Concurrency: fix inconsistent _asyncLet_get signatures

[kateinoigakukun]

Resolve #4250

How did I find the root issue (memo)

Given this snippet,

func getX() async -> Int { return 0 }

func yay() async {
  async let x = getX()
  _ = await x
}

Run $ swiftc -emit-ir -Xfrontend -disable-swift-specific-llvm-optzns -parse-as-library test.swift -o test.ll to check LLVM IR before CoroSplit pass. Then, it outputs the below instruction.

define hidden swifttailcc void @"$s4test10EntrypointV3yayyyYaFZ"(%swift.context* swiftasync %0) #0 {
entry:
  ...
  %21 = call { i8*, i8* } (i32, i8*, i8*, ...) @llvm.coro.suspend.async.sl_p0i8p0i8s(
    i32 0,
    i8* %18,
    i8* bitcast (i8* (i8*)* @__swift_async_resume_get_context to i8*),
    i8* bitcast (void (i8*, %swift.context*, i8*, i8*, i8*, %swift.context*)* @__swift_suspend_dispatch_5 to i8*),
    i8* bitcast (void (%swift.context*, i8*, i8*, i8*, %swift.context*)* @swift_asyncLet_get to i8*),
    %swift.context* %19,
    i8* %16,
    i8* %11,
    i8* %18,
    %swift.context* %20
  )
  ...
}

@llvm.coro.suspend.async is an LLVM intrinsic which indicates a suspension point. The CoroSplit pass splits a function into several resumption functions at the suspension points. The generated function takes the value returned by the function passed to the suspension asynchronously. ({ i8*, i8* }) Note that the asynchronously returned type is not the type returned synchronously. swift_asyncLet_get returns nothing synchronously, but it passes two i8* to the resumption function, so llvm.coro.suspend.async's result type is declared as { i8*, i8* }.

In the above example, the function and the resumption function generated from the suspension point would be transformed by CoroSplit pass:

define hidden swifttailcc void @"$s4test10EntrypointV3yayyyYaFZ"(%swift.context* swiftasync %0) #0 {
entry:
  ...
  musttail call swifttailcc void @swift_asyncLet_get(%swift.context* swiftasync %16, i8* %14, i8* %9, i8* bitcast (void (i8*)* @"$s4test10EntrypointVAAyyYaFZTY1_" to i8*), %swift.context* %17) #1
  ...
}

define internal swiftcc void @"$s4test10EntrypointVAAyyYaFZTY1_"(i8* swiftasync %0, i8* %1) #0 {
entryresume.1:
  ...
}

As I said before, the resumption function (s4test10EntrypointVAAyyYaFZTY1_) takes two parameters.
However, swift_asyncLet_get passes only an async context, so it crashes on wasm due to signature mismatch.

So, who determines the async result type of swift_asyncLet_get? The result type is computed by SignatureExpansion::expandAsyncAwaitType and its input is the intrinsic function declaration at stdlib. The Swift level declaration says that it returns async context and a pointer asynchronously, but its implementation returns only async context. That's the mismatch.

[MaxDesiatov]

👍