Protocol conformance cache for generic types #82818

DmT021 · 2025-07-05T19:33:24Z

This change adds a new type of cache (cache by type descriptor) to the protocol conformance lookup system. This optimization is beneficial for generic types, where the same conformance can be reused across different instantiations of the generic type.

Key changes:

Add a GetOrInsertManyScope class to ConcurrentReadableHashMap for performing multiple insertions under a single lock
Add type descriptor-based caching for protocol conformances
Add environment variables for controlling and debugging the conformance cache
Add tests to verify the behavior of the conformance cache
Fix for Runtime protocol conformance check unexpected first success #82889

The implementation is controlled by the SWIFT_DEBUG_ENABLE_CACHE_PROTOCOL_CONFORMANCES_BY_TYPE_DESCRIPTOR environment variable, which is enabled by default.

Here are some numbers for a synthetic example:

// slow_protocol_conformance.swift

import Foundation

func measureWithDispatchTime<T>(_ f: () -> T) -> (T, TimeInterval) {
  let start = DispatchTime.now()
  let result = f()
  let finish = DispatchTime.now()
  let delta = finish.uptimeNanoseconds - start.uptimeNanoseconds
  let time = Double(delta) / Double(NSEC_PER_SEC)
  return (result, time)
}

protocol Proto {}
struct S0<T>: Proto {}
struct S1<T>: Proto {}

func wrapEachInS(types: [Any.Type], depth: Int = 1) -> [Any.Type] {
  if depth == 0 {
    return types
  }
  func handleExistential<T>(_ type: T.Type) -> [Any.Type] {
    [
      S0<T>.self,
      S1<T>.self,
    ]
  }
  let wrapped = types.flatMap {
    _openExistential($0, do: handleExistential)
  }
  return wrapEachInS(types: wrapped, depth: depth - 1)
}

@_optimize(none)
public func blackHole(_: some Any) {}

let depth = if CommandLine.arguments.count == 2, 
  let value = Int(CommandLine.arguments[1]) {
  value
} else {
  20
}
let types = wrapEachInS(types: [Void.self], depth: depth)
func measure() -> TimeInterval {
  measureWithDispatchTime({
    for type in types {
      blackHole(type as? Proto.Type)
    }
  }).1
}
let n = 1 << depth
let firstMeasure = measure()
let secondMeasure = measure()
print("Time for \(n) types:")
print("  First = \(firstMeasure) s, or \(Int(firstMeasure / Double(n) * Double(NSEC_PER_SEC))) ns per type")
print("  Second = \(secondMeasure) s, or \(Int(secondMeasure / Double(n) * Double(NSEC_PER_SEC))) ns per type")

swiftc -c slow_protocol_conformance.swift -module-name main -O

On main:

/usr/bin/env \
  SWIFT_DEBUG_ENABLE_SHARED_CACHE_PROTOCOL_CONFORMANCES=0 \
  DYLD_LIBRARY_PATH=/Users/dmt021/w/swift-project/build/Ninja+cmark-RelWithDebInfoAssert+llvm-RelWithDebInfo+swift-RelWithDebInfoAssert+stdlib-RelWithDebInfoAssert/swift-macosx-arm64/lib/swift/macosx \
  ./slow_protocol_conformance
Time for 1048576 types:
  First = 18.618541792 s, or 17756 ns per type
  Second = 0.059922125 s, or 57 ns per type

hyperfine '/usr/bin/env \
  SWIFT_DEBUG_ENABLE_SHARED_CACHE_PROTOCOL_CONFORMANCES=0 \
  DYLD_LIBRARY_PATH=/Users/dmt021/w/swift-project/build/Ninja+cmark-RelWithDebInfoAssert+llvm-RelWithDebInfo+swift-RelWithDebInfoAssert+stdlib-RelWithDebInfoAssert/swift-macosx-arm64/lib/swift/macosx \
  ./slow_protocol_conformance'
Benchmark 1: /usr/bin/env \
  SWIFT_DEBUG_ENABLE_SHARED_CACHE_PROTOCOL_CONFORMANCES=0 \
  DYLD_LIBRARY_PATH=/Users/dmt021/w/swift-project/build/Ninja+cmark-RelWithDebInfoAssert+llvm-RelWithDebInfo+swift-RelWithDebInfoAssert+stdlib-RelWithDebInfoAssert/swift-macosx-arm64/lib/swift/macosx \
  ./slow_protocol_conformance
  Time (mean ± σ):     20.356 s ±  0.264 s    [User: 20.032 s, System: 0.172 s]
  Range (min … max):   20.111 s … 20.950 s    10 runs

On the PR branch with the feature disabled:

/usr/bin/env \
  SWIFT_DEBUG_ENABLE_SHARED_CACHE_PROTOCOL_CONFORMANCES=0 \
  SWIFT_DEBUG_ENABLE_CACHE_PROTOCOL_CONFORMANCES_BY_TYPE_DESCRIPTOR=0 \
  DYLD_LIBRARY_PATH=/Users/dmt021/w/swift-project/build/Ninja+cmark-RelWithDebInfoAssert+llvm-RelWithDebInfo+swift-RelWithDebInfoAssert+stdlib-RelWithDebInfoAssert/swift-macosx-arm64/lib/swift/macosx \
  ./slow_protocol_conformance
Time for 1048576 types:
  First = 18.620672959 s, or 17758 ns per type
  Second = 0.066553542 s, or 63 ns per type

hyperfine '/usr/bin/env \
  SWIFT_DEBUG_ENABLE_SHARED_CACHE_PROTOCOL_CONFORMANCES=0 \
  SWIFT_DEBUG_ENABLE_CACHE_PROTOCOL_CONFORMANCES_BY_TYPE_DESCRIPTOR=0 \
  DYLD_LIBRARY_PATH=/Users/dmt021/w/swift-project/build/Ninja+cmark-RelWithDebInfoAssert+llvm-RelWithDebInfo+swift-RelWithDebInfoAssert+stdlib-RelWithDebInfoAssert/swift-macosx-arm64/lib/swift/macosx \
  ./slow_protocol_conformance'
Benchmark 1: /usr/bin/env \
  SWIFT_DEBUG_ENABLE_SHARED_CACHE_PROTOCOL_CONFORMANCES=0 \
  SWIFT_DEBUG_ENABLE_CACHE_PROTOCOL_CONFORMANCES_BY_TYPE_DESCRIPTOR=0 \
  DYLD_LIBRARY_PATH=/Users/dmt021/w/swift-project/build/Ninja+cmark-RelWithDebInfoAssert+llvm-RelWithDebInfo+swift-RelWithDebInfoAssert+stdlib-RelWithDebInfoAssert/swift-macosx-arm64/lib/swift/macosx \
  ./slow_protocol_conformance
  Time (mean ± σ):     20.335 s ±  0.129 s    [User: 20.108 s, System: 0.173 s]
  Range (min … max):   20.169 s … 20.542 s    10 runs

On the PR branch with the feature enabled:

/usr/bin/env \
  SWIFT_DEBUG_ENABLE_SHARED_CACHE_PROTOCOL_CONFORMANCES=0 \
  SWIFT_DEBUG_ENABLE_CACHE_PROTOCOL_CONFORMANCES_BY_TYPE_DESCRIPTOR=1 \
  DYLD_LIBRARY_PATH=/Users/dmt021/w/swift-project/build/Ninja+cmark-RelWithDebInfoAssert+llvm-RelWithDebInfo+swift-RelWithDebInfoAssert+stdlib-RelWithDebInfoAssert/swift-macosx-arm64/lib/swift/macosx \
  ./slow_protocol_conformance
Time for 1048576 types:
  First = 0.293545208 s, or 279 ns per type
  Second = 0.069242875 s, or 66 ns per type

hyperfine '/usr/bin/env \
  SWIFT_DEBUG_ENABLE_SHARED_CACHE_PROTOCOL_CONFORMANCES=0 \
  SWIFT_DEBUG_ENABLE_CACHE_PROTOCOL_CONFORMANCES_BY_TYPE_DESCRIPTOR=1 \
  DYLD_LIBRARY_PATH=/Users/dmt021/w/swift-project/build/Ninja+cmark-RelWithDebInfoAssert+llvm-RelWithDebInfo+swift-RelWithDebInfoAssert+stdlib-RelWithDebInfoAssert/swift-macosx-arm64/lib/swift/macosx \
  ./slow_protocol_conformance'
Benchmark 1: /usr/bin/env \
  SWIFT_DEBUG_ENABLE_SHARED_CACHE_PROTOCOL_CONFORMANCES=0 \
  SWIFT_DEBUG_ENABLE_CACHE_PROTOCOL_CONFORMANCES_BY_TYPE_DESCRIPTOR=1 \
  DYLD_LIBRARY_PATH=/Users/dmt021/w/swift-project/build/Ninja+cmark-RelWithDebInfoAssert+llvm-RelWithDebInfo+swift-RelWithDebInfoAssert+stdlib-RelWithDebInfoAssert/swift-macosx-arm64/lib/swift/macosx \
  ./slow_protocol_conformance
  Time (mean ± σ):      1.571 s ±  0.033 s    [User: 1.483 s, System: 0.078 s]
  Range (min … max):    1.514 s …  1.629 s    10 runs

DmT021 · 2025-07-05T19:33:37Z

@swift-ci please test

DmT021 · 2025-07-05T19:35:41Z

@swift-ci Please benchmark

DmT021 · 2025-07-06T03:39:48Z

@swift-ci please test

DmT021 · 2025-07-06T12:09:09Z

@swift-ci please smoke test

DmT021 · 2025-07-06T22:29:22Z

@swift-ci please test macOS

DmT021 · 2025-07-09T00:33:51Z

@swift-ci please test

DmT021 · 2025-07-09T12:27:07Z

@swift-ci please smoke test macOS

DmT021 · 2025-07-09T14:04:03Z

@swift-ci please smoke test

DmT021 · 2025-07-09T19:19:15Z

@swift-ci please smoke test Linux

mikeash

This is great! Nice idea. Hopefully the memory impact is small, should be if there are on average many generic types used for a given descriptor. I left a few very minor comments, but overall this looks really good.

stdlib/public/runtime/ProtocolConformance.cpp

include/swift/Runtime/Concurrent.h

This change adds a new type of cache (cache by type descriptor) to the protocol conformance lookup system. This optimization is beneficial for generic types, where the same conformance can be reused across different instantiations of the generic type. Key changes: - Add a `GetOrInsertManyScope` class to `ConcurrentReadableHashMap` for performing multiple insertions under a single lock - Add type descriptor-based caching for protocol conformances - Add environment variables for controlling and debugging the conformance cache - Add tests to verify the behavior of the conformance cache - Fix for swiftlang#82889 The implementation is controlled by the `SWIFT_DEBUG_ENABLE_CACHE_PROTOCOL_CONFORMANCES_BY_TYPE_DESCRIPTOR` environment variable, which is enabled by default.

DmT021 · 2025-07-11T01:56:34Z

@swift-ci please smoke test

DmT021 · 2025-07-11T06:51:51Z

@swift-ci please smoke test Windows

DmT021 force-pushed the wp/conformance-cache-by-descriptor branch from 712d217 to 645d438 Compare July 6, 2025 03:38

DmT021 force-pushed the wp/conformance-cache-by-descriptor branch from 645d438 to e42a0e0 Compare July 9, 2025 00:31

DmT021 force-pushed the wp/conformance-cache-by-descriptor branch from e42a0e0 to 6b0c1ec Compare July 9, 2025 14:03

DmT021 marked this pull request as ready for review July 10, 2025 01:44

DmT021 requested review from rjmccall, mikeash, al45tair and compnerd as code owners July 10, 2025 01:44

mikeash requested changes Jul 10, 2025

View reviewed changes

DmT021 force-pushed the wp/conformance-cache-by-descriptor branch from 6b0c1ec to 7989dbe Compare July 11, 2025 01:54

DmT021 requested a review from mikeash July 11, 2025 12:06

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Protocol conformance cache for generic types #82818

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