedStringKey	no	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
12	AttributeScopes.UIKitAttributes.TrackingAttribute	AttributedStringKey	YES	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
13	AttributeScopes.UIKitAttributes.TrackingAttribute	MarkdownDecodableAttributedStringKey	no	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
14	AttributeScopes.UIKitAttributes.StrikethroughStyleAttribute	AttributedStringKey	YES	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
15	AttributeScopes.UIKitAttributes.StrikethroughStyleAttribute	MarkdownDecodableAttributedStringKey	no	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
16	AttributeScopes.UIKitAttributes.UnderlineStyleAttribute	AttributedStringKey	YES	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
17	AttributeScopes.UIKitAttributes.UnderlineStyleAttribute	MarkdownDecodableAttributedStringKey	no	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
18	AttributeScopes.UIKitAttributes.StrokeColorAttribute	AttributedStringKey	YES	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
19	AttributeScopes.UIKitAttributes.StrokeColorAttribute	MarkdownDecodableAttributedStringKey	no	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
20	AttributeScopes.UIKitAttributes.StrokeWidthAttribute	AttributedStringKey	YES	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
21	AttributeScopes.UIKitAttributes.StrokeWidthAttribute	MarkdownDecodableAttributedStringKey	no	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
22	AttributeScopes.UIKitAttributes.ShadowAttribute	AttributedStringKey	YES	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
23	AttributeScopes.UIKitAttributes.ShadowAttribute	MarkdownDecodableAttributedStringKey	no	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
24	AttributeScopes.UIKitAttributes.TextEffectAttribute	AttributedStringKey	YES	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
25	AttributeScopes.UIKitAttributes.TextEffectAttribute	MarkdownDecodableAttributedStringKey	no	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
26	AttributeScopes.UIKitAttributes.BaselineOffsetAttribute	AttributedStringKey	YES	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
27	AttributeScopes.UIKitAttributes.BaselineOffsetAttribute	MarkdownDecodableAttributedStringKey	no	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
28	AttributeScopes.UIKitAttributes.UnderlineColorAttribute	AttributedStringKey	YES	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
29	AttributeScopes.UIKitAttributes.UnderlineColorAttribute	MarkdownDecodableAttributedStringKey	no	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
30	AttributeScopes.UIKitAttributes.StrikethroughColorAttribute	AttributedStringKey	YES	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
31	AttributeScopes.UIKitAttributes.StrikethroughColorAttribute	MarkdownDecodableAttributedStringKey	no	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
32	AttributeScopes.UIKitAttributes.AttachmentAttribute	AttributedStringKey	YES	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
33	AttributeScopes.UIKitAttributes.AttachmentAttribute	MarkdownDecodableAttributedStringKey	no	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
34	AttributeScopes.UIKitAttributes.ObliquenessAttribute	AttributedStringKey	YES	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
35	AttributeScopes.UIKitAttributes.ObliquenessAttribute	MarkdownDecodableAttributedStringKey	no	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
36	AttributeScopes.UIKitAttributes.ExpansionAttribute	AttributedStringKey	YES	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
37	AttributeScopes.UIKitAttributes.ExpansionAttribute	MarkdownDecodableAttributedStringKey	no	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
38	AttributeScopes.UIKitAttributes.TextItemTagAttribute	AttributedStringKey	YES	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
39	AttributeScopes.UIKitAttributes.TextItemTagAttribute	MarkdownDecodableAttributedStringKey	no	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
40	AttributeScopes.UIKitAttributes.AdaptiveImageGlyphAttribute	AttributedStringKey	YES	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
41	AttributeScopes.UIKitAttributes.AdaptiveImageGlyphAttribute	MarkdownDecodableAttributedStringKey	no	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
42	AttributeScopes.AccessibilityAttributes	AttributedStringKey	no	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
43	AttributeScopes.AccessibilityAttributes	AttributeScope	YES	false	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
44	AttributeScopes.AccessibilityAttributes.HeadingLevelAttribute	AttributedStringKey	YES	false	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
45	AttributeScopes.AccessibilityAttributes.HeadingLevelAttribute	MarkdownDecodableAttributedStringKey	YES	false	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
46	AttributeScopes.AccessibilityAttributes.TextCustomAttribute	AttributedStringKey	YES	false	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
47	AttributeScopes.AccessibilityAttributes.TextCustomAttribute	MarkdownDecodableAttributedStringKey	YES	false	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
48	AttributeScopes.AccessibilityAttributes.TextualContextAttribute	AttributedStringKey	YES	false	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
49	AttributeScopes.AccessibilityAttributes.TextualContextAttribute	MarkdownDecodableAttributedStringKey	YES	false	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
50	AttributeScopes.AccessibilityAttributes.IncludesPunctuationAttribute	AttributedStringKey	YES	false	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
51	AttributeScopes.AccessibilityAttributes.IncludesPunctuationAttribute	MarkdownDecodableAttributedStringKey	YES	false	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
52	AttributeScopes.AccessibilityAttributes.AdjustedPitchAttribute	AttributedStringKey	YES	false	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
53	AttributeScopes.AccessibilityAttributes.AdjustedPitchAttribute	MarkdownDecodableAttributedStringKey	YES	false	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
54	AttributeScopes.AccessibilityAttributes.SpellOutAttribute	AttributedStringKey	YES	false	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
55	AttributeScopes.AccessibilityAttributes.SpellOutAttribute	MarkdownDecodableAttributedStringKey	YES	false	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
56	AttributeScopes.AccessibilityAttributes.QueueAnnouncementAttribute	AttributedStringKey	YES	false	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
57	AttributeScopes.AccessibilityAttributes.QueueAnnouncementAttribute	MarkdownDecodableAttributedStringKey	YES	false	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
58	AttributeScopes.AccessibilityAttributes.IPANotationAttribute	AttributedStringKey	YES	false	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
59	AttributeScopes.AccessibilityAttributes.IPANotationAttribute	MarkdownDecodableAttributedStringKey	YES	false	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
60	AttributeScopes.AccessibilityAttributes.AnnouncementPriorityAttribute	AttributedStringKey	YES	false	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
61	AttributeScopes.AccessibilityAttributes.AnnouncementPriorityAttribute	MarkdownDecodableAttributedStringKey	YES	false	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
62	AttributeScopes.FoundationAttributes	AttributedStringKey	no	true	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
63	AttributeScopes.FoundationAttributes	AttributeScope	YES	true	1	static AttributeScope.scopeDescription.getter (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_:including:) (Foundation)
64	AttributeScopes.FoundationAttributes.LinkAttribute	AttributedStringKey	YES	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
65	AttributeScopes.FoundationAttributes.LinkAttribute	MarkdownDecodableAttributedStringKey	no	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
66	AttributeScopes.FoundationAttributes.LanguageIdentifierAttribute	AttributedStringKey	YES	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
67	AttributeScopes.FoundationAttributes.LanguageIdentifierAttribute	MarkdownDecodableAttributedStringKey	YES	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
68	AttributeScopes.FoundationAttributes.PersonNameComponentAttribute	AttributedStringKey	YES	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
69	AttributeScopes.FoundationAttributes.PersonNameComponentAttribute	MarkdownDecodableAttributedStringKey	no	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
70	AttributeScopes.FoundationAttributes.NumberFormatAttributes	AttributedStringKey	no	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
71	AttributeScopes.FoundationAttributes.NumberFormatAttributes	AttributeScope	YES	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
72	AttributeScopes.FoundationAttributes.NumberFormatAttributes.SymbolAttribute	AttributedStringKey	YES	true	3	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
73	AttributeScopes.FoundationAttributes.NumberFormatAttributes.SymbolAttribute	MarkdownDecodableAttributedStringKey	no	true	3	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
74	AttributeScopes.FoundationAttributes.NumberFormatAttributes.NumberPartAttribute	AttributedStringKey	YES	true	3	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
75	AttributeScopes.FoundationAttributes.NumberFormatAttributes.NumberPartAttribute	MarkdownDecodableAttributedStringKey	no	true	3	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
76	AttributeScopes.FoundationAttributes.DateFieldAttribute	AttributedStringKey	YES	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
77	AttributeScopes.FoundationAttributes.DateFieldAttribute	MarkdownDecodableAttributedStringKey	no	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
78	AttributeScopes.FoundationAttributes.AlternateDescriptionAttribute	AttributedStringKey	YES	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
79	AttributeScopes.FoundationAttributes.AlternateDescriptionAttribute	MarkdownDecodableAttributedStringKey	no	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
80	AttributeScopes.FoundationAttributes.ImageURLAttribute	AttributedStringKey	YES	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
81	AttributeScopes.FoundationAttributes.ImageURLAttribute	MarkdownDecodableAttributedStringKey	no	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
82	AttributeScopes.FoundationAttributes.ReplacementIndexAttribute	AttributedStringKey	YES	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
83	AttributeScopes.FoundationAttributes.ReplacementIndexAttribute	MarkdownDecodableAttributedStringKey	no	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
84	AttributeScopes.FoundationAttributes.MeasurementAttribute	AttributedStringKey	YES	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
85	AttributeScopes.FoundationAttributes.MeasurementAttribute	MarkdownDecodableAttributedStringKey	no	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
86	AttributeScopes.FoundationAttributes.ByteCountAttribute	AttributedStringKey	YES	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
87	AttributeScopes.FoundationAttributes.ByteCountAttribute	MarkdownDecodableAttributedStringKey	no	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
88	AttributeScopes.FoundationAttributes.DurationFieldAttribute	AttributedStringKey	YES	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
89	AttributeScopes.FoundationAttributes.DurationFieldAttribute	MarkdownDecodableAttributedStringKey	no	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
90	AttributeScopes.FoundationAttributes.AgreementConceptAttribute	AttributedStringKey	YES	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
91	AttributeScopes.FoundationAttributes.AgreementConceptAttribute	MarkdownDecodableAttributedStringKey	YES	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
92	AttributeScopes.FoundationAttributes.AgreementArgumentAttribute	AttributedStringKey	YES	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
93	AttributeScopes.FoundationAttributes.AgreementArgumentAttribute	MarkdownDecodableAttributedStringKey	YES	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
94	AttributeScopes.FoundationAttributes.ReferentConceptAttribute	AttributedStringKey	YES	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
95	AttributeScopes.FoundationAttributes.ReferentConceptAttribute	MarkdownDecodableAttributedStringKey	YES	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
96	AttributeScopes.FoundationAttributes.LocalizedNumberFormatAttribute	AttributedStringKey	YES	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
97	AttributeScopes.FoundationAttributes.LocalizedNumberFormatAttribute	MarkdownDecodableAttributedStringKey	YES	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
98	AttributeScopes.FoundationAttributes.InlinePresentationIntentAttribute	AttributedStringKey	YES	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
99	AttributeScopes.FoundationAttributes.InlinePresentationIntentAttribute	MarkdownDecodableAttributedStringKey	no	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
100	AttributeScopes.FoundationAttributes.PresentationIntentAttribute	AttributedStringKey	YES	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
101	AttributeScopes.FoundationAttributes.PresentationIntentAttribute	MarkdownDecodableAttributedStringKey	no	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
102	AttributeScopes.FoundationAttributes.MarkdownSourcePositionAttribute	AttributedStringKey	YES	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
103	AttributeScopes.FoundationAttributes.MarkdownSourcePositionAttribute	MarkdownDecodableAttributedStringKey	no	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
104	AttributeScopes.FoundationAttributes.LocalizedStringArgumentAttributes	AttributedStringKey	no	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
105	AttributeScopes.FoundationAttributes.LocalizedStringArgumentAttributes	AttributeScope	no	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
106	AttributeScopes.FoundationAttributes.InflectionAlternativeAttribute	AttributedStringKey	YES	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
107	AttributeScopes.FoundationAttributes.InflectionAlternativeAttribute	MarkdownDecodableAttributedStringKey	YES	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
108	AttributeScopes.FoundationAttributes.MorphologyAttribute	AttributedStringKey	YES	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
109	AttributeScopes.FoundationAttributes.MorphologyAttribute	MarkdownDecodableAttributedStringKey	YES	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
110	AttributeScopes.FoundationAttributes.InflectionRuleAttribute	AttributedStringKey	YES	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
111	AttributeScopes.FoundationAttributes.InflectionRuleAttribute	MarkdownDecodableAttributedStringKey	YES	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
112	AttributeScopes.FoundationAttributes.AssumedFallbackInflectionAttribute	AttributedStringKey	YES	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
113	AttributeScopes.FoundationAttributes.AssumedFallbackInflectionAttribute	MarkdownDecodableAttributedStringKey	YES	true	2	static AttributeScope.scopeDescription.getter (Foundation) ← static AttributeScope.scopeDescription.getter (Foundation)
114	AttributeScopes.UIKitAttributes.FontAttribute	ObjectiveCConvertibleAttributedStringKey	no	false	0	specialized Dictionary<>.init(\*\\\_:attributeTable:options:) (Foundation) ← NSAttributedString.init<τ\\\_0_0>(\\\_\*:including:) (Foundation)

By suggested optimizations we can get rid almost half of them: 52 of 114. So depending on approximation of total casting overhead inside AttributeContainer.init:

1. If we suppose that total casting overhead is 90% of AttributeContainer.init - we will get 52 / 114 * 0.9 = ~40% boost
2. if 50% - then 52 / 114 * 0.5 = ~ 22% boost.

That's how I've got my 20-40% approximation.

I'd be curious to better understand how much of a performance benefit this gives us

Lets come up with some basic approximation for p10 and p50 (median).

Percentile	Today	−20%	−40%
p10	44 ms	35 ms (−9 ms)	26 ms (−18 ms)
p50	66 ms	53 ms (−13 ms)	40 ms (−26 ms)

It may seem insignificant, but I suppose that large portion of all iOS apps is using modern AttributeContainer API and face this slowdown somewhere at app startup / navigation.

On the other hand, our app is enourmously big and we have 205k protocol conformance descriptors. Average enterprise app would have about 80-100k. So we need to downscale our boost by 30% (why not 50%? Because 205k protocol conformance descriptors are introduced by our statically linked binary, and when each dynamic library is loaded, its set of protocol conformance descriptors is appended to the existing list in Swift Runtime, lets say all system dynamic libraries hold about 100k protocol conformance descriptors). So new boost approximation for average enterprise app would be:

Percentile	Today	−20%	−40%
p10	31 ms	25 ms (−6 ms)	18 ms (−13 ms)
p50	46 ms	37 ms (−9 ms)	28 ms (−18 ms)

In my opinion, even with such insignificant numbers optimization is worth it because of scale: it is 13-18 ms boost on main thread in almost every app.

P.S. Based on our app measurements in production environment, p50 (median) for successful swift_conformsToProtocolMaybeInstantiateSuperclasses (first call for given type and protocol) is about 1.3-1.5 ms (it depends on where specific protocol conformance descriptor is
located in binary). For unsuccessful casts - even more, 2.4-2.8 ms (because we are doomed to scan all protocol conformance descriptors, rather than stopping at first matching one). These measurements are correct only when single cast is performed. If multiple casts are performed consequently (this case), overhead drops significantly, due to cache locality. That’s why p50 for AttributeContainer.init is significantly less than 1.3 ms * 112. And single cast inside AttributeContainer.init is only about 500 microseconds