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[SR-14102] Idiomatic way of checking if an Array<UInt8> starts with a bunch of bytes is slow #56488

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weissi opened this issue Jan 25, 2021 · 3 comments
Open

[SR-14102] Idiomatic way of checking if an Array<UInt8> starts with a bunch of bytes is slow #56488

weissi opened this issue Jan 25, 2021 · 3 comments
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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. performance

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@weissi
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weissi commented Jan 25, 2021

Previous ID SR-14102
Radar rdar://problem/73570855
Original Reporter @weissi
Type Bug

Attachment: Download

Environment 
docker.io/swiftlang/swift:nightly-main-bionic
Swift version 5.4-dev (LLVM 7459512566cf7f4, Swift 8b5869ce6217b1f)
Target: x86_64-unknown-linux-gnu
Additional Detail from JIRA
Votes 0
Component/s
Labels Bug
Assignee None
Priority Medium

md5: cbaa9373048befa9b532f9d0f5ae9bbb

Issue Description:

Consider this code:

@inline(never)
func doIt(_ data: [UInt8]) -> Int {
    if data.starts(with: [1, 2, 3]) {
        return 1
    } else if data.starts(with: [4, 5, 6]) {
        return 2
    } else if data.starts(with: [0xff, 0xef, 0xdd]) {
        return 3
    } else {
        return 4
    }
}

You'd assume that Swift can do better than building three arrays and then doing starts(with🙂 on them. However, that's not the case...

swiftc -emit-assembly -O test.swift  | swift demangle | grep -A200 ^test.doIt | grep -e call -e 'retq'
    callq   __swift_instantiateConcreteTypeFromMangledName
    callq   swift_initStaticObject@PLT
    callq   swift_retain@PLT
    callq   swift_release@PLT
    callq   swift_initStaticObject@PLT
    callq   swift_retain@PLT
    callq   swift_release@PLT
    callq   swift_initStaticObject@PLT
    callq   swift_retain@PLT
    callq   swift_release@PLT
    retq

What's incredibly baffling is that if you rewrite it using Strings, then it generates much better code:

@inline(never)
func stringIt(_ data: [UInt8]) -> Int {
    if data.starts(with: "\u{0001}\u{00002}\u{00003}".utf8) {
        return 1
    } else if data.starts(with: "\u{0004}\u{0005}\u{0006}".utf8) {
        return 2
    // can't do this: } else if data.starts(with: [0xff, 0xef, 0xdd]) {
    // can't do this:    return 3
    } else {
        return 4
    }
}

Unfortunately that's super awkward and only works for unicode code units that are the same as their ASCII value, so I think that's 0 ... 127.

See here: the stringIt code has a retq before any callq

swiftc -emit-assembly -O test.swift  | swift demangle | grep -A200 ^test.stringIt | grep -e call -e 'retq'
    retq
        [...]
@weissi
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weissi commented Jan 25, 2021

@swift-ci create

@weissi
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weissi commented Jan 25, 2021

@milseman or maybe ravikandhadai (JIRA User)? good job in any case 🙂

@weissi
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weissi commented Jan 25, 2021

this leads to ugly code in https://github.com/apple/swift-corelibs-foundation/pull/2966/files#r562223942

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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@weissi @AnthonyLatsis