relates to:

• SR-10604 dictionary modify accessor allocates every single time
• SR-10708 switch over Dictionary lookup retains ref count
• SR-10028 one single 'handler as? _ChannelOutboundHandler' allocates 144 times, every time it's executed
• SR-10346 excessive reference counting, leading to CoW copy for every heap insert in NIO :|
• SR-10682 Swift master, ARC performance regression
• SR-11231 ManagedBufferPointer's suggested deinit allocates

Issue Description:

Frequently we see Swift regressions in allocations that get introduced in by newer Swift versions in the SwiftNIO allocation counter tests or by just manually debugging things. This is quite costly and Swift should IMHO have their own allocation counter tests that make sure allocations from certain pieces of code only ever go down.

Examples:

• https://bugs.swift.org/browse/SR-10708

• https://bugs.swift.org/browse/SR-10682

• https://bugs.swift.org/browse/SR-10604

• https://bugs.swift.org/browse/SR-10346

• https://bugs.swift.org/browse/SR-10028

• https://bugs.swift.org/browse/SR-10708

• https://bugs.swift.org/browse/SR-11231

SwiftNIO examples:

• the allocation counter framework: https://github.com/apple/swift-nio/tree/master/IntegrationTests/allocation-counter-tests-framework

• how you'd run it for a given SwiftPM package: https://github.com/apple/swift-nio/blob/master/IntegrationTests/tests_04_performance/test_01_resources/run-nio-alloc-counter-tests.sh

• example test case: https://github.com/apple/swift-nio/blob/master/IntegrationTests/tests_04_performance/test_01_resources/test_future_lots_of_callbacks.swift

• configured limits in our CI: https://github.com/apple/swift-nio/blob/master/docker/docker-compose.1804.50.yaml#L28-L38

• the guts are here: https://github.com/apple/swift-nio/blob/master/IntegrationTests/allocation-counter-tests-framework/template/HookedFunctionsDoHook/Sources/HookedFunctions/src/hooked-functions.c